Introduction

Architecture

Design

MCP Rotuer

Implementing an MCP (Model Context Protocol) Router inside light-4j gateway is a visionary and highly strategic idea.

The industry is currently struggling with “Tool Sprawl”—where every microservice needs to be manually taught to an LLM. By placing an MCP Router at the gateway level, we effectively turn our entire microservice ecosystem into a single, searchable library of capabilities for AI agents.

Here is a breakdown of why this is a good idea and how to design it for the light-4j ecosystem.

1. Why it is a Strategic Win

• Discovery at Scale: Instead of configuring 50 tools for an agent, the agent connects to our gateway via MCP. The gateway then “advertises” the available tools based on the services it already knows.
• Protocol Translation: Our backend services don’t need to know what MCP is. The gateway handles the conversion from MCP (JSON-RPC over SSE/HTTP) to REST (OpenAPI) or GraphQL.
• Security & Governance: We can apply light-4j’s existing JWT validation, rate limiting, and audit logging to AI interactions. We control which agents have access to which “tools” (APIs).
• Schema Re-use: We already have OpenAPI specs or GraphQL schemas in light-4j. We can dynamically generate the MCP “Tool Definitions” from these existing schemas.

2. Design Considerations

A. The Transport Layer

MCP supports two primary transports: Stdio (for local scripts) and HTTP with SSE (Server-Sent Events) (for remote services).

• Decision: For a gateway, we must use the HTTP + SSE transport.
• Implementation: Light-4j is built on Undertow, which has excellent support for SSE. We will need to implement an MCP endpoint (e.g., /mcp/message) and an SSE endpoint (e.g., /mcp/sse).

B. Tool Discovery & Dynamic Mapping

How does the Gateway decide which APIs to expose as MCP Tools?

• Metadata Driven: Use light-4j configuration or annotations in the OpenAPI files to mark specific endpoints as “AI-enabled.”
• The Mapper: Create a component that converts an OpenAPI Operation into an MCP Tool Definition.
  • description in OpenAPI becomes the tool’s description (crucial for LLM reasoning).
  • requestBody schema becomes the tool’s inputSchema.

C. Authentication & Context Pass-through

This is the hardest part.

• The Problem: The LLM agent connects to the Gateway, but the Backend Microservice needs a user-specific JWT.
• The Solution: The MCP Router must be able to take the identity from the MCP connection (initial handshake) and either pass it through or exchange it for a backend token (OAuth2 Token Exchange).

D. Statefulness vs. Statelessness

MCP is often stateful (sessions).

• Implementation: Since light-4j is designed for high performance and statelessness, we may need light-session-4j a small Session Manager (potentially backed by Redis or PostgresQL) to keep track of which MCP Client is mapped to which internal context during the SSE connection.

3. Implementation Plan for light-4j

Step 1: Create the McpHandler

Create a new middleware handler in light-4j that intercepts calls to /mcp.

• This handler must implement the MCP lifecycle: initialize -> list_tools -> call_tool.

Step 2: Tool Registry

Implement a registry that scans our gateway’s internal routing table.

• REST: For every path (e.g., GET /customers/{id}), generate an MCP tool named get_customers_by_id.
• GraphQL: For every Query/Mutation, generate a corresponding MCP tool.

Step 3: JSON-RPC over SSE

MCP uses JSON-RPC 2.0. We will need a simple parser that:

1. Receives an MCP call_tool request.
2. Identifies the internal REST/GraphQL route.
3. Executes a local dispatch (internal call) to the existing light-4j handler for that service.
4. Wraps the response in an MCP content object and sends it back via SSE.

Step 4: Governance (The “Agentic” Layer)

Add a “Critique” or “Guardrail” check. Since this is at the gateway, we can inspect the tool output. If the LLM requested sensitive data, the Gateway can mask it before the agent sees it.

Step 5: MCP Proxy Support

The MCP Router supports acting as a proxy for backend MCP servers.

• Configuration: Tools can be configured with a protocol field (http or mcp).
• Behavior:
  • http (default): Transforming MCP tool calls to REST requests (JSON translation).
  • mcp: Forwarding JSON-RPC requests directly to a backend MCP server via HTTP POST.
• Use Case: This allows integrating existing MCP servers (e.g., Node.js, Python) into the light-4j gateway without rewriting them implementation.

4. Potential Challenges to Watch

1. Context Window Overload: If our gateway has 500 APIs, sending 500 tool definitions to the LLM will crash its context window.
   • Solution: Implement Categorization. When an agent connects, it should specify a “Scope” (e.g., mcp?scope=accounting), and the gateway only returns tools relevant to that scope.
2. Latency: Adding a protocol translation layer at the gateway adds milliseconds.
   • Solution: Light-4j’s native performance is our advantage here. Minimize JSON serialization/deserialization by using direct buffer access where possible.
3. Complex Schemas: Some API payloads are too complex for an LLM to understand.
   • Solution: Provide a “Summary” view. Allow our MCP router to transform a complex 100-field JSON response into a 5-field summary that the LLM can actually use.

Conclusion

Building an MCP Router for light-4j transforms our API Gateway from a “Traffic Cop” into an “AI Brain Center.” It allows our Java-based enterprise services to be “Agent-Ready” without touching the underlying microservice code.

Configuration Hot Reload Design

Introduction

In the light-4j framework, minimizing downtime is crucial for microservices. The Configuration Hot Reload feature allows services to update their configuration at runtime without restarting the server. This design document outlines the centralized caching architecture used to achieve consistent and efficient hot reloads.

Architecture Evolution

Previous Approach (Decentralized)

Initially, configuration reload was handled in a decentralized manner:

• Each handler maintained its own static configuration object.
• A reload() method was required on every handler to manually refresh this object.
• The ConfigReloadHandler used reflection to search for and invoke these reload() methods.

Drawbacks:

• Inconsistency: Different parts of the application could hold different versions of the configuration.
• Complexity: Every handler needed boilerplate code for reloading.
• State Management: Singleton classes (like ClientConfig) often held stale references that were difficult to update.

Current Approach (Centralized Cache)

The new architecture centralizes the “source of truth” within the Config class itself.

• Centralized Cache: The Config class maintains a ConcurrentHashMap of all loaded configurations.
• Cache Invalidation: Instead of notifying components to reload, we simply invalidate the specific entry in the central cache.
• Lazy Loading: Consumers (Handlers, Managers) fetch the configuration from the Config class at the moment of use. If the cache is empty (cleared), Config reloads it from the source files.

Detailed Design

1. The Config Core (Config.java)

The Config class is enhanced to support targeted cache invalidation.

public abstract class Config {
    // ... existing methods
    
    // New method to remove a specific config from memory
    public abstract void clearConfigCache(String configName);
}

When clearConfigCache("my-config") is called:

1. The entry for “my-config” is removed from the internal configCache.
2. The next time getJsonMapConfig("my-config") or getJsonObjectConfig(...) is called, the Config class detects the miss and reloads the file from the filesystem or external config server.

2. The Admin Endpoint (ConfigReloadHandler)

The ConfigReloadHandler exposes the /adm/config-reload endpoint. Its responsibility has been simplified:

1. Receive Request: Accepts a list of modules/plugins to reload.
2. Resolve Config Names: Looks up the configuration file names associated with the requested classes using the ModuleRegistry.
3. Invalidate Cache: Calls Config.getInstance().clearConfigCache(configName) for each identified module.

It no longer relies on reflection to call methods on the handlers.

3. Configuration Consumers

Handlers and other components must follow a stateless pattern regarding configuration.

Anti-Pattern (Old Way):

public class MyHandler {
    static MyConfig config = MyConfig.load(); // Static load at startup
    
    public void handleRequest(...) {
        // Use static config - will remain stale even if file changes
        if (config.isEnabled()) ... 
    }
}

Recommended Pattern (New Way):

public class MyHandler {
    // No static config field
    
    public void handleRequest(...) {
        // Fetch fresh config from central cache every time
        // This is fast due to HashMap lookup
        MyConfig config = MyConfig.load(); 
        
        if (config.isEnabled()) ...
    }
}

Implementation in Config Classes: The load() method in configuration classes (e.g., CorrelationConfig) simply delegates to the cached Config methods:

private CorrelationConfig(String configName) {
    // Always ask Config class for the Map. 
    // If cache was cleared, this call triggers a file reload.
    mappedConfig = Config.getInstance().getJsonMapConfig(configName); 
}

4. Handling Singletons (e.g., ClientConfig)

For Singleton classes that parse configuration into complex objects, they must check if the underlying configuration has changed.

public static ClientConfig get() {
    // Check if the Map instance in Config.java is different from what we typically hold
    Map<String, Object> currentMap = Config.getInstance().getJsonMapConfig(CONFIG_NAME);
    if (instance == null || instance.mappedConfig != currentMap) {
        synchronized (ClientConfig.class) {
            if (instance == null || instance.mappedConfig != currentMap) {
                instance = new ClientConfig(); // Re-parse and create new instance
            }
        }
    }
    return instance;
}

}
return instance;

}

5. Lazy Rebuild on Config Change

Some handlers (like LightProxyHandler) maintain expensive internal objects that depend on the configuration (e.g., LoadBalancingProxyClient, ProxyHandler). Recreating these on every request is not feasible due to performance. However, they must still react to configuration changes.

For these cases, we use a Lazy Rebuild pattern:

1. Volatile Config Reference: The handler maintains a volatile reference to its configuration object.
2. Check on Request: At the start of handleRequest, it checks if the cached config object is the same as the one returned by Config.load().
3. Rebuild if Changed: If the reference has changed (identity check), it synchronizes and rebuilds the internal components.

Example Implementation (LightProxyHandler):

public class LightProxyHandler implements HttpHandler {
    private volatile ProxyConfig config;
    private volatile ProxyHandler proxyHandler;

    public LightProxyHandler() {
        this.config = ProxyConfig.load();
        buildProxy(); // Initial build
    }

    private void buildProxy() {
        // Expensive object creation based on config
        this.proxyHandler = ProxyHandler.builder()
                .setProxyClient(new LoadBalancingProxyClient()...)
                .build();
    }

    @Override
    public void handleRequest(HttpServerExchange exchange) throws Exception {
        ProxyConfig newConfig = ProxyConfig.load();
        // Identity check: ultra-fast
        if (newConfig != config) {
            synchronized (this) {
                newConfig = ProxyConfig.load(); // Double-check
                if (newConfig != config) {
                    config = newConfig;
                    buildProxy(); // Rebuild internal components
                }
            }
        }
        // Use the (potentially new) proxyHandler
        proxyHandler.handleRequest(exchange);
    }
}

This pattern ensures safe updates without the overhead of rebuilding on every request, and without requiring a manual reload() method.

6. Config Class Implementation Pattern (Singleton with Caching)

For configuration classes that are frequently accessed (per request), instantiating a new object each time can be expensive. We recommend implementing a Singleton pattern that caches the configuration object and only invalidates it when the underlying configuration map changes.

Example Implementation (ApiKeyConfig):

public class ApiKeyConfig {
    private static final String CONFIG_NAME = "apikey";
    // Cache the instance
    private static ApiKeyConfig instance;
    private final Map<String, Object> mappedConfig;
    
    // Private constructor to force use of load()
    private ApiKeyConfig(String configName) {
        mappedConfig = Config.getInstance().getJsonMapConfig(configName);
        setConfigData();
    }

    public static ApiKeyConfig load() {
        return load(CONFIG_NAME);
    }

    public static ApiKeyConfig load(String configName) {
        // optimistically check if we have a valid cached instance
        Map<String, Object> mappedConfig = Config.getInstance().getJsonMapConfig(configName);
        if (instance != null && instance.getMappedConfig() == mappedConfig) {
            return instance;
        }
        
        // Double-checked locking for thread safety
        synchronized (ApiKeyConfig.class) {
            mappedConfig = Config.getInstance().getJsonMapConfig(configName);
            if (instance != null && instance.getMappedConfig() == mappedConfig) {
                return instance;
            }
            instance = new ApiKeyConfig(configName);
            // Register the module with the configuration. masking the apiKey property.
            // As apiKeys are in the config file, we need to mask them.
            List<String> masks = new ArrayList<>();
            // if hashEnabled, there is no need to mask in the first place.
            if(!instance.hashEnabled) {
                masks.add("apiKey");
            }
            ModuleRegistry.registerModule(configName, ApiKeyConfig.class.getName(), Config.getNoneDecryptedInstance().getJsonMapConfigNoCache(configName), masks);

            return instance;
        }
    }
    
    public Map<String, Object> getMappedConfig() {
        return mappedConfig;
    }
}

This pattern ensures that:

1. Performance: Applications use the cached instance for the majority of requests (fast reference check).
2. Freshness: If Config.getInstance().getJsonMapConfig(name) returns a new Map object (due to a reload), the equality check fails, and a new ApiKeyConfig is created.
3. Consistency: The handleRequest method still calls ApiKeyConfig.load(), but receives the singleton instance transparently.

6. Thread Safety

The Config class handles concurrent access using the Double-Checked Locking pattern to ensure that the configuration file is loaded exactly once, even if multiple threads request it simultaneously immediately after the cache is cleared.

Scenario:

1. Thread A and Thread B both handle a request for MyHandler.
2. Both call MyConfig.load(), which calls Config.getJsonMapConfig("my-config").
3. Both see that the cache is empty (returning null) because it was just cleared by the reload handler.
4. Thread A acquires the lock (synchronized). Thread B waits.
5. Thread A checks the cache again (still null), loads the file from disk, puts it in the configCache, and releases the lock.
6. Thread B acquires the lock.
7. Thread B checks the cache again. This time it finds the config loaded by Thread A.
8. Thread B uses the existing config without loading from disk.

This ensures no race conditions or redundant file I/O operations occur in high-concurrency environments.

Workflow Summary

1. Update: User updates values.yml or a config file on the server/filesystem.
2. Trigger: User calls POST https://host:port/adm/config-reload with the module name.
3. Clear: ConfigReloadHandler tells Config.java to clearConfigCache for that module.
4. Processing:
   • Step 4a: Request A arrives at MyHandler.
   • Step 4b: MyHandler calls MyConfig.load().
   • Step 4c: MyConfig calls Config.getJsonMapConfig().
   • Step 4d: Config sees cache miss, reads file from disk, parses it, puts it in cache, and returns it.
   • Step 4e: MyHandler processes request with NEW configuration.
5. Subsequent Requests: Step 4d is skipped; data is served instantly from memory.

Configuration Consistency During Request Processing

Can Config Objects Change During a Request?

A common question arises: Can the config object created in handleRequest be changed during the request/response exchange?

Short Answer: Theoretically possible but extremely unlikely, and the design handles this correctly.

Understanding the Behavior

When a handler processes a request using the recommended pattern:

public void handleRequest(HttpServerExchange exchange) {
    MyConfig config = MyConfig.load(); // Creates local reference
    
    // Use config throughout request processing
    if (config.isEnabled()) {
        // ... process request
    }
}

The config variable is a local reference to a configuration object. Here’s what happens:

1. Cache Hit: MyConfig.load() calls Config.getJsonMapConfig(configName) which returns a reference to the cached Map<String, Object>.
2. Object Construction: A new MyConfig object is created, wrapping this Map reference.
3. Local Scope: The config variable holds this reference for the duration of the request.

Scenario: Reload During Request Processing

Consider this timeline:

1. T1: Request A starts, calls MyConfig.load(), gets reference to Config Object v1
2. T2: Admin calls /adm/config-reload, cache is cleared
3. T3: Request B starts, calls MyConfig.load(), triggers reload, gets reference to Config Object v2
4. T4: Request A continues processing with Config Object v1
5. T5: Request A completes successfully with Config Object v1

Key Points:

• Request A maintains its reference to the original config object throughout its lifecycle
• Request B gets a new config object with reloaded values
• Both requests process correctly with consistent configuration for their entire duration
• No race conditions or inconsistent state within a single request

Why This Design is Safe

1. Immutable Config Objects

Configuration objects are effectively immutable once constructed:

private MyConfig(String configName) {
    mappedConfig = Config.getInstance().getJsonMapConfig(configName);
    setConfigData(); // Parses and sets final fields
}

Fields are set during construction and never modified afterward.

2. Local Variable Isolation

Each request has its own local config variable:

• The reference is stored on the thread’s stack
• Even if the cache is cleared, the reference remains valid
• The underlying Map object continues to exist until no references remain (garbage collection)

3. Per-Request Consistency

This design ensures that each request has a consistent view of configuration from start to finish:

• No mid-request configuration changes
• Predictable behavior throughout request processing
• Easier debugging and reasoning about request flow

4. Graceful Transition

The architecture enables zero-downtime config updates:

• In-flight requests: Complete with the config they started with
• New requests: Use the updated configuration
• No interruption: No requests fail due to config reload

Edge Cases and Considerations

Long-Running Requests

For requests that take significant time to process (e.g., minutes):

• The request will complete with the configuration it started with
• If config is reloaded during processing, the request continues with “old” config
• This is correct behavior - we want consistent config per request

High-Concurrency Scenarios

During a config reload under heavy load:

• Multiple threads may simultaneously detect cache miss
• Double-checked locking ensures only one thread loads from disk
• All threads eventually get the same new config instance
• No duplicate file I/O or parsing overhead

Memory Implications

Question: If old config objects are still referenced by in-flight requests, do we have memory leaks?

Answer: No, this is handled by Java’s garbage collection:

1. Request A holds reference to Config Object v1
2. Cache is cleared and reloaded with Config Object v2
3. Request A completes and goes out of scope
4. Config Object v1 has no more references
5. Garbage collector reclaims Config Object v1

The memory overhead is minimal and temporary, lasting only as long as the longest in-flight request.

Best Practices

To ensure optimal behavior with hot reload:

1. Always Load Fresh: Call MyConfig.load() at the start of handleRequest, not in constructor

   // ✅ GOOD
   public void handleRequest(...) {
       MyConfig config = MyConfig.load();
   }
   
   // ❌ BAD
   private static MyConfig config = MyConfig.load();
   

2. Use Local Variables: Store config in local variables, not instance fields

   // ✅ GOOD
   MyConfig config = MyConfig.load();
   
   // ❌ BAD
   this.config = MyConfig.load();
   

3. Don’t Cache in Handlers: Let the Config class handle caching

   // ✅ GOOD - Load on each request
   public void handleRequest(...) {
       MyConfig config = MyConfig.load();
   }
   
   // ❌ BAD - Caching in handler
   private MyConfig cachedConfig;
   public void handleRequest(...) {
       if (cachedConfig == null) cachedConfig = MyConfig.load();
   }
   

Summary

The centralized cache design ensures:

• Thread Safety: Multiple threads can safely reload and access config
• Request Consistency: Each request has a stable config view from start to finish
• Zero Downtime: Config updates don’t interrupt in-flight requests
• Performance: HashMap lookups are extremely fast (O(1))
• Simplicity: No complex synchronization needed in handlers

Benefits

1. Performance: Only one disk read per reload cycle. Subsequent accesses are Hash Map lookups.
2. Reliability: Config state is consistent. No chance of “half-reloaded” application state.
3. Simplicity: drastic reduction in boilerplate code across the framework.

Module Registry Design

Introduction

The Module Registry is a core component in the light-4j framework that tracks all active modules (middleware handlers, plugins, utilities) and their current configurations. This info is primarily exposed via the /server/info admin endpoint, allowing operators to verify the runtime state of the service.

With the introduction of Centralized Configuration Management and Hot Reload, the Module Registry design has been updated to ensure consistency updates without manual intervention from handlers.

Architecture

Centralized Registration

Previously, each Handler was responsible for registering its configuration in its constructor or reload() method. This led to decentralized logic and potential inconsistencies during hot reloads.

In the new design, Configuration Classes (*Config.java) are responsible for registering themselves with the ModuleRegistry immediately upon instantiation.

Workflow:

1. Handler requests config: MyConfig.load().
2. Config class loads data from the central cache.
3. MyConfig constructor initializes fields.
4. MyConfig constructor calls ModuleRegistry.registerModule(...).

Caching and Optimization

Since MyConfig objects are instantiated per-request (to ensure fresh config is used), the registration call happens frequently. To prevent performance degradation, ModuleRegistry implements an Identity Cache.

// Key: configName + ":" + moduleClass
private static final Map<String, Object> registryCache = new HashMap<>();

public static void registerModule(String configName, String moduleClass, Map<String, Object> config, List<String> masks) {
    String key = configName + ":" + moduleClass;
    
    // Optimization: Identity Check
    if (config != null && registryCache.get(key) == config) {
        return; // Exact same object already registered, skip overhead
    }
    // ... proceed with registration
}

This ensures that while the registration intent is declared on every request, the heavy lifting (deep copying, masking) only happens when the configuration object actually changes (i.e., after a reload).

Security and Masking

Configurations often contain sensitive secrets (passwords, API keys). The ModuleRegistry must never store or expose these in plain text.

1. Non-Decrypted Config

The Config framework supports auto-decryption of CRYPT:... values. However, server/info should show the original encrypted value (or a mask), not the decrypted secret.

Config classes register the Non-Decrypted version of the config map:

// Inside MyConfig constructor
ModuleRegistry.registerModule(
    CONFIG_NAME, 
    MyConfig.class.getName(), 
    Config.getNoneDecryptedInstance().getJsonMapConfigNoCache(CONFIG_NAME), 
    List.of("secretKey", "password")
);

2. Deep Copy & Masking

To prevent the ModuleRegistry from accidentally modifying the active configuration object (or vice versa), and to safely apply masks without affecting the runtime application:

1. ModuleRegistry creates a Deep Copy of the configuration map.
2. Masks (e.g., replacing values with *) are applied to the copy.
3. The masked copy is stored in the registry for display.

Best Practices for Module Developers

When creating a new module with a configuration file:

1. Self-Register in Config Constructor: Call ModuleRegistry.registerModule inside your Config class constructor.
2. Use Non-Decrypted Instance: Always fetch the config for registration using Config.getNoneDecryptedInstance().
3. Define Masks: specific attributes that should be masked (e.g., passwords, tokens) in the registration call.
4. Remove Handler Registration: Do not call register in your Handler, static blocks, or reload() methods.

Example

public class MyConfig {
    private MyConfig(String configName) {
        // 1. Load runtime config
        config = Config.getInstance();
        mappedConfig = config.getJsonMapConfig(configName);
        setConfigData();
        
        // 2. Register with ModuleRegistry
        ModuleRegistry.registerModule(
            configName, 
            MyConfig.class.getName(), 
            Config.getNoneDecryptedInstance().getJsonMapConfigNoCache(configName), 
            List.of("clientSecret") // Mask sensitive fields
        );
    }
}

HttpClient Retry

Since the JDK HttpClient supports proxy configuration, we use it to connect to external services through NetSkope or McAfee gateways within an enterprise environment. When comparing it with the light-4j Http2Client, we identified several important behavioral differences and usage considerations.

Unresolved InetSocketAddress

When configuring a proxy for the JDK HttpClient, it is important to use an unresolved socket address for the proxy host.

        if (config.getProxyHost() != null && !config.getProxyHost().isEmpty()) {
            clientBuilder.proxy(ProxySelector.of(
                    InetSocketAddress.createUnresolved(
                            config.getProxyHost(),
                            config.getProxyPort() == 0 ? 443 : config.getProxyPort()
                    )
            ));
        }

Using InetSocketAddress.createUnresolved() ensures that DNS resolution occurs when a new connection is established, rather than at client creation time. This allows DNS lookups to respect TTL values and return updated IP addresses when DNS records change.

Retry with Temporary HttpClient

The JDK HttpClient is a heavyweight, long-lived object and should not be created frequently. It also does not expose APIs to directly manage connections (for example, closing an existing connection and retrying with a fresh one).

A naive retry strategy is to create a new HttpClient instance for retries. Below is an implementation we used for testing purposes:

                    /*
                    A new client is created from the second attempt onwards as there is no way we can create new connections. Since this handler
                    is a singleton, it would be a bad idea to repeatedly abandon HttpClient instances in a singleton handler. It will cause leak
                    of resource. We need a clean connection without abandoning the shared resource. The solution is to create a temporary fresh
                    client from the second attempt and discard it immediately to minimize resource footprint.
                    */
                    int maxRetries = config.getMaxConnectionRetries();
                    HttpResponse<byte[]> response = null;

                    for (int attempt = 0; attempt < maxRetries; attempt++) {

                        try {
                            if (attempt == 0) {
                                // First attempt: Use the long-lived, shared client
                                response = this.client.send(request, HttpResponse.BodyHandlers.ofByteArray());
                            } else {
                                // Subsequent attempts: Create a fresh, temporary client inside a try-with-resources
                                logger.info("Attempt {} failed. Creating fresh client for retry.", attempt);

                                try (HttpClient temporaryClient = createJavaHttpClient()) {
                                    response = temporaryClient.send(request, HttpResponse.BodyHandlers.ofByteArray());
                                } // temporaryClient.close() called here if inner block exits normally/exceptionally
                            }
                            break; // Success! Exit the loop.
                        } catch (IOException | InterruptedException e) {
                            // Note: The exception could be from .send() OR from createJavaHttpClient() (if attempt > 0)
                            if (attempt >= maxRetries - 1) {
                                throw e; // Rethrow exception on final attempt failure
                            }
                            logger.warn("Attempt {} failed ({}). Retrying...", attempt + 1, e.getMessage());
                            // Loop continues to next attempt
                        }
                    }

Load Test Findings

We created a test project under light-example-4j/instance-variable consisting of a caller service and a callee service. The caller created a new HttpClient per request.

During load testing, we observed:

• Extremely high thread and memory utilization

• Very poor throughput

• Sustained 100% CPU usage for several minutes after traffic stopped, as the JVM attempted to clean up HttpClient resources

This behavior exposes several serious problems.

Identified Problems

The following is the problems:

1. Resource Exhaustion (Threads & Native Memory) The java.net.http.HttpClient is designed to be a long-lived, heavy object. When you create an instance, it spins up a background thread pool (specifically a SelectorManager thread) to handle async I/O. If you create a new client for every HTTP request, you are spawning thousands of threads. Even though the client reference is overwritten and eventually Garbage Collected, the background threads may not shut down immediately, leading to OutOfMemoryError: unable to create new native thread or high CPU usage.

2. Loss of Connection Pooling (Performance Killer) The HttpClient maintains an internal connection pool to reuse TCP connections (Keep-Alive) to the downstream service (localhost:7002). By recreating the client every time, you force a new TCP handshake (and SSL handshake if using HTTPS) for every single request. This drastically increases latency.

3. Thread Safety (Race Condition) Your handler is a singleton, meaning one instance of DataGetHandler serves all requests. You have defined private HttpClient client; as an instance variable.

   • Scenario: Request A comes in, creates a client, and assigns it to this.client. Immediately after, Request B comes in and overwrites this.client with a new client.
   • Because handleRequest is called concurrently by multiple Undertow worker threads, having a mutable instance variable shared across requests is unsafe. While it might not crash immediately, it is poor design to share state this way without synchronization (though in this specific logic, you don’t actually need to share the state, which makes it even worse).

Connection: close Header

Given the above findings, creating temporary HttpClient instances for retries is too risky. Further investigation revealed a safer and more efficient approach: forcing a new connection using the Connection: close header.

By disabling persistent connections for a retry request, the existing HttpClient can be reused while ensuring the next request establishes a fresh TCP connection.

Why this is better than creating a new HttpClient

• Lower Resource Overhead: Recreating HttpClient creates a new SelectorManager thread and internal infrastructure, which is a heavy operation.

• Prevents Thread Leaks: Repeatedly creating and discarding clients can lead to “SelectorManager” thread buildup if not closed properly.

• Clean Socket Management: The Connection: close header handles the lifecycle at the protocol level, allowing the existing client’s thread pool to remain stable.

How does the header work

The Connection: close header instructs both the client and the server (or load balancer) to terminate the TCP connection immediately after the current request/response exchange is finished. When you use this header on a retry, the following happens:

• Current Request: The client established a connection (or reused one) to send the request.

• After Response: Once the response is received (or the request fails), the client’s internal pool will not return that connection to the pool; it will close the socket.

• Next Attempt: Any subsequent request made by that same HttpClient instance will be forced to open a brand-new connection. This fresh connection will hit your load balancer again, which can then route it to a different, healthy node.

Recommended Retry Strategy (Three Attempts)

In most high-availability scenarios, retrying a third time (or using a “3-strikes” policy) is recommended. Here is the optimal sequence:

• 1st Attempt (Normal): Use the default persistent connection. This is fast and efficient.

• 2nd Attempt (The “Fresh Connection” Retry): Use the Connection: close header. This forces the load balancer to re-evaluate the target node if the first one was dead or hanging.

• 3rd Attempt (Final Safeguard): If the second attempt fails, it is often worth one final try with a small exponential backoff (e.g., 500ms–1s). This helps in cases of transient network congestion or when the load balancer hasn’t yet updated its health checks to exclude the failing node.

                    /*
                     * 1st Attempt (Normal): Use the default persistent connection. This is fast and efficient.

                     * 2nd Attempt (The "Fresh Connection" Retry): Use the Connection: close header. This forces
                     *  the load balancer to re-evaluate the target node if the first one was dead or hanging.

                     * 3rd Attempt (Final Safeguard): It will use a new connection to send the request.
                     */
                    int maxRetries = config.getMaxConnectionRetries();
                    HttpResponse<byte[]> response = null;

                    for (int attempt = 0; attempt < maxRetries; attempt++) {
                        try {
                            HttpRequest finalRequest;

                            if (attempt == 0) {
                                // First attempt: Use original request (Keep-Alive enabled by default)
                                finalRequest = request;
                            } else {
                                // Subsequent attempts: Force a fresh connection by adding the 'Connection: close' header.
                                // This ensures the load balancer sees a new TCP handshake and can route to a new node.
                                logger.info("Attempt {} failed. Retrying with 'Connection: close' to force fresh connection.", attempt);

                                finalRequest = HttpRequest.newBuilder(request, (name, value) -> true)
                                        .header("Connection", "close")
                                        .build();
                            }

                            // Always use the same shared, long-lived client
                            response = this.client.send(finalRequest, HttpResponse.BodyHandlers.ofByteArray());
                            break; // Success! Exit the loop.

                        } catch (IOException | InterruptedException e) {
                            if (attempt >= maxRetries - 1) {
                                throw e; // Rethrow on final attempt
                            }
                            logger.warn("Attempt {} failed ({}). Retrying...", attempt + 1, e.getMessage());

                            // Optional: Add a small sleep/backoff here to allow LB health checks to update
                        }
                    }

Header Restriction

Header Restriction: Ensure your JVM is started with -Djdk.httpclient.allowRestrictedHeaders=connection to allow the HttpClient to modify the Connection header.

To make it easier, we have added the following code to the handler.

    static {
        System.setProperty("jdk.httpclient.allowRestrictedHeaders", "host,connection");
    }

Light GenAI Client Design

Introduction

The light-genai-4j library provides a standardized way for Light-4j applications to interact with various Generative AI (GenAI) providers. By abstracting the underlying client implementations behind a common interface, applications can support dynamic model switching and simplified integration for different environments (e.g., local development vs. production).

Architecture

The project is structured into a core module and provider-specific implementation modules.

Modules

1. genai-core: Defines the common interfaces and shared utilities.
2. genai-ollama: Implementation for the Ollama API, suitable for local LLM inference.
3. genai-bedrock: Implementation for AWS Bedrock, suitable for enterprise-grade managed LLMs.

Interface Design

The core interaction is defined by the GenAiClient interface in the genai-core module.

package com.networknt.genai;

import java.util.List;

public interface GenAiClient {
    /**
     * Generates a text completion for the given list of chat messages.
     * 
     * @param messages The list of chat messages (history).
     * @return The generated text response from the model.
     */
    String chat(List<ChatMessage> messages);

    /**
     * Generates a text completion stream for the given list of chat messages.
     * 
     * @param messages The list of chat messages (history).
     * @param callback The callback to receive chunks, completion, and errors.
     */
    void chatStream(List<ChatMessage> messages, StreamCallback callback);
}

The StreamCallback interface:

public interface StreamCallback {
    void onEvent(String content);
    void onComplete();
    void onError(Throwable t);
}

This simple interface allows for “drop-in” replacements of the backend model without changing the application logic.

ChatMessage

A simple POJO to represent a message in the conversation.

public class ChatMessage {
    private String role; // "user", "assistant", "system"
    private String content;
    // constructors, getters, setters
}

Implementations

Ollama (genai-ollama)

Connects to a local or remote Ollama instance.

• Configuration: ollama.yml
  • ollamaUrl: URL of the Ollama server (e.g., http://localhost:11434).
  • model: The model name to use (e.g., llama3.1, mistral).
• Protocol: Uses the /api/generate endpoint via HTTP/2.

AWS Bedrock (genai-bedrock)

Connects to Amazon Bedrock using the AWS SDK for Java v2.

• Configuration: bedrock.yml
  • region: AWS Region (e.g., us-east-1).
  • modelId: The specific model ID (e.g., anthropic.claude-v2, amazon.titan-text-express-v1).
• Authentication: Uses the standard AWS Default Credentials Provider Chain (Environment variables, Profile, IAM Roles).

OpenAI (genai-openai)

Connects to the OpenAI Chat Completions API.

• Configuration: openai.yml
  • url: API endpoint (e.g., https://api.openai.com/v1/chat/completions).
  • model: The model to use (e.g., gpt-3.5-turbo, gpt-4).
  • apiKey: Your OpenAI API key.
• Protocol: Uses standard HTTP/2 (or HTTP/1.1) to send JSON payloads.

Gemini (genai-gemini)

Connects to Google’s Gemini models (via Vertex AI or AI Studio).

• Configuration: gemini.yml
  • url: API endpoint structure.
  • model: The model identifier (e.g., gemini-pro).
  • apiKey: Your Google API key.
• Protocol: REST API with JSON payloads.

Code Example

The following example demonstrates how to use the interface to interact with a model, regardless of the underlying implementation.

// Logic to instantiate the correct client based on external configuration (e.g. from service.yml or reflection)
GenAiClient client;
if (useBedrock) {
    client = new BedrockClient();
} else if (useOpenAi) {
    client = new OpenAiClient();
} else if (useGemini) {
    client = new GeminiClient();
} else {
    client = new OllamaClient();
}

// Application logic remains agnostic
List<ChatMessage> messages = new ArrayList<>();
messages.add(new ChatMessage("user", "Explain quantum computing in 50 words."));
String response = client.chat(messages);
System.out.println(response);

// For subsequent turns:
messages.add(new ChatMessage("assistant", response));
messages.add(new ChatMessage("user", "What about entanglement?"));
String response2 = client.chat(messages);

Future Enhancements

• Streaming Support: Add generateStream to support token streaming.
• Chat Models: Add support for structured chat history (System, User, Assistant messages).
• Tool Use: Support for function calling and tool use with models that support it.
• More Providers: Integrations for OpenAI (ChatGPT), Google Vertex AI (Gemini), and others.

Technical Decisions

Use of Http2Client over JDK HttpClient

The implementation uses the light-4j Http2Client (wrapping Undertow) instead of the standard JDK HttpClient for the following reasons:

1. Framework Consistency: Http2Client is the standard client within the light-4j ecosystem. Using it ensures consistent configuration, management, and behavior across all modules of the framework.
2. Performance: It leverages the non-blocking I/O capabilities of the underlying Undertow server, sharing the same XNIO worker threads as the server components. This minimizes context switching and optimizes resource usage in a microservices environment.
3. Callback Pattern: The ClientCallback and ChannelListener patterns are idiomatic to light-4j/Undertow. While they differ from the CompletableFuture style of the JDK client, using them maintains architectural uniformity for developers familiar with the framework’s internals.
4. Integration: Utilizing the framework’s client allows for seamless integration with other light-4j features such as centralized SSL context management, connection pooling, and client-side observability.

For implementations that require vendor-specific logic (like AWS signing), we utilize the official vendor SDKs (e.g., AWS SDK for Java v2 for Bedrock) to handle complex authentication and protocol details efficiently.

Client Credentials to Authorization Code Token Exchange

Introduction

This document outlines the design for exchanging an external Client Credentials (CC) token (e.g., from Okta) for an internal Authorization Code (AC) style token (containing user/function identity) within the light-4j ecosystem.

Use Case

External partners or applications integrate with light-portal using their own Identity Provider (e.g., Okta) via the Client Credentials flow.

• External Token: A JWT issued by Okta representing the External App.
• Internal Requirement: light-portal services require a JWT containing internal userId, roles, and custom_claims to perform business logic (Event Sourcing/CQRS).
• Goal: Bridge the external identity to the internal identity transparently at the ingress/handler layer.

Architecture

The solution uses RFC 8693 OAuth 2.0 Token Exchange.

High-Level Flow

1. External Request: The external app calls light-portal with Authorization: Bearer <Okta-CC-Token>.
2. Interception: A TokenExchangeHandler in light-portal intercepts the request.
3. Validation: The handler validates the Okta token (signature, expiration) using JwtVerifier and Okta’s JWK.
4. Exchange:
   • The handler requests a token exchange from the internal light-oauth2 service.
   • Grant Type: urn:ietf:params:oauth:grant-type:token-exchange
   • Subject Token: The verified Okta token.
   • Subject Token Type: urn:ietf:params:oauth:token-type:jwt
5. Minting:
   • light-oauth2 validates the request.
   • It identifies the “Shadow Project” mapped to the external client ID.
   • It issues a new internal JWT containing the mapped internal userId and roles.
6. Injection: The handler replaces the Authorization header with the new internal token.
7. Processing: Downstream services processing the request see a standard internal user token.

Configuration

The TokenExchangeHandler configures the exchange parameters via client.yml or values.yml using OAuthTokenExchangeConfig.

client:
  tokenExUri: "https://light-oauth2/oauth2/token"
  tokenExClientId: "${portal.client_id}"
  tokenExClientSecret: "${portal.client_secret}"
  tokenExScope: 
    - "portal.w"
    - "portal.r"

Client Identity Mapping

A critical component is mapping the External Client ID (from Okta) to an Internal Function ID.

Recommended Approach: Database (Shadow Client)

Maintain a “Shadow Client” record in the light-oauth2 database for each external partner.

1. Registration: Create a client in light-oauth2 where client_id matches the Okta Client ID.
2. Mapping: Populate the custom_claims column for this client with the internal identity:

   { "functionId": "acme-integration-user", "internalRole": "partner" }
   

3. Execution: When light-oauth2 performs the exchange, it looks up this client record and automatically injects these custom claims into the new token.

Pros:

• Scalable: Manage thousands of partners via Portal UI/Database without restarts.
• Standard: Uses existing light-oauth2 features.
• Decoupled: The handler code remains generic.

Alternative Approach: Configuration

Map IDs locally in the handler configuration.

tokenExchangeMapping:
  "okta-client-id-1": "internal-id-1"

Pros: Simple for MVP. Cons: Requires restart to add partners; hard to manage at scale.

Security Considerations

1. Trust: The internal light-oauth2 must trust the Portal Client to perform exchanges.
2. Validation: The TokenExchangeHandler must validate the external token before attempting exchange to prevent garbage requests from reaching the OAuth server.
3. Scope: The exchanged token should have scopes limited to what the external partner is allowed to do.

Client SimplePool Design

Overview

The SimplePool is a lightweight HTTP connection pooling implementation in the light-4j client module. It was contributed by a customer to address connection management issues in high-throughput scenarios. The implementation provides a robust connection pooling mechanism with support for both HTTP/1.1 and HTTP/2 connections.

Architecture

The SimplePool follows a layered architecture with clear separation of concerns:

graph TB
    subgraph "Public API"
        SCP[SimpleConnectionPool]
    end
    
    subgraph "URI-Level Pool"
        SUCP[SimpleURIConnectionPool]
    end
    
    subgraph "Connection Management"
        SCH[SimpleConnectionHolder]
        CT[ConnectionToken]
    end
    
    subgraph "Abstraction Layer"
        SC[SimpleConnection Interface]
        SCM[SimpleConnectionMaker Interface]
    end
    
    subgraph "Undertow Implementation"
        SCC[SimpleClientConnection]
        SCCM[SimpleClientConnectionMaker]
    end
    
    SCP --> SUCP
    SUCP --> SCH
    SCH --> CT
    SCH --> SC
    SCH --> SCM
    SC --> SCC
    SCM --> SCCM

Core Components

SimpleConnection (Interface)

A protocol-agnostic interface that wraps raw connections:

• isOpen() - Check if connection is still open
• getRawConnection() - Access the underlying connection object
• isMultiplexingSupported() - Detect HTTP/2 capability
• getLocalAddress() - Get client-side address
• safeClose() - Safely close the connection

SimpleConnectionHolder

The central state management component that wraps a SimpleConnection and tracks its lifecycle:

Connection States:

• NOT_BORROWED_VALID - Available for borrowing
• BORROWED_VALID - Currently in use
• NOT_BORROWED_EXPIRED - Expired, ready for cleanup
• BORROWED_EXPIRED - Expired but still in use
• CLOSED - Connection terminated

State Machine:

                    |
                   \/
         [ NOT_BORROWED_VALID ] --(borrow)-->   [ BORROWED_VALID ]
                   |            <-(restore)--           |
                   |                                    |
                (expire)                             (expire)
                   |                                    |
                  \/                                   \/
         [ NOT_BORROWED_EXPIRED ] <-(restore)-- [ BORROWED_EXPIRED ]
                  |
               (close)
                 |
                \/
             [ CLOSED ]

Key Features:

• Connection Tokens: Track borrows with ConnectionToken objects
• Time-Freezing: All state checks use a consistent “now” timestamp to prevent race conditions
• Thread-Safe: All state transitions are synchronized
• HTTP/2 Multiplexing: HTTP/2 connections support unlimited borrows; HTTP/1.1 limited to 1

SimpleURIConnectionPool

Manages connections for a single URI with multiple tracking sets:

• allCreatedConnections - All connections created by connection makers
• allKnownConnections - All connections tracked by the pool
• borrowable - Connections available for borrowing
• borrowed - Connections with outstanding tokens
• notBorrowedExpired - Connections ready for cleanup

Key Features:

• Random selection from borrowable connections for load distribution
• Automatic cleanup of expired connections during borrow/restore operations
• Leaked connection detection and cleanup

SimpleConnectionPool

Top-level pool that manages SimpleURIConnectionPool instances per URI:

• Thread-safe map of URI to connection pools
• Lazy initialization of per-URI pools
• Delegates borrow/restore to appropriate URI pool

SimpleConnectionMaker (Interface)

Factory interface for creating connections with two creation modes:

1. Simplified mode using isHttp2 boolean
2. Full control mode with XnioWorker, SSL, ByteBufferPool, and OptionMap

Undertow Implementation

• SimpleClientConnection: Wraps Undertow’s ClientConnection
• SimpleClientConnectionMaker: Creates connections using UndertowClient or Http2Client

Integration with Http2Client

The Http2Client class integrates SimplePool via:

• borrow(URI, XnioWorker, ByteBufferPool, ...) methods - Get connection token
• restore(ConnectionToken) method - Return connection to pool

// Usage pattern
ConnectionToken token = http2Client.borrow(uri, worker, bufferPool, isHttp2);
try {
    ClientConnection connection = (ClientConnection) token.getRawConnection();
    // Use connection...
} finally {
    http2Client.restore(token);
}

Configuration

The pool behavior is controlled via ClientConfig:

• connectionExpireTime - How long connections remain valid (default from config)
• connectionPoolSize - Maximum connections per URI (default from config)
• request.connectTimeout - Connection creation timeout

Code Review Findings

Strengths

1. Well-Documented State Machine: The connection state transitions are clearly documented with diagrams in code comments.

2. Thread Safety: Proper synchronization with synchronized methods and ConcurrentHashMap usage.

3. Time-Freezing Pattern: Excellent approach to prevent time-of-check-time-of-use (TOCTOU) race conditions by passing a consistent now value.

4. Leak Detection: The findAndCloseLeakedConnections() method handles edge cases where connections are created but not properly tracked.

5. HTTP/2 Multiplexing Support: Proper differentiation between HTTP/1.1 (single borrow) and HTTP/2 (unlimited borrows).

6. Random Connection Selection: Uses ThreadLocalRandom for efficient, fair distribution of connections.

7. Comprehensive Logging: Detailed debug logging with connection port and state information.

Resolved Issues

The following issues were identified during code review and have been fixed:

1. Race Condition in SimpleConnectionPool.borrow() ✅ Fixed

Location: SimpleConnectionPool.java

Issue: Double-checked locking had a subtle race condition.

Fix: Replaced with atomic computeIfAbsent():

SimpleURIConnectionPool pool = pools.computeIfAbsent(uri,
    u -> new SimpleURIConnectionPool(u, expireTime, poolSize, connectionMaker));
return pool.borrow(createConnectionTimeout);

2. Unused isHttp2 Parameter ✅ Fixed

Location: SimpleConnectionPool.java

Fix: Removed the unused isHttp2 parameter from the borrow() method signature.

3. NPE Risk in Http2Client.restore() ✅ Fixed

Location: Http2Client.java

Fix: Added null check before calling restore:

SimpleURIConnectionPool pool = pools.get(token.uri());
if(pool != null) pool.restore(token);

4. Singleton Pattern Thread Safety ✅ Fixed

Location: SimpleClientConnectionMaker.java

Fix: Implemented thread-safe singleton using the Holder pattern:

private static class Holder {
    static final SimpleClientConnectionMaker INSTANCE = new SimpleClientConnectionMaker();
}
public static SimpleConnectionMaker instance() {
    return Holder.INSTANCE;
}

5. Missing Null Check in SimpleConnectionPool.restore() ✅ Fixed

Location: SimpleConnectionPool.java

Fix: Added null checks for both the connection token and the pool:

if(connectionToken == null) return;
SimpleURIConnectionPool pool = pools.get(connectionToken.uri());
if(pool != null) pool.restore(connectionToken);

6. Hardcoded Worker Configuration ✅ Improved

Location: SimpleClientConnectionMaker.java

Fix: Extracted hardcoded value to a named constant for clarity:

private static final int DEFAULT_WORKER_IO_THREADS = 8;

Note

The Static Worker and SSL Reuse concern (TODO comments about reusing WORKER and SSL) remains as a documented consideration for future enhancement if configuration reloading becomes a requirement.

Best Practices Observed

1. Functional Interface with Lambda: The RemoveFromAllKnownConnections interface provides flexibility for Iterator-based or direct removal.

2. Explicit State Assertions: Using IllegalStateException for invalid state transitions aids debugging.

3. Comprehensive Javadoc: Methods include detailed documentation about thread safety and usage patterns.

4. Connection Token Pattern: Ensures connections can be tracked and returned correctly, preventing leaks when used properly.

Summary

The SimplePool implementation is a well-designed, production-quality connection pool with excellent documentation and careful attention to thread safety. All identified issues have been resolved. The implementation successfully handles:

• Connection lifecycle management
• HTTP/1.1 and HTTP/2 protocol differences
• Connection expiration and cleanup
• Leak detection and prevention
• Thread-safe concurrent access

New Features

Pool Metrics

Connection pool metrics can be enabled via configuration to track:

• Total borrows and restores per URI
• Connection creation and closure counts
• Borrow failures
• Current active connection count

client:
  request:
    poolMetricsEnabled: true

Metrics Usage Examples

Basic metrics access:

Http2Client client = Http2Client.getInstance();
SimplePoolMetrics metrics = client.getPoolMetrics();
if (metrics != null) {
    // Get summary for logging
    logger.info(metrics.getSummary());
    
    // Access per-URI metrics
    for (Map.Entry<URI, SimplePoolMetrics.UriMetrics> entry : metrics.getAllMetrics().entrySet()) {
        URI uri = entry.getKey();
        SimplePoolMetrics.UriMetrics uriMetrics = entry.getValue();
        
        logger.info("Pool {} - active: {}, borrows: {}, restores: {}, created: {}, closed: {}, failures: {}",
            uri,
            uriMetrics.getActiveConnections(),
            uriMetrics.getTotalBorrows(),
            uriMetrics.getTotalRestores(),
            uriMetrics.getTotalCreated(),
            uriMetrics.getTotalClosed(),
            uriMetrics.getBorrowFailures());
    }
}

Periodic metrics logging (e.g., every 5 minutes):

ScheduledExecutorService scheduler = Executors.newSingleThreadScheduledExecutor();
scheduler.scheduleAtFixedRate(() -> {
    SimplePoolMetrics metrics = Http2Client.getInstance().getPoolMetrics();
    if (metrics != null) {
        logger.info(metrics.getSummary());
    }
}, 5, 5, TimeUnit.MINUTES);

Exposing metrics via REST endpoint:

@GET
@Path("/pool/metrics")
public Response getPoolMetrics() {
    SimplePoolMetrics metrics = Http2Client.getInstance().getPoolMetrics();
    if (metrics == null) {
        return Response.status(503).entity("Metrics not enabled").build();
    }
    
    Map<String, Object> result = new HashMap<>();
    for (Map.Entry<URI, SimplePoolMetrics.UriMetrics> entry : metrics.getAllMetrics().entrySet()) {
        SimplePoolMetrics.UriMetrics m = entry.getValue();
        result.put(entry.getKey().toString(), Map.of(
            "active", m.getActiveConnections(),
            "borrows", m.getTotalBorrows(),
            "restores", m.getTotalRestores(),
            "created", m.getTotalCreated(),
            "closed", m.getTotalClosed(),
            "failures", m.getBorrowFailures()
        ));
    }
    return Response.ok(result).build();
}

Pool Warm-Up

Pre-establish connections to reduce latency on first request:

client:
  request:
    poolWarmUpEnabled: true
    poolWarmUpSize: 2

Programmatic warm-up:

Http2Client client = Http2Client.getInstance();
client.warmUpPool(URI.create("https://api.example.com:8443"));

Connection Health Checks

Background thread validates idle connections and removes stale ones:

client:
  request:
    healthCheckEnabled: true
    healthCheckIntervalMs: 30000

Health Check Usage Examples

Graceful shutdown:

// In your application shutdown hook
Runtime.getRuntime().addShutdownHook(new Thread(() -> {
    Http2Client.getInstance().shutdown();
    logger.info("Http2Client shutdown complete");
}));

With Server module shutdown listener:

public class ClientShutdownListener implements ShutdownHookProvider {
    @Override
    public void onShutdown() {
        Http2Client.getInstance().shutdown();
    }
}

Monitoring pool health programmatically:

Http2Client client = Http2Client.getInstance();
Map<URI, SimpleURIConnectionPool> pools = client.getPools();

for (Map.Entry<URI, SimpleURIConnectionPool> entry : pools.entrySet()) {
    SimpleURIConnectionPool pool = entry.getValue();
    logger.info("Pool {} - active: {}, borrowable: {}, borrowed: {}",
        entry.getKey(),
        pool.getActiveConnectionCount(),
        pool.getBorrowableCount(),
        pool.getBorrowedCount());
}

Complete configuration example:

client:
  request:
    # Connection timeouts
    connectTimeout: 10000
    timeout: 3000
    
    # Pool sizing
    connectionPoolSize: 10
    connectionExpireTime: 1800000
    
    # Metrics (disabled by default)
    poolMetricsEnabled: true
    
    # Warm-up (disabled by default)
    poolWarmUpEnabled: true
    poolWarmUpSize: 2
    
    # Health checks (enabled by default)
    healthCheckEnabled: true
    healthCheckIntervalMs: 30000

Consolidating AbstractJwtVerifyHandler and AbstractSimpleJwtVerifyHandler

Introduction

The light-4j framework previously had two abstract base classes for JWT verification: AbstractJwtVerifyHandler and AbstractSimpleJwtVerifyHandler.

• AbstractJwtVerifyHandler: Used by JwtVerifyHandler (in light-rest-4j) and HybridJwtVerifyHandler (in light-hybrid-4j). It included logic for verifying OAuth 2.0 scopes against an OpenAPI specification.
• AbstractSimpleJwtVerifyHandler: Used by SimpleJwtVerifyHandler (in light-rest-4j). It provided basic JWT verification (signature, expiration) but skipped all scope verification logic.

This design document outlines the consolidation of these two classes into a single AbstractJwtVerifyHandler to reduce code duplication and simplify maintenance.

Motivation

The two abstract classes shared approximately 85% of their code, including token extraction, JWT signature verification using JwtVerifier, and audit info population. The primary difference was the presence of scope verification logic in AbstractJwtVerifyHandler.

Maintaining two separate hierarchies for largely identical logic increased the risk of inconsistencies (e.g., bug fixes applied to one but missed in the other) and added unnecessary complexity.

Changes

1. AbstractJwtVerifyHandler Updates

The AbstractJwtVerifyHandler in light-4j/unified-security has been updated to support scopeless verification, effectively merging the behavior of AbstractSimpleJwtVerifyHandler.

• getSpecScopes() is no longer abstract: It now has a default implementation that returns null.

  public List<String> getSpecScopes(HttpServerExchange exchange, Map<String, Object> auditInfo) throws Exception {
      return null;  // Default: no scope verification (simple JWT behavior)
  }
  

• Conditional Scope Verification: The handleJwt method now checks if getSpecScopes() returns null. If it does, the scope verification logic (checking secondary scopes and matching against spec scopes) is skipped.
• Enriched Audit Info: AbstractJwtVerifyHandler extracts additional claims like email, host, and role into the audit info map. By using this class for simple JWT verification, these claims are now populated for SimpleJwtVerifyHandler as well, improving audit capabilities.

2. Removal of AbstractSimpleJwtVerifyHandler

The AbstractSimpleJwtVerifyHandler class in light-4j/unified-security has been deleted.

3. SimpleJwtVerifyHandler Update

The SimpleJwtVerifyHandler in light-rest-4j/openapi-security has been updated to extend AbstractJwtVerifyHandler directly.

• It inherits the default getSpecScopes() implementation (returning null), which triggers the scopeless verification path in the base class.
• It implements the required isSkipAuth() method, same as before.

4. UnifiedSecurityHandler Update

The UnifiedSecurityHandler in light-4j/unified-security has been updated to cast the “sjwt” (Simple JWT) handler to AbstractJwtVerifyHandler instead of the removed AbstractSimpleJwtVerifyHandler.

Impact

Simplified Hierarchy

There is now a single abstract base class (AbstractJwtVerifyHandler) for all JWT verification handlers in the framework.

Backward Compatibility

• Functionality: Existing handlers (JwtVerifyHandler, SimpleJwtVerifyHandler, HybridJwtVerifyHandler) continue to work as before.
• Configuration: No changes to security.yml or openapi-security.yml are required.
• Behavior: SimpleJwtVerifyHandler now extracts more comprehensive audit information (e.g., user email, host, role) if present in the token, which is a beneficial side effect.

Conclusion

This refactoring simplifies the codebase, removes duplication, and ensures consistent JWT handling across different modules while maintaining full backward compatibility.

Cross-Cutting-Concerns

Light-4j

Http Handler

Path Resource Handler

The PathResourceHandler is a middleware handler in light-4j that provides an easy way to serve static content from a specific filesystem path. It wraps Undertow’s PathHandler and ResourceHandler, allowing you to expose local directories via HTTP with simple configuration.

Features

• External Configuration: Define path mapping and base directory in a YAML file.
• Path Matching: Supports both exact path matching and prefix-based matching.
• Directory Listing: Optional support for listing directory contents.
• Performance: Integrated with Undertow’s PathResourceManager for efficient file serving.
• Auto-Registration: Automatically registers with ModuleRegistry for runtime visibility.

Configuration (path-resource.yml)

The behavior of the handler is controlled by path-resource.yml.

# Path Resource Configuration
---
# The URL path at which the static content will be exposed (e.g., /static)
path: ${path-resource.path:/}

# The absolute filesystem path to the directory containing the static files
base: ${path-resource.base:/var/www/html}

# If true, the handler matches all requests starting with the 'path'.
# If false, it only matches exact hits on the 'path'.
prefix: ${path-resource.prefix:true}

# The minimum file size for transfer optimization (in bytes).
transferMinSize: ${path-resource.transferMinSize:1024}

# Whether to allow users to see a listing of files if they access a directory.
directoryListingEnabled: ${path-resource.directoryListingEnabled:false}

Setup

1. Add Dependency

Include the resource module in your pom.xml:

<dependency>
    <groupId>com.networknt</groupId>
    <artifactId>resource</artifactId>
    <version>${version.light-4j}</version>
</dependency>

2. Register Handler

In your handler.yml, register the PathResourceHandler and add it to your handler chain or path mappings.

handlers:
  - com.networknt.resource.PathResourceHandler@pathResource

chains:
  default:
    - ...
    - pathResource
    - ...

Or, if you want it on a specific path:

paths:
  - path: '/static'
    method: 'GET'
    exec:
      - pathResource

Operational Visibility

The path-resource module integrates with the ModuleRegistry. You can verify the active path and base directory mapping at runtime via the Server Info endpoint.

Virtual Host Handler

The VirtualHostHandler is a middleware handler in light-4j that enables name-based virtual hosting. It allows a single server instance to serve different content or applications based on the Host header in the incoming HTTP request. This is a wrapper around Undertow’s NameVirtualHostHandler.

Features

• Domain-Based Routing: Route requests to different resource sets based on the domain name.
• Flexible Mappings: Each virtual host can define its own path, base directory, and performance settings.
• Centralized Configuration: All virtual hosts are defined in a single virtual-host.yml file.
• Auto-Registration: Automatically registers with ModuleRegistry for administrative oversight.

Configuration (virtual-host.yml)

The configuration contains a list of host definitions.

# Virtual Host Configuration
---
hosts:
  - domain: dev.example.com
    path: /
    base: /var/www/dev
    transferMinSize: 1024
    directoryListingEnabled: true
  - domain: prod.example.com
    path: /app
    base: /var/www/prod/dist
    transferMinSize: 1024
    directoryListingEnabled: false

Host Parameters:

• domain: The domain name to match (exact match against the Host header).
• path: The URL prefix path within that domain to serve resources from.
• base: The absolute filesystem path to the directory containing the static content.
• transferMinSize: Minimum file size for transfer optimization (in bytes).
• directoryListingEnabled: Whether to allow directory browsing for this specific host.

Setup

1. Add Dependency

Include the resource module in your pom.xml:

<dependency>
    <groupId>com.networknt</groupId>
    <artifactId>resource</artifactId>
    <version>${version.light-4j}</version>
</dependency>

2. Register Handler

In your handler.yml, register the VirtualHostHandler.

handlers:
  - com.networknt.resource.VirtualHostHandler@virtualHost

chains:
  default:
    - ...
    - virtualHost
    - ...

How it Works

When a request arrives, the VirtualHostHandler:

1. Inspects the Host header of the request.
2. Matches it against the domain entries in the configuration.
3. If a match is found, it delegates the request to a subdomain-specific PathHandler and ResourceHandler configured with the corresponding base and path.
4. If no match is found, the request typically proceeds down the chain or returns a 404 depending on the rest of the handler configuration.

Operational Visibility

The virtual-host module registers itself with the ModuleRegistry during initialization. The full list of configured virtual hosts and their mapping settings can be inspected via the Server Info endpoint.

Router Handler

The RouterHandler is a key component in the Light-Router and http-sidecar services. It acts as a reverse proxy request handler, forwarding incoming requests to downstream services based on service discovery and routing rules.

Introduction

In a microservices architecture, a gateway or sidecar often needs to route requests to various backend services. The RouterHandler fulfills this role by:

1. Proxying: It uses a LoadBalancingRouterProxyClient to forward requests.
2. Protocol Support: It supports HTTP/1.1 and HTTP/2, with optional TLS (HTTPS) for downstream connections.
3. ** rewriting**: It offers powerful capabilities to rewrite URLs, methods, headers, and query parameters before forwarding the request.
4. Resilience: It handles connection pooling, retries, and timeouts (both global and path-specific).

Configuration

The handler is configured via the router.yml file. This configuration file allows you to fine-tune the connection behavior and define rewrite rules.

Key Configuration Options

Path-Specific Timeouts

You can override the global maxRequestTime for specific paths using pathPrefixMaxRequestTime.

pathPrefixMaxRequestTime:
  /v1/heavy-processing: 5000
  /v2/report: 10000

Rewrite Rules

The router supports extensive rewrite rules to adapt legacy clients or unify API surfaces.

URL Rewrite Rewrite specific paths using Regex.

urlRewriteRules:
  # Regex Pattern -> Replacement
  - /listings/(.*)$ /listing.html?listing=$1

Method Rewrite Convert methods (e.g., for clients that don’t support DELETE/PUT).

methodRewriteRules:
  # Endpoint Pattern -> Source Method -> Target Method
  - /v1/pets/{petId} GET DELETE

Header & Query Param Rewrite Rename keys or replace values for headers and query parameters.

headerRewriteRules:
  /v1/old-api:
    - oldK: X-Old-Header
      newK: X-New-Header

queryParamRewriteRules:
  /v1/search:
    - oldK: q
      newK: query

Example router.yml

# Router Configuration
http2Enabled: true
httpsEnabled: true
maxRequestTime: 2000
rewriteHostHeader: true
reuseXForwarded: false
maxConnectionRetries: 3
connectionsPerThread: 20

# Host Whitelist (Regex)
hostWhitelist:
  - 192\.168\..*
  - networknt\.com

# Metrics
metricsInjection: true
metricsName: router-response

Usage

The RouterHandler is typically used as the terminal handler in a gateway or sidecar chain. It effectively “exits” the Light-4j processing chain and hands the request over to the external service.

handler.yml

handlers:
  - com.networknt.router.RouterHandler@router
  # ... other handlers

chains:
  default:
    - correlation
    - limit
    - router # The request is forwarded here

Metrics Injection

If metricsInjection is enabled, the router can inject response time metrics into the configured Metrics Handler. This helps in tracking the latency introduced by downstream services separately from the gateway’s own processing time.

OAuth Server Handler

The OAuthServerHandler is a specialized handler designed to simulate an OAuth 2.0 provider endpoint (e.g., /oauth/token). Its primary purpose is to facilitate the migration of legacy applications that expect to exchange client credentials for a token, without requiring immediate code changes in those applications.

Introduction

In a typical migration scenario, you might have legacy clients that are hardcoded to call a specific token endpoint to get an access token before calling an API. The OAuthServerHandler intercepts these requests at the gateway.

It supports the client_credentials grant type and can operate in two modes:

1. Dummy Mode: Returns a generated dummy token. This is useful when the gateway itself handles authentication/authorization using a different mechanism (e.g., Mutual TLS or a centralized token) and the client just needs some token to proceed.
2. Pass-Through Mode: Validates the credentials locally and then fetches a real JWT from a backend OAuth 2.0 provider (using TokenService configuration) and returns it to the client.

Configuration

The handler is configured via the oauthServer.yml file.

Example oauthServer.yml

# OAuth Server Configuration
enabled: true
getMethodEnabled: false
passThrough: true
tokenServiceId: aad-token-service

# List of valid credentials (clientId:clientSecret)
client_credentials:
  - 3848203948:2881938882
  - client_App1:pass1234

Handlers

OAuthServerHandler

• Path: Typically mapped to /oauth2/token or similar.
• Method: POST
• Content-Type: application/json, application/x-www-form-urlencoded, multipart/form-data.
• Logic:
  • Extracts client_id and client_secret from the request body OR Authorization: Basic header.
  • Validates them against the client_credentials list.
  • If valid, returns a JSON response with access_token, token_type, and expires_in.

OAuthServerGetHandler

• Path: Configurable (e.g., /oauth2/token).
• Method: GET
• Logic:
  • Extracts credentials from Query Parameters (client_id, client_secret).
  • Warning: This is highly insecure as secrets are exposed in the URL. Use only for legacy migration where absolutely necessary.
  • Requires getMethodEnabled: true in config.

Usage

To use these handlers, register them in handler.yml.

handler.yml

paths:
  - path: '/oauth2/token'
    method: 'POST'
    exec:
      - com.networknt.router.OAuthServerHandler
  - path: '/oauth2/token'
    method: 'GET'
    exec:
      - com.networknt.router.OAuthServerGetHandler

Proxy Handler

The Proxy Handler (LightProxyHandler) is a core component of the Light-Gateway and Sidecar patterns. It acts as a reverse proxy, forwarding incoming requests to backend services (downstream APIs) and returning the responses to the client.

Introduction

In a microservices architecture, a reverse proxy is often needed to route traffic from the internet to internal services, or to act as a sidecar handling cross-cutting concerns (security, metrics, etc.) for a specific service instance. The LightProxyHandler is built on top of Undertow’s ProxyHandler and provides features like load balancing, connection pooling, and protocol translation (HTTP/1.1 to HTTP/2).

Configuration

The handler is configured via proxy.yml.

proxy.yml Example

enabled: true
http2Enabled: false
hosts: http://localhost:8081,http://localhost:8082
connectionsPerThread: 20
maxRequestTime: 5000
rewriteHostHeader: true
reuseXForwarded: false
maxConnectionRetries: 3
maxQueueSize: 0
forwardJwtClaims: true
metricsInjection: true
metricsName: proxy-response

Features

Load Balancing

The handler supports client-side load balancing if multiple URLs are provided in hosts. It uses a round-robin strategy by default via LoadBalancingProxyClient.

Protocol Support

• HTTP/1.1: Default behavior.
• HTTP/2: Enable http2Enabled to communicate with backends via HTTP/2 (requires HTTPS).
• HTTPS: Supports invalid certificate verification (for internal self-signed certs) if configured in client.yml (TLS verification disabled).

JWT Claims Forwarding

If forwardJwtClaims is enabled, the handler extracts the JWT from the Authorization header, decodes the claims (without signature verification, assuming it was verified by SecurityHandler previously), and injects them as a JSON header jwtClaims to the downstream service. This is useful for backend services that need user context but don’t want to re-verify or parse JWTs.

Metrics Injection

The handler can measure the time taken for the downstream call (including network latency) and inject this data into the MetricsHandler. This allows distinguishing between the gateway’s overhead and the backend’s processing time.

Usage

In handler.yml, register the LightProxyHandler.

handlers:
  - com.networknt.proxy.LightProxyHandler@proxy

Then use it in paths, typically as the terminal handler.

paths:
  - path: '/v1/pets'
    method: 'get'
    exec:
      - security
      - metrics
      - proxy

Middleware Handler

APM Metrics Handler

The APM Metrics Handler (APMMetricsHandler) is a specialized metrics handler designed to integrate with Broadcom APM (formerly CA APM). It collects application metrics and sends them to the APM EPAgent (Enterprise Performance Agent) via its RESTful interface.

Introduction

Many enterprise customers use Broadcom APM for monitoring and performance management. This handler allows light-4j services (microsrevices, gateways, sidecars) to push metrics directly to an EPAgent deployed as a sidecar or a common service in the Kubernetes cluster or on the VM host.

It extends the AbstractMetricsHandler and shares the same core logic for collecting request/response metrics, but implements a specific sender for the APM protocol.

Configuration

The handler uses the metrics.yml configuration file, which is shared by all push-based metrics handlers.

metrics.yml Example

enabled: true
enableJVMMonitor: true
serverProtocol: http
# Hostname of the Broadcom APM EPAgent service
serverHost: opentracing.ccaapm.svc.cluster.local
serverPort: 8888
# Default path for EPAgent REST interface
serverPath: /apm/metricFeed
# Name of your service/component in APM
productName: my-service-name
reportInMinutes: 1

Usage

1. Register the Handler

In handler.yml, you must register the APMMetricsHandler specifically. Since there can only be one metrics handler active in the chain usually, you should use the alias metrics.

handlers:
  - com.networknt.metrics.APMMetricsHandler@metrics

2. Configure the Chain

Add the metrics alias to your default chain. It should be placed early in the chain to capture the full duration of requests.

chains:
  default:
    - exception
    - metrics
    - header
    # ... other handlers

3. Update values.yml

In your deployment’s values.yml, override the default metrics.yml values to point to your actual EPAgent.

metrics.serverHost: epagent.monitoring.svc
metrics.serverPort: 8080
metrics.productName: payment-service

Metrics Collected

The handler collects the following metrics:

• Response Time: Duration of the request.
• Success Count: Number of successful requests (2xx).
• Error Counts: 4xx (request errors) and 5xx (server errors).
• JVM Metrics: (Optional) Heap usage, Thread count, CPU usage.

Tags

Common tags injected into metrics:

• api: Service ID.
• env: Environment (dev, test, prod).
• host: Hostname / Container ID.
• clientId: Calling client ID (if available).
• endpoint: The API endpoint being accessed.

API Key Handler

For some legacy applications to migrate from a monolithic gateway to the light-gateway without changing any code on the consumer application, we need to support the API Key authentication on the light-gateway (LG) or client-proxy (LCP). The consumer application sends the API key in a header to authenticate itself on the light-gateway. Then the light-gateway will retrieve a JWT token to access the downstream API.

Only specific paths will have API Key set up, and the header name for each application might be different. To support all use cases, we add a list of maps to the configuration apikey.yml to pathPrefixAuths property.

Each config item will have pathPrefix, headerName, and apiKey. The handler will try to match the path prefix first and then get the input API Key from the header. After comparing with the configured API Key, the handler will return either ERR10075 API_KEY_MISMATCH or pass the control to the next handler in the chain.

Configuration

The configuration is managed by ApiKeyConfig and corresponds to apikey.yml (or apikey.json/apikey.properties).

Configuration Properties

Configuration Example (apikey.yml)

# ApiKey Authentication Security Configuration for light-4j
# Enable ApiKey Authentication Handler, default is false.
enabled: ${apikey.enabled:false}

# If API key hash is enabled. The API key will be hashed with PBKDF2WithHmacSHA1 before it is
# stored in the config file. It is more secure than put the encrypted key into the config file.
# The default value is false. If you want to enable it, you need to use the light-hash command line tool.
hashEnabled: ${apikey.hashEnabled:false}

# path prefix to the api key mapping. It is a list of map between the path prefix and the api key
# for apikey authentication. In the handler, it loops through the list and find the matching path
# prefix. Once found, it will check if the apikey is equal to allow the access or return an error.
pathPrefixAuths: ${apikey.pathPrefixAuths:}

Values Example (values.yml)

To enable the handler and configure paths, add the following to your values.yml:

# apikey.yml
apikey.enabled: true
apikey.pathPrefixAuths:
  - pathPrefix: /test1
    headerName: x-gateway-apikey
    apiKey: abcdefg
  - pathPrefix: /test2
    headerName: x-apikey
    apiKey: mykey

JSON format example for pathPrefixAuths (useful for config server):

[{"pathPrefix":"/test1","headerName":"x-gateway-apikey","apiKey":"abcdefg"},{"pathPrefix":"/test2","headerName":"x-apikey","apiKey":"mykey"}]

Multiple Consumers for Same Path

Most services will have multiple consumers, and each consumer might have its own API key for authentication. You can define multiple entries for the same path prefix:

apikey.pathPrefixAuths:
  - pathPrefix: /test1
    headerName: x-gateway-apikey
    apiKey: abcdefg
  # The same prefix has another apikey header and value.
  - pathPrefix: /test1
    headerName: authorization
    apiKey: xyz

Security with Hash

Storing API keys in clear text is only recommended for testing. For production, enable hashing:

1. Enable hash in values.yml:

   apikey.hashEnabled: true
   

2. Generate the hash of your API key using the light-hash utility.
3. Use the generated hash string as the apiKey value in your configuration.

Error Response

If the request path matches a configured prefix but the API key verification fails, the handler returns error ERR10075.

Status Code: 401 Code: ERR10075 Message: API_KEY_MISMATCH Description: APIKEY from header %s is not matched for request path prefix %s.

To prevent leaking sensitive information, the expected apiKey from the config file is not revealed in the error message.

Usage

Register the ApiKeyHandler in your handler.yml chain.

handler.handlers:
  .
  - com.networknt.apikey.ApiKeyHandler@apikey

handler.chains.default:
  .
  - apikey
  .

If you are using the Unified Security Handler, you can integrate the API Key handler there as well to support multiple authentication methods (ApiKey, Basic, OAuth2, SWT) simultaneously.

Audit Handler

The AuditHandler is a middleware component responsible for logging request and response details for auditing purposes. It captures important information such as headers, execution time, status codes, and even request/response bodies if configured.

The audit logs are typically written to a separate log file (e.g., audit.log) via slf4j/logback, allowing them to be ingested by centralized logging systems like ELK or Splunk.

Configuration

The configuration is managed by AuditConfig and corresponds to audit.yml (or audit.json/audit.properties).

Configuration Properties

Configuration Example (audit.yml)

# Enable Audit Logging
enabled: ${audit.enabled:true}

# Enable mask in the audit log
mask: ${audit.mask:true}

# Output response status code
statusCode: ${audit.statusCode:true}

# Output response time
responseTime: ${audit.responseTime:true}

# when auditOnError is true:
#  - it will only log when status code >= 400
# when auditOnError is false:
#  - it will log on every request
auditOnError: ${audit.auditOnError:false}

# log level; by default it is set to info. If you want to change it to error, set to true.
logLevelIsError: ${audit.logLevelIsError:false}

# timestamp format
timestampFormat: ${audit.timestampFormat:}

# headers to audit
headers: ${audit.headers:X-Correlation-Id, X-Traceability-Id,caller_id}

# audit fields. Add requestBody or responseBody here to enable payload logging.
audit: ${audit.audit:client_id, user_id, scope_client_id, endpoint, serviceId}

Values Example (values.yml)

You can customize the audit behavior per environment using values.yml. For example, to enable full body logging in a dev environment:

# audit.yml
audit.audit:
  - client_id
  - user_id
  - endpoint
  - requestBody
  - responseBody
  - queryParameters
  - pathParameters

Logging Body and Parameters

The AuditHandler supports logging additional details if they are added to the audit list in the configuration:

• requestBody: Logs the request body.
• responseBody: Logs the response body.
• queryParameters: Logs query parameters.
• pathParameters: Logs path parameters.
• requestCookies: Logs request cookies.

Note: Enabling body logging (requestBody or responseBody) can significantly impact performance and is generally not recommended for production unless auditOnError is enabled or for specific debugging purposes.

Usage

Register the AuditHandler in your handler.yml chain. It is usually placed near the beginning of the chain to capture the start time and ensure it runs even if downstream handlers fail.

handler.handlers:
  .
  - com.networknt.audit.AuditHandler@audit

handler.chains.default:
  - exception
  - traceability
  - correlation
  - audit
  .

Logback Configuration

The AuditHandler writes to a logger named audit. You should configure a specific appender in your logback.xml to route these logs to a separate file or system.

Example logback.xml snippet:

<appender name="audit" class="ch.qos.logback.core.rolling.RollingFileAppender">
    <file>log/audit.log</file>
    <encoder>
        <pattern>%msg%n</pattern>
    </encoder>
    <rollingPolicy class="ch.qos.logback.core.rolling.FixedWindowRollingPolicy">
        <fileNamePattern>log/audit.log.%i.zip</fileNamePattern>
        <minIndex>1</minIndex>
        <maxIndex>10</maxIndex>
    </rollingPolicy>
    <triggeringPolicy class="ch.qos.logback.core.rolling.SizeBasedTriggeringPolicy">
        <maxFileSize>10MB</maxFileSize>
    </triggeringPolicy>
</appender>

<logger name="audit" level="INFO" additivity="false">
    <appender-ref ref="audit"/>
</logger>

Log Format

The audit log is written as a JSON object.

Example output:

{
  "timestamp": "2023-10-27T10:00:00.123-0400",
  "serviceId": "com.networknt.petstore-1.0.0",
  "X-Correlation-Id": "12345",
  "endpoint": "/v1/pets@get",
  "statusCode": 200,
  "responseTime": 45
}

Basic Auth

The Basic Authentication middleware provides a mechanism to authenticate users using the standard HTTP Basic Authentication scheme (Authorization header with Basic base64(username:password)).

While OAuth 2.0 (JWT) is the recommended security standard for microservices, Basic Auth is useful for:

• IoT devices that do not support OAuth flows.
• Integrating legacy systems.
• Simple authentication requirements where a full OAuth provider is overkill.

Configuration

The configuration is managed by BasicAuthConfig and corresponds to basic-auth.yml (or basic-auth.json/basic-auth.properties). The settings are injected under the basic key in values.yml.

Configuration Properties

User Definition

Each user object in the users list/map contains:

• username: The login username.
• password: The password (clear text or encrypted).
• paths: A list of URL path prefixes this user is allowed to access.

Special usernames:

• anonymous: Used when allowAnonymous is true. Defines paths accessible without authentication.
• bearer: Used when allowBearerToken is true. Defines paths accessible with a Bearer token (verification delegated to downstream or another handler).

Configuration Example (basic-auth.yml)

# Basic Authentication Security Configuration for light-4j
enabled: ${basic.enabled:false}
enableAD: ${basic.enableAD:true}
allowAnonymous: ${basic.allowAnonymous:false}
allowBearerToken: ${basic.allowBearerToken:false}
users: ${basic.users:}

Values Example (values.yml)

You typically configure users in values.yml:

basic.enabled: true
basic.users:
  - username: user1
    password: user1pass
    paths:
      - /v1/address
  - username: user2
    # Encrypted password
    password: CRYPT:0754fbc37347c136be7725cbf62b6942:71756e13c2400985d0402ed6f49613d0
    paths:
      - /v2/pet
      - /v2/address
  # Paths allowed for anonymous access (if allowAnonymous: true)
  - username: anonymous
    paths:
      - /v1/party
      - /info

Features

Static User Authentication

Verify username and password against the configured list. If matched, checks if the request path matches one of the user’s allowed paths.

LDAP Authentication

If enableAD is true and a configured user has an empty password, the handler attempts to authenticate against the configured LDAP server (using LdapUtil). Context:

1. User user1 is defined in basic.users with password: "" (empty).
2. Incoming request has Basic base64(user1:secret).
3. Handler skips local password check and calls LdapUtil.authenticate("user1", "secret").
4. If LDAP auth succeeds, it checks the allowed paths for user1 in the config.

Anonymous Access

If allowAnonymous is true, requests without an Authorization header are checked against the paths defined for the special anonymous user.

• If path matches anonymous.paths, request proceeds.
• If path does not match, returns ERR10071 (NOT_AUTHORIZED_REQUEST_PATH) or ERR10002 (MISSING_AUTH_TOKEN).

Bearer Token Pass-through

If allowBearerToken is true, requests with Authorization: Bearer <token> are checked against the paths defined for the special bearer user.

• If path matches bearer.paths, request proceeds (skipping Basic Auth check).
• This is useful in a gateway/proxy where some endpoints use Basic Auth and others use OAuth2, and you want to route them through the same chain conditionally.

Usage

Register the BasicAuthHandler in your handler.yml chain.

handler.handlers:
  .
  - com.networknt.basicauth.BasicAuthHandler@basic

handler.chains.default:
  .
  - basic
  .

Error Codes

• ERR10002: MISSING_AUTH_TOKEN - The basic authentication header is missing (and anonymous not allowed).
• ERR10046: INVALID_BASIC_HEADER - The format of the basic authentication header is not valid.
• ERR10047: INVALID_USERNAME_OR_PASSWORD - Invalid username or password.
• ERR10071: NOT_AUTHORIZED_REQUEST_PATH - Authenticated user is not authorized for the requested path.
• ERR10072: BEARER_USER_NOT_FOUND - Bearer token allowed but bearer user configuration missing.

Body Handler

The BodyHandler is a middleware component designed primarily for light-rest-4j services. It automatically parses the incoming request body for methods like POST, PUT, and PATCH, and attaches the parsed object to the exchange for subsequent handlers to consume.

Note: This handler is not suitable for light-gateway or http-sidecar as those services typically stream the request body directly to downstream services without consuming it. For gateway scenarios, use RequestBodyInterceptor and ResponseBodyInterceptor if inspection is required.

Functionality

The handler inspects the Content-Type header and parses the body accordingly:

1. application/json:

   • Parsed into a Map (if starts with {) or List (if starts with [).
   • Attached to exchange with key BodyHandler.REQUEST_BODY.
   • Optionally caches the raw string body to BodyHandler.REQUEST_BODY_STRING if cacheRequestBody is enabled (useful for auditing).

2. text/plain:

   • Parsed into a String.
   • Attached to exchange with key BodyHandler.REQUEST_BODY.

3. multipart/form-data / application/x-www-form-urlencoded:

   • Parsed into FormData.
   • Attached to exchange with key BodyHandler.REQUEST_BODY.

4. Other / Missing Content-Type:

   • Attached as an InputStream.
   • Attached to exchange with key BodyHandler.REQUEST_BODY.

Configuration

The configuration is managed by BodyConfig and corresponds to body.yml (or body.json/body.properties).

Configuration Properties

Configuration Example (body.yml)

# Enable body parse flag
enabled: ${body.enabled:true}

# Cache request body as a string along with JSON object.
# Required for audit logging of request body.
cacheRequestBody: ${body.cacheRequestBody:false}

# Cache response body as a string.
# Required for audit logging of response body.
cacheResponseBody: ${body.cacheResponseBody:false}

# Log full bodies for debugging (use carefully)
logFullRequestBody: ${body.logFullRequestBody:false}
logFullResponseBody: ${body.logFullResponseBody:false}

Usage

Register the BodyHandler in your handler.yml chain. It should be placed early in the chain, before any validation or business logic handlers that need access to the body.

handler.handlers:
  .
  - com.networknt.body.BodyHandler@body

handler.chains.default:
  .
  - exception
  - correlation
  - body
  .

Retrieving the Body

In your business handler or subsequent middleware:

import com.networknt.body.BodyHandler;

// ...

// Get the parsed body (Map, List, String, FormData, or InputStream)
Object body = exchange.getAttachment(BodyHandler.REQUEST_BODY);

if (body instanceof List) {
    // Handle JSON List
} else if (body instanceof Map) {
    // Handle JSON Map
} else {
    // Handle other types
}

Retrieving the Cached String

If cacheRequestBody is enabled:

// Get the raw JSON string
String bodyString = exchange.getAttachment(BodyHandler.REQUEST_BODY_STRING);

Content Handler

The ContentHandler is a middleware handler designed to manage the Content-Type header in HTTP responses. It ensures that a valid content type is always set, either by reflecting the request’s Content-Type or by applying a configured default.

Introduction

In some scenarios, backend services or handlers might not explicitly set the Content-Type header in their responses. This can lead to clients misinterpreting the response body. The ContentHandler solves this by:

1. Checking if the request has a Content-Type header. If so, it uses that same value for the response Content-Type.
2. If the request does not have a Content-Type, it sets the response Content-Type to a configured default value (usually application/json).

This is particularly useful when working with clients that expect specific content types or when enforcing a default format for your API.

Configuration

The handler is configured via the content.yml file.

Example content.yml

# Content middleware configuration
enabled: true
# The default content type to be used in the response if request content type is missing
contentType: application/json

Usage

To use the ContentHandler, you need to register it in your handler.yml configuration file and add it to the middleware chain.

handler.yml

handlers:
  - com.networknt.content.ContentHandler@content
  # ... other handlers

chains:
  default:
    - content
    # ... other handlers

Logic

The handleRequest method performs the following logic:

1. Check Request Header: It checks if the Content-Type header exists in the incoming request.
2. Reflect Context Type: If the request has a Content-Type, the handler sets the response Content-Type to match the request’s Content-Type.
3. Apply Default: If the request header is missing, the handler sets the response Content-Type to the value specified in contentType from the configuration (defaulting to application/json).
4. Next Handler: Finally, it passes control to the next handler in the chain.

if (exchange.getRequestHeaders().contains(Headers.CONTENT_TYPE)) {
    exchange.getResponseHeaders().put(Headers.CONTENT_TYPE, exchange.getRequestHeaders().get(Headers.CONTENT_TYPE).element());
} else {
    exchange.getResponseHeaders().put(Headers.CONTENT_TYPE, config.getContentType());
}
Handler.next(exchange, next);

Correlation Handler

The CorrelationHandler is a middleware handler designed to ensure that every request contains a unique correlation ID (X-Correlation-Id). This ID is propagated across service-to-service calls, allowing for centralized logging and traceability in a microservices architecture.

Introduction

In a distributed system, a single user request often triggers a chain of calls across multiple services. To debug issues or trace the flow of execution, it is critical to have a unique identifier that ties all these logs together. The CorrelationHandler ensures this mechanism by:

1. Checking: Looking for an existing X-Correlation-Id header in the incoming request.
2. Generating: If the header is missing and configuration allows, generating a new UUID for the request.
3. Logging: Injecting the Correlation ID (and optional Traceability ID) into the SLF4J MDC (Mapped Diagnostic Context) so it appears in specific log patterns.

Configuration

The handler is configured via the correlation.yml file.

Example correlation.yml

# Correlation Id Handler Configuration
enabled: true
# If set to true, it will auto-generate the correlationID if it is not provided in the request
autogenCorrelationID: true
# MDC context keys usually match your logback.xml pattern
correlationMdcField: cId
traceabilityMdcField: tId

Usage

To use the CorrelationHandler, register it in handler.yml and add it to the middleware chain. It should be placed early in the chain so that subsequent handlers can benefit from the MDC context.

handler.yml

handlers:
  - com.networknt.correlation.CorrelationHandler@correlation
  # ... other handlers

chains:
  default:
    - correlation
    # ... derived handlers

Logging Integration

The handler puts the Correlation ID into the logging context (MDC) using the key defined in correlationMdcField (default cId). To see this in your logs, your logback.xml must utilize this key in its pattern.

logback.xml Example

<appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <encoder>
        <!-- The %X{cId} pattern pulls the value from the MDC -->
        <pattern>%d{HH:mm:ss.SSS} [%thread] %X{cId} %-5level %logger{36} - %msg%n</pattern>
    </encoder>
</appender>

When properly configured, your logs will include the Correlation ID, making it easy to filter logs in tools like Splunk or ELK (Elasticsearch, Logstash, Kibana).

Interaction with Traceability

The handler also acknowledges the X-Traceability-Id header.

• If X-Traceability-Id is present, it is added to the MDC under the traceabilityMdcField (default tId).
• If a new Correlation ID is generated and a Traceability ID exists, the handler logs an info message associating the two IDs: Associate traceability Id {tId} with correlation Id {cId}.

Client Propagation

When making downstream calls using light-4j Client modules (like Http2Client), it is best practice to propagate the Correlation ID. The Client module provides helper methods (e.g., propagateHeaders) to automatically copy the X-Correlation-Id from the current request to the downstream client request.

CORS Handler

The CorsHttpHandler is a middleware handler that manages Cross-Origin Resource Sharing (CORS) headers. It handles both pre-flight OPTIONS requests and actual HTTP requests, ensuring that browsers can securely interact with your API from different origins.

Introduction

Single Page Applications (SPAs) often run on a different domain or port than the API server they consume. Browsers enforce the Same-Origin Policy, which restricts resources from being loaded from a different origin. To allow this, the API server must implement the CORS protocol.

The CorsHttpHandler simplifies this by:

1. Handling Pre-flight Requests: Automatically responding to OPTIONS requests with the correct Access-Control-Allow-* headers.
2. Handling Actual Requests: Injecting the necessary CORS headers into standard responses so the browser accepts the result.
3. Path-Specific Configuration: allowing different CORS rules for different API paths (useful in gateway scenarios).

Configuration

The handler is configured via the cors.yml file.

Example cors.yml

# CORS Handler Configuration
enabled: true

# Global Configuration
# Allowed origins. Wildcards (*) are NOT supported for security.
allowedOrigins:
  - http://localhost:3000
  - https://my-app.com

# Allowed methods
allowedMethods:
  - GET
  - POST
  - PUT
  - DELETE
  - OPTIONS

# Path-Specific Configuration (Optional)
# Use this if you are running a Gateway and need different rules for different downstream services.
pathPrefixAllowed:
  /v1/pets:
    allowedOrigins:
      - https://petstore.com
    allowedMethods:
      - GET
  /v1/market:
    allowedOrigins:
      - https://market.com
    allowedMethods:
      - POST

Usage

To use the CorsHttpHandler, register it in your handler.yml configuration file and add it to the middleware chain.

handler.yml

handlers:
  - com.networknt.cors.CorsHttpHandler@cors
  # ... other handlers

chains:
  default:
    - cors
    # ... other handlers

Logic

The handler operates with the following logic:

1. Check Request: It determines if the incoming request is a CORS request (contains Origin header).
2. Match Config:
   • It checks pathPrefixAllowed first. If the request path matches a configured prefix, it uses that specific configuration.
   • Otherwise, it uses the global allowedOrigins and allowedMethods.
3. Process Pre-flight (OPTIONS):
   • If the request is an OPTIONS request, it validates the Origin and Access-Control-Request-Method.
   • It sets the Access-Control-Allow-Origin, Access-Control-Allow-Methods, Access-Control-Allow-Headers, Access-Control-Allow-Credentials, and Access-Control-Max-Age headers.
   • It returns a 200 OK response immediately, stopping the chain.
4. Process Actual Request:
   • For non-OPTIONS requests, it validates the Origin.
   • It adds the Access-Control-Allow-Origin and Access-Control-Allow-Credentials headers to the response.
   • It then passes control to the next handler in the chain.

Origin Matching

The handler performs strict origin matching. If the Origin header matches one of the allowedOrigins, that specific origin is reflected in the Access-Control-Allow-Origin header. If it does not match, the request may be rejected (403) or handled without CORS headers depending on the flow.

DeRef Token Middleware

The DerefMiddlewareHandler is designed to handle “By Reference” tokens (opaque tokens) at the edge of your microservices architecture (e.g., in a BFF or Gateway). It exchanges these opaque tokens for “By Value” tokens (JWTs) by communicating with an OAuth 2.0 provider.

Introduction

In some security architectures, organizations prefer not to expose JWTs (which contain claims and potentially sensitive information) to public clients (browsers, mobile apps). Instead, they issue opaque “Reference Tokens”.

However, internal microservices typically rely on JWTs for stateless authentication and authorization. The DerefMiddlewareHandler bridges this gap by intercepting requests with reference tokens and “dereferencing” them into JWTs before passing the request to downstream services.

Logic Flow

The handler performs the following steps:

1. Check Header: It looks for the Authorization header.
2. Validate Existence: If the header is missing, it returns an error (ERR10002).
3. Format Check:
   • It checks if the token contains a dot (.).
   • If existing dot: It assumes the token is already a JWT (Signed JWTs always have 3 parts separated by dots). The handler ignores it and passes the request to the next handler.
   • If no dot: It treats the token as a Reference Token.
4. Dereference:
   • It calls the OAuth 2.0 provider (via OauthHelper) to exchange the reference token for a JWT.
   • If the exchange fails or returns an error, it terminates the request with an appropriate error status (ERR10044 or ERR10045).
   • If successful, it replaces the Authorization header in the current request with the new Bearer <JWT>.
5. Proceed: The request continues to the next handler in the chain with the valid JWT.

Configuration

The handler is configured via the deref.yml file.

Example deref.yml

# Dereference Token Middleware Configuration
enabled: true

Note: The actual connection details for the OAuth 2.0 provider (URL, credentials) are typically handled by the client module configuration and OauthHelper internal settings, not directly in deref.yml.

Usage

To use this handler, register it in handler.yml and place it before the JwtVerifyHandler or any other handler that requires a valid JWT.

handler.yml

handlers:
  - com.networknt.deref.DerefMiddlewareHandler@deref
  # ... other handlers

chains:
  default:
    - deref
    - security # (JwtVerifyHandler)
    # ... other handlers

Error Codes

• ERR10002: MISSING_AUTH_TOKEN - The Authorization header is missing.
• ERR10044: EMPTY_TOKEN_DEREFERENCE_RESPONSE - The OAuth provider returned an empty response.
• ERR10045: TOKEN_DEREFERENCE_ERROR - The OAuth provider returned an error message.

Dump Handler

The DumpHandler is a middleware handler used to log the detailed request and response information. It is primarily used for debugging and troubleshooting in development or testing environments.

Warning: detailed logging of requests and responses can be very slow and may consume significant storage. It is generally not recommended for production environments unless explicitly needed for brief diagnostic sessions.

Introduction

Why requests fail is not always obvious. Sometimes, you need to see exactly what headers, cookies, or body content were sent by the client, or exactly what the server responded with. The DumpHandler captures this information and writes it to the logs.

Configuration

The handler is configured via the dump.yml file.

Detailed Configuration (request & response)

You can granularly control which parts of the HTTP message are logged using the request and response objects in the config.

Request Options:

• url: Log the request URL.
• headers: Log headers.
• cookies: Log cookies.
• queryParameters: Log query parameters.
• body: Log the request body.
• Filters: filteredHeaders, filteredCookies, filteredQueryParameters allow you to exclude specific sensitive keys.

Response Options:

• headers: Log headers.
• cookies: Log cookies.
• statusCode: Log the HTTP status code.
• body: Log the response body.

Example dump.yml

# Dump Handler Configuration
enabled: true
logLevel: INFO
indentSize: 4
useJson: false
mask: false

# Enable Request Dumping
requestEnabled: true
request:
  url: true
  headers: true
  filteredHeaders:
    - Authorization
    - X-Correlation-Id
  cookies: true
  filteredCookies:
    - JSESSIONID
  queryParameters: true
  filteredQueryParameters:
    - password
  body: true

# Enable Response Dumping
responseEnabled: true
response:
  headers: true
  cookies: true
  body: true
  statusCode: true

Usage

To use the DumpHandler, register it in handler.yml and add it to the middleware chain.

Placement: It is highly recommended to place the DumpHandler after the BodyHandler. If placed before BodyHandler, the request body input stream might not be readable or might be consumed prematurely if not handled correctly.

handler.yml

handlers:
  - com.networknt.dump.DumpHandler@dump
  # ... other handlers

chains:
  default:
    - correlation
    - body
    - dump  # Place after BodyHandler
    # ... other handlers

Logging

The handler writes to the standard SLF4J logger. You should configure your logback.xml to capture these logs, potentially directing them to a separate file to avoid cluttering your main application logs.

logback.xml Example

<appender name="DUMP_FILE" class="ch.qos.logback.core.rolling.RollingFileAppender">
    <file>target/dump.log</file>
    <encoder>
        <pattern>%-5level [%thread] %date{ISO8601} %msg%n</pattern>
    </encoder>
    <rollingPolicy class="ch.qos.logback.core.rolling.FixedWindowRollingPolicy">
        <fileNamePattern>target/dump.log.%i.zip</fileNamePattern>
        <minIndex>1</minIndex>
        <maxIndex>5</maxIndex>
    </rollingPolicy>
    <triggeringPolicy class="ch.qos.logback.core.rolling.SizeBasedTriggeringPolicy">
        <maxFileSize>10MB</maxFileSize>
    </triggeringPolicy>
</appender>

<!-- Logger for the dump handler package -->
<logger name="com.networknt.dump" level="INFO" additivity="false">
    <appender-ref ref="DUMP_FILE"/>
</logger>

Output Example

INFO  [XNIO-1 task-1] 2026-02-03 10:15:30 DumpHelper.java:23 - Http request/response information:
request:
    url: /v1/pets
    headers:
        Host: localhost:8080
        Content-Type: application/json
    body: {
        "id": 123,
        "name": "Sparky"
    }
response:
    statusCode: 200
    headers:
        Content-Type: application/json
    body: {
        "status": "success"
    }

Encode Decode Middleware

The encode-decode module provides two middleware handlers to handle compression and decompression of HTTP request and response bodies. This is crucial for optimizing bandwidth usage and supporting clients that send compressed data.

Request Decode Handler

The RequestDecodeHandler is responsible for handling requests where the body content is compressed (e.g., GZIP or Deflate). It checks the Content-Encoding header and automatically wraps the request stream to decompress the content on the fly, allowing subsequent handlers to read the plain content.

Configuration

The handler is configured via request-decode.yml.

Usage

Register com.networknt.decode.RequestDecodeHandler in your handler.yml. It should be placed early in the chain, before any handler that needs to read the request body (like BodyHandler).

handlers:
  - com.networknt.decode.RequestDecodeHandler@decode
  # ... other handlers

chains:
  default:
    - decode
    - body
    # ...

Response Encode Handler

The ResponseEncodeHandler is responsible for compressing the response body before sending it to the client. It automatically negotiates the compression method based on the client’s Accept-Encoding header.

Configuration

The handler is configured via response-encode.yml.

Usage

Register com.networknt.encode.ResponseEncodeHandler in your handler.yml. It is typically placed early in the chain so that it wraps the response exchange for all subsequent handlers.

handlers:
  - com.networknt.encode.ResponseEncodeHandler@encode
  # ... other handlers

chains:
  default:
    - encode
    # ... other handlers

Summary

• Request Decode: Decompresses incoming request bodies (Client -> Server).
• Response Encode: Compresses outgoing response bodies (Server -> Client).

Exception Handler

The ExceptionHandler is a critical middleware component that sits at the beginning of the request/response chain. Its primary role is to catch any unhandled exceptions thrown by subsequent handlers, log them, and return a standardized error response to the client.

Introduction

In Light-4j, exceptions are used to signal errors. While business handlers are encouraged to handle known exceptions locally to provide context-specific responses, the ExceptionHandler acts as a safety net (or “last line of defense”) to ensure that:

1. No exception crashes the server or returns a raw stack trace to the client.
2. All unhandled exceptions result in a proper JSON error response.
3. Requests are dispatched from the IO thread to the Worker thread (if not already there).

Thread Model

One important feature of this handler is that it explicitly checks if the request is currently running in the IO Thread. If so, it dispatches the request to the Worker Thread.

if (exchange.isInIoThread()) {
    exchange.dispatch(this);
    return;
}

This ensures that business logic, which might block or throw exceptions, runs in the worker pool, preventing the non-blocking IO threads from hanging.

Logic Flow

1. Dispatch: Logic moves execution to a worker thread.
2. Chain Execution: It wraps Handler.next(exchange, next) in a try-catch block.
3. Exception Catching: If an exception bubbles up, it is categorized and handled:
   • FrameworkException (Runtime): Returns the specific Status code defined in the exception.
   • Other RuntimeExceptions: Returns generic error ERR10010 (Status 500).
   • ApiException (Checked): Returns the specific Status code defined in the exception.
   • ClientException (Checked): Returns the specific Status code if set, otherwise ERR10011 (Status 400).
   • Other Checked Exceptions: Returns generic error ERR10011 (Status 400).

Configuration

The handler is configured via exception.yml. By default, it is enabled.

# Exception Handler Configuration
enabled: true

Error Codes

The handler relies on status.yml to map error codes to messages.

Usage

This handler should be the first (or one of the very first) handlers in your chain defined in handler.yml.

handler.yml

handlers:
  - com.networknt.exception.ExceptionHandler@exception
  # ... other handlers

chains:
  default:
    - exception
    - metrics
    - trace
    - correlation
    # ... business handlers

Best Practices

• Keep it Enabled: Do not disable this handler in production unless you have a specific reason and alternative exception handling mechanism.
• Handle Business Exceptions: Try to catch and handle semantic exceptions (like “User Not Found”) in your business handler to return a more meaningful error code than the generic fallbacks provided here.

Expect100 Continue Handler

The Expect100ContinueHandler is a middleware handler designed to manage the HTTP Expect: 100-continue protocol. This protocol allows a client to send a header with the request headers to ask the server if it is willing to accept the request body before sending it.

Introduction

Some HTTP clients (like curl or Apache HttpClient) automatically send an Expect: 100-continue header when the request body is large (usually > 1024 bytes). They then wait for the server to reply with HTTP/1.1 100 Continue before transmitting the body.

If the backend or the gateway does not handle this properly, the client might hang waiting for the 100 response, or the connection might be mishandled. This handler provides proper support for this protocol within the Light-4j handler chain.

Logic Flow

When the handler detects Expect: 100-continue in the request header:

1. Check Ignored Paths: If the request path matches ignoredPathPrefixes, the handler simply removes the Expect header and lets the request proceed. This is useful if downstream services don’t support or understand the header.
2. Check In-Place Paths: If the request path matches inPlacePathPrefixes, the handler immediately sends a 100 Continue response to the client and then removes the header. This signals the client to start sending the body immediately.
3. Default Behavior: If neither of the above applies, it adds a request wrapper (Expect100ContinueConduit) and a response commit listener to manage the 100-continue handshake transparently during the request lifecycle.

Configuration

The handler is configured via expect-100-continue.yml.

expect-100-continue.yml

# Expect100Continue Handler Configuration
enabled: true

# Remove Expect header for these paths (simulating it never existed)
ignoredPathPrefixes:
  - /v1/old-service

# Send 100 Continue immediately for these paths
inPlacePathPrefixes:
  - /v1/upload

Usage

Register com.networknt.expect100continue.Expect100ContinueHandler in your handler.yml. It should be placed early in the chain, before any handler that reads the body.

handler.yml

handlers:
  - com.networknt.expect100continue.Expect100ContinueHandler@expect
  # ... other handlers

chains:
  default:
    - expect
    # ...

Why use this handler?

Without this handler, Undertow (the underlying server) might handle 100-continue automatically in a way that doesn’t fit a proxy/gateway scenario, or downstream services might reject the request with the Expect header. This handler gives you fine-grained control over how to negotiate this protocol.

External Service Handler

The External Service Handler (ExternalServiceHandler) enables the gateway or service to access third-party services, potentially through a corporate proxy (forward proxy).

Introduction

In enterprise environments, internal services often need to call external APIs (e.g., Salesforce, Slack) but are restricted by firewalls and MUST go through a corporate web gateway (like McAfee Web Gateway). The default RouterHandler or ProxyHandler may not support this specific forward proxy configuration easily or may be too heavy-weight.

The ExternalServiceHandler uses the JDK 11+ HttpClient to make these calls. It supports:

• Configuring a forward proxy (host and port).
• Routing requests based on path prefixes to different external hosts.
• URL rewriting.
• Custom timeouts per path.
• Connection retries.

Configuration

The handler is configured via external-service.yml.

external-service.yml Example

enabled: true
# Corporate Proxy Settings
proxyHost: proxy.corp.example.com
proxyPort: 8080

# Global Timeouts
connectTimeout: 2000
timeout: 5000

# Mapping Paths to External Hosts
pathPrefixes:
  - pathPrefix: /sharepoint
    host: https://sharepoint.microsoft.com
    timeout: 10000
  - pathPrefix: /salesforce
    host: https://api.salesforce.com
    timeout: 5000
  - pathPrefix: /openai
    host: https://api.openai.com

# URL Rewriting (Optional)
urlRewriteRules:
  - /openai/(.*) /$1

Features

Forward Proxy Support

If proxyHost and proxyPort are configured, all outgoing requests will be routed through this proxy. This is essential for accessing the internet from within a DMZ or secure corporate network.

Path-Based Routing

The handler intercepts requests matching a configured prefix (e.g., /sharepoint) and routes them to the corresponding target host (e.g., https://sharepoint.microsoft.com).

• pathPrefixes: The recommended config, allowing specific timeouts per route.
• pathHostMappings: Simple space-separated string mappings (Legacy).

URL Rewriting

You can rewrite the path before it is sent to the target. For example, stripping the /openai prefix so that a request to /openai/v1/chat becomes /v1/chat at the target https://api.openai.com.

Metrics Injection

Like the Proxy Handler, this handler can inject metrics into the MetricsHandler to track the latency of external calls separately from internal processing.

Body Handling

It supports buffering request bodies for POST, PUT, and PATCH methods to forward them to the external service.

Usage

1. Register the handler in handler.yml.

   handlers:
     - com.networknt.proxy.ExternalServiceHandler@external
   

2. Add it to the default chain or specific paths. Since it works based on path prefixes, it is often placed in the default chain before the router or other terminal handlers, so it can intercept specific traffic.

   chains:
     default:
       - exception
       - metrics
       - header
       - external  # <--- Intercepts configured paths (e.g. /sharepoint)
       - prefix
       - router
   

   If the request path matches one of the pathPrefixes, the ExternalServiceHandler takes over, makes the remote call, and ends the exchange (it acts as a terminal handler for those requests). If the path does not match, it calls Handler.next(exchange, next) to pass the request down the chain.

Limitations

• It acts as a simple pass-through. Complex logic (aggregation, transformation) should be handled by an orchestrator or specialized handler.
• It creates a new HttpRequest for each call but reuses the HttpClient.

Header Handler

The HeaderHandler is a middleware component in the light-4j framework designed to modify request and response headers as they pass through the handler chain. This is particularly useful for cross-cutting concerns such as:

• Security: Updating or removing authorization headers (e.g., converting a Bearer token to a Basic Authorization header in a proxy scenario).
• Privacy: Removing sensitive internal headers before sending a response to the client.
• Context Propagation: Injecting correlation IDs or other context information into headers.
• API Management: Standardizing headers across different microservices.

The handler is highly configurable, allowing for global header manipulation as well as path-specific configurations.

Configuration

The HeaderHandler uses a configuration file named header.yml to define its behavior.

Configuration Options

The configuration supports the following sections:

1. enabled: A boolean flag to enable or disable the handler globally. Default is false.
2. request: Defines header manipulations for incoming requests.
   • remove: A list of header names to remove from the request.
   • update: A map of key/value pairs to add or update in the request headers. If a key exists, its value is replaced.
3. response: Defines header manipulations for outgoing responses.
   • remove: A list of header names to remove from the response.
   • update: A map of key/value pairs to add or update in the response headers.
4. pathPrefixHeader: A map where keys are URL path prefixes and values are request and response configurations specific to that path. This allows granular control over header manipulation based on the endpoint being accessed.

Example header.yml (Fully Expanded)

# Enable header handler or not, default to false
enabled: false

# Global Request header manipulation
request:
  # Remove all the headers listed here
  remove:
  - header1
  - header2
  # Add or update the header with key/value pairs
  update:
    key1: value1
    key2: value2

# Global Response header manipulation
response:
  # Remove all the headers listed here
  remove:
  - header1
  - header2
  # Add or update the header with key/value pairs
  update:
    key1: value1
    key2: value2

# Per-path header manipulation
pathPrefixHeader:
  /petstore:
    request:
      remove:
        - headerA
        - headerB
      update:
        keyA: valueA
        keyB: valueB
    response:
      remove:
        - headerC
        - headerD
      update:
        keyC: valueC
        keyD: valueD
  /market:
    request:
      remove:
        - headerE
        - headerF
      update:
        keyE: valueE
        keyF: valueF
    response:
      remove:
        - headerG
        - headerH
      update:
        keyG: valueG
        keyH: valueH

Reference header.yml (Template)

This is the default configuration file packaged with the module (src/main/resources/config/header.yml). It uses placeholders that can be overridden by values.yml or handling external configurations.

# Enable header handler or not. The default to false and it can be enabled in the externalized
# values.yml file. It is mostly used in the http-sidecar, light-proxy or light-router.
enabled: ${header.enabled:false}
# Request header manipulation
request:
  # Remove all the request headers listed here. The value is a list of keys.
  remove: ${header.request.remove:}
  # Add or update the header with key/value pairs. The value is a map of key and value pairs.
  # Although HTTP header supports multiple values per key, it is not supported here.
  update: ${header.request.update:}
# Response header manipulation
response:
  # Remove all the response headers listed here. The value is a list of keys.
  remove: ${header.response.remove:}
  # Add or update the header with key/value pairs. The value is a map of key and value pairs.
  # Although HTTP header supports multiple values per key, it is not supported here.
  update: ${header.response.update:}
# requestPath specific header configuration. The entire object is a map with path prefix as the
# key and request/response like above as the value. For config format, please refer to test folder.
pathPrefixHeader: ${header.pathPrefixHeader:}

Configuring via values.yml

You can use values.yml to override specific properties. The properties can be defined in various formats (YAML, JSON string, comma-separated string) to suit different configuration sources (file system, config server).

# header.yml overrides
header.enabled: true

# List of strings (JSON format)
header.request.remove: ["header1", "header2"]

# Map (JSON format)
header.request.update: {"key1": "value1", "key2": "value2"}

# List (Comma separated string)
header.response.remove: header1,header2

# Map (Comma and colon separated string)
header.response.update: key1:value1,key2:value2

# Map (YAML format)
# Note: YAML format is suitable for file-based configuration. 
# For config server usage, use a JSON string representation.
header.pathPrefixHeader:
  /petstore:
    request:
      remove:
        - headerA
        - headerB
      update:
        keyA: valueA
        keyB: valueB
    response:
      remove:
        - headerC
        - headerD
      update:
        keyC: valueC
        keyD: valueD
  /market:
    request:
      remove:
        - headerE
        - headerF
      update:
        keyE: valueE
        keyF: valueF
    response:
      remove:
        - headerG
        - headerH
      update:
        keyG: valueG
        keyH: valueH

Usage

To use the HeaderHandler in your application:

1. Add the Dependency: Ensure the header module is included in your project’s pom.xml.
2. Register the Handler: Add com.networknt.header.HeaderHandler to your middleware chain in handler.yml.
3. Configure: Provide a header.yml or configured values.yml in your src/main/resources/config folder (or external config directory) to define the desired header manipulations.

IP Whitelist

The ip-whitelist middleware handler is designed to secure specific endpoints (e.g., health checks, metric endpoints, admin screens) by allowing access only from trusted IP addresses. This is often used for endpoints that cannot be easily secured via OAuth 2.0 or other standard authentication mechanisms, or as an additional layer of security.

It serves as a critical component in scenarios where services like Consul, Prometheus, or internal orchestration tools need access to technical endpoints without user-level authentication.

Requirements

The handler supports:

• Per-Path Configuration: Rules are defined for specific url path prefixes.
• IPv4 and IPv6: Full support for both IP versions.
• Flexible matching:
  • Exact match: e.g., 127.0.0.1 or FE45:00:00:000:0:AAA:FFFF:0045
  • Wildcard match: e.g., 10.10.*.* or FE45:00:00:000:0:AAA:FFFF:*
  • CIDR Notation (Slash): e.g., 127.0.0.48/30 or FE45:00:00:000:0:AAA:FFFF:01F4/127
• Default Policy: Configurable default behavior (allow or deny) when no specific rule is matched.

Configuration

The configuration is managed via whitelist.yml.

Configuration Options

• enabled: (boolean) Enable or disable the handler globally. Default true.
• defaultAllow: (boolean) Determines the behavior for the IP addresses listed in the configuration.
  • true (Whitelist Mode - Most Common): If defaultAllow is true, the IPs listed for a path are ALLOWED. Any IP accessing that path that is not in the list is DENIED. If a path is not defined in the config, access is ALLOWED globally for that path.
  • false (Blacklist Mode): If defaultAllow is false, the IPs listed for a path are DENIED. Any IP accessing that path that is not in the list is ALLOWED. If a path is not defined in the config, access is DENIED globally for that path.
• paths: A map where keys are request path prefixes and values are lists of IP patterns.

Example whitelist.yml (Template)

# IP Whitelist configuration

# Indicate if this handler is enabled or not.
enabled: ${whitelist.enabled:true}

# Default allowed or denied behavior.
defaultAllow: ${whitelist.defaultAllow:true}

# List of path prefixes and their access rules.
paths: ${whitelist.paths:}

Configuring via values.yml

You can define the rules in values.yml using either YAML or JSON format.

YAML Format

Suitable for file-based configuration.

whitelist.enabled: true
whitelist.defaultAllow: true
whitelist.paths:
  /health:
    - 127.0.0.1
    - 10.10.*.*
  /prometheus:
    - 192.168.1.5
    - 10.10.*.*
  /admin:
    - 127.0.0.1

JSON Format

Suitable for config servers or environment variables where a single string is required.

whitelist.paths: {"/health":["127.0.0.1","10.10.*.*"],"/prometheus":["192.168.1.5"]," /admin":["127.0.0.1"]}

Logic Flow

1. Extract IP: The handler extracts the source IP address from the request.
2. Match Path: It checks if the request path starts with any of the configured prefixes.
3. Find ACL: If a matching path prefix is found, it retrieves the corresponding Access Control List containing rules (IP patterns).
4. Evaluate Rules:
   • It iterates through the rules for the IP version (IPv4/IPv6).
   • If a rule matches the source IP:
     • It returns !rule.isDeny(). (In defaultAllow=true mode, rules are allow rules, so deny is false, returning true).
   • If no rule matches the source IP:
     • It returns !defaultAllow. (In defaultAllow=true mode, if IP is not in the allow list, it returns false (deny)).
5. No Path Match: If the request path is not configured:
   • It returns defaultAllow. (In defaultAllow=true mode, unknown paths are open).

Error Handling

If access is denied, the handler terminates the exchange and returns an error.

• Status Code: 403 Forbidden
• Error Code: ERR10049
• Message: INVALID_IP_FOR_PATH
• Description: Peer IP %s is not in the whitelist for the endpoint %s

Usage

1. Add the ip-whitelist module dependency to your pom.xml.
2. Add com.networknt.whitelist.WhitelistHandler to your handler.yml chain.
3. Configure whitelist.yml (or values.yml) with your specific IP rules.

Path Prefix Service Handler

The PathPrefixServiceHandler is a middleware handler primarily used in the Light-Gateway or http-sidecar. It maps a request path prefix to a serviceId, allowing the router to discover and invoke the correct downstream service without requiring the caller to provide the service_id header or perform full OpenAPI validation.

Introduction

In the Light-4j architecture, the Router Handler typically relies on a service_id header to locate the target service in the service registry (like Consul).

The PathPrefixServiceHandler simplifies this by enabling path-based routing. If your downstream services have unique path prefixes (e.g., /v1/users -> user-service, /v1/orders -> order-service), this handler can automatically lookup the serviceId based on the request URL and inject it into the request header.

Logic Flow

1. Intercept Request: The handler intercepts the incoming request.
2. Match Prefix: It checks the request path against the configured mapping table.
3. Inject Header:
   • If a matching prefix is found, it retrieves the corresponding serviceId.
   • It injects this serviceId into the service_id header (if not already present).
   • It populates the endpoint in the audit data for logging/metrics.
4. No Match: If no match is found, it marks the endpoint as Unknown in the audit data but allows the request to proceed (best-effort).

Configuration

The handler is configured via the pathPrefixService.yml file.

Example pathPrefixService.yml

# Path Prefix Service Configuration
enabled: true

# Mapping: Path Prefix -> Service ID
mapping:
  /v1/address: party.address-1.0.0
  /v2/address: party.address-2.0.0
  /v1/contact: party.contact-1.0.0
  /v1/pets: petstore-3.0.1

Configuration Formats

The mapping can be defined in multiple formats to support different configuration sources (like standard config files or a config server).

1. YAML Format (Standard):

mapping:
  /v1/address: party.address-1.0.0
  /v2/address: party.address-2.0.0

2. JSON Format (String): Useful for environment variables or config servers that expect strings.

mapping: '{"/v1/address": "party.address-1.0.0", "/v2/address": "party.address-2.0.0"}'

3. Java Map Format (String): Key-value pairs separated by &.

mapping: /v1/address=party.address-1.0.0&/v2/address=party.address-2.0.0

Usage

To use this handler, register it in handler.yml and place it before the RouterHandler.

handler.yml

handlers:
  - com.networknt.router.middleware.PathPrefixServiceHandler@prefix
  # ... other handlers

chains:
  default:
    - correlation
    - metrics
    - prefix   # Place before router
    - router

Note: Do not map system endpoints like /health or /server/info in this handler, as they are common to all services.

Comparison with PathServiceHandler

• PathPrefixServiceHandler: Simple prefix matching. Does not require OpenAPI specifications. Fast and lightweight. Best for simple routing requirements.
• PathServiceHandler: Requires OpenAPI handlers. Can perform more complex matching based on specific endpoints (Method + Path).

Path Service Handler

The PathServiceHandler is a middleware handler used to discover the serviceId based on the request endpoint (Method + Path). It is similar to the PathPrefixServiceHandler but offers finer granularity by leveraging the endpoint information from the audit attachment (populated by OpenAPI/Swagger handlers).

Introduction

In a microservices architecture, the router needs to know the serviceId to route the request to the correct downstream service. While PathPrefixServiceHandler uses a simple path prefix, PathServiceHandler uses the exact endpoint (e.g., /v1/pets/{petId}@GET) to map to a serviceId.

This is particularly useful when:

1. You have multiple services sharing the same path prefix but serving different endpoints.
2. You want to route specific endpoints to different services (e.g., read vs. write separation).

Prerequisite: This handler typically comes after the OpenApiHandler (or SwaggerHandler) in the chain because it relies on the Audit Info attachment (auditInfo) where the normalized endpoint string is stored.

Logic Flow

1. Check Service ID: It first checks if the service_id header is already present. If so, it skips logic.
2. Retrieve Endpoint: It retrieves the endpoint string from the Audit Info attachment (e.g., /v1/pets/123@get -> normalized to /v1/pets/{petId}@get).
3. Lookup Mapping: It looks up this endpoint in the configured mapping.
4. Inject Header: If a match is found, it injects the mapped serviceId into the service_id header.

Configuration

The handler is configured via the pathService.yml file.

Configuration Format

The mapping can be defined in YAML, JSON, or Java Map string format.

Endpoint Format: [path]@[method] (method must be lowercase).

Example pathService.yml:

# Path Service Configuration
enabled: true

# Mapping: Endpoint -> Service ID
mapping:
  /v1/address/{id}@get: party.address-1.0.0
  /v2/address@get: party.address-2.0.0
  /v1/contact@post: party.contact-1.0.0

Usage

To use this handler, register it in handler.yml and place it after the OpenApiHandler and before the RouterHandler.

handler.yml

handlers:
  - com.networknt.router.middleware.PathServiceHandler@path
  # ... other handlers

chains:
  default:
    - correlation
    - openapi-handler
    - path   # Place after OpenAPI and before Router
    - router

Comparison

Rate Limit

The rate-limit middleware handler is designed to protect your APIs from being overwhelmed by too many requests. It can be used for two primary purposes:

1. DDoS Protection: Limiting concurrent requests from the public internet to prevent denial-of-service attacks.
2. Throttling: Protecting slow backend services or managing API quotas for different clients or users.

This handler impacts performance slightly, so it is typically not enabled by default. It should be placed early in the middleware chain, typically after ExceptionHandler and MetricsHandler, to fail fast and still allow metrics to capture rate-limiting events.

Dependency

To use the handler, add the following dependency to your pom.xml:

<dependency>
    <groupId>com.networknt</groupId>
    <artifactId>rate-limit</artifactId>
    <version>${version.light-4j}</version>
</dependency>

Note: The configuration class is located in the limit-config module.

Handler Configuration

Register the handler in handler.yml:

handlers:
  - com.networknt.limit.LimitHandler@limit

chains:
  default:
    - exception
    - metrics
    - limit
    - ...

Configuration (limit.yml)

The handler is configured via limit.yml.

# Rate Limit Handler Configuration

# Enable or disable the handler
enabled: ${limit.enabled:false}

# Error code returned when limit is reached. 
# Use 503 for DDoS protection (to trick attackers) or 429 for internal throttling.
errorCode: ${limit.errorCode:429}

# Default rate limit: e.g., 10 requests per second and 10000 quota per day.
rateLimit: ${limit.rateLimit:10/s 10000/d}

# If true, rate limit headers are always returned, even for successful requests.
headersAlwaysSet: ${limit.headersAlwaysSet:false}

# Key of the rate limit: server, address, client, user
# server: Shared limit for the entire server.
# address: Limit per IP address.
# client: Limit per client ID (from JWT).
# user: Limit per user ID (from JWT).
key: ${limit.key:server}

# Custom limits can be defined for specific keys
server: ${limit.server:}
address: ${limit.address:}
client: ${limit.client:}
user: ${limit.user:}

# Key Resolvers
clientIdKeyResolver: ${limit.clientIdKeyResolver:com.networknt.limit.key.JwtClientIdKeyResolver}
addressKeyResolver: ${limit.addressKeyResolver:com.networknt.limit.key.RemoteAddressKeyResolver}
userIdKeyResolver: ${limit.userIdKeyResolver:com.networknt.limit.key.JwtUserIdKeyResolver}

Rate Limit Keys

1. Server Key

The limit is shared across all incoming requests. You can define specific limits for different path prefixes:

limit.server:
  /v1/address: 10/s
  /v2/address: 1000/s

2. Address Key

The source IP address is used as the key. Each IP gets its own quota.

limit.address:
  192.168.1.100: 10/h 1000/d
  192.168.1.102:
    /v1: 10/s

3. Client Key

The client_id from the JWT token is used as the key. Note: JwtVerifierHandler must be in the chain before the limit handler.

limit.client:
  my-client-id: 100/m 10000/d

4. User Key

The user_id from the JWT token is used as the key. Note: JwtVerifierHandler must be in the chain before the limit handler.

limit.user:
  [email protected]: 10/m 10000/d

Rate Limit Headers

When a limit is reached (or if headersAlwaysSet is true), the following headers are returned:

• X-RateLimit-Limit: The configured limit (e.g., 10/s).
• X-RateLimit-Remaining: The number of requests remaining in the current time window.
• X-RateLimit-Reset: The number of seconds until the current window resets.
• Retry-After: (Only when limit is reached) The timestamp when the client can retry.

Module Registration

The limit-config module automatically registers itself with the ModuleRegistry when LimitConfig.load() is called. This allows the configuration to be visible via the Server Info endpoint and supports hot reloading of configurations.

Request Injection Middleware

The Request Injection middleware (RequestInterceptorInjectionHandler) is designed to inject implementations of the RequestInterceptor interface into the request chain. This allows developers to modify request metadata and body transparently before the request reaches the business handler.

Introduction

In many scenarios, you may need to inspect or modify the request body or headers based on complex rules or external data. While the request-transformer middleware handles rule-based transformation, the request-injection middleware provides a programmatic way to inject custom logic via the RequestInterceptor interface.

A key feature of this handler is its ability to buffer the request body (even for large requests using chunked transfer encoding), allowing interceptors to read and modify the full body content.

Configuration

The handler is configured via request-injection.yml.

• appliedBodyInjectionPathPrefixes: Body injection involves buffering the entire request in memory, which is expensive. It should only be enabled for specific paths that require it. For other paths, the interceptors are still called, but shouldReadBody will return false, skipping the buffering logic unless an interceptor specifically demands it and the path matches.

request-injection.yml Example

enabled: true
maxBuffers: 1024
appliedBodyInjectionPathPrefixes:
  - /v1/pets
  - /v1/store

RequestInterceptor Interface

To use this middleware, you must implement the RequestInterceptor interface and register your implementation via the service provider interface (SPI) or service.yml if using the SingletonServiceFactory.

public interface RequestInterceptor {
    /**
     * Handle the request.
     * @param exchange The HttpExchange
     * @throws Exception Exception
     */
    void handleRequest(HttpServerExchange exchange) throws Exception;

    /**
     * Indicate if the interceptor requires the request body.
     * @return boolean
     */
    default boolean isRequiredContent() {
        return false;
    }
}

• handleRequest: Implement your logic here. You can modify headers, check security, or rewrite the body (if available in attachment).
• isRequiredContent: Return true if your interceptor needs to inspect the request body. This combined with appliedBodyInjectionPathPrefixes config determines if the handler will buffer the request stream.

Logic Flow

1. Check Config: The handler takes RequestInjectionConfig to check if the current request path matches appliedBodyInjectionPathPrefixes.
2. Check Interceptors: It checks if any registered RequestInterceptor returns true for isRequiredContent().
3. Buffer Body: If both conditions are met (and method has content like POST/PUT), it reads the request channel into a PooledByteBuffer array.
4. Invoke Interceptors: It calls handleRequest() on all registered interceptors.
   • Interceptors can access the buffered body via exchange.getAttachment(AttachmentConstants.BUFFERED_REQUEST_DATA_KEY).
5. Continue: After all interceptors run (and assuming none terminated the exchange), the request processing continues to the next handler.

Payload Size Limit

The maxBuffers configuration limits the maximum size of the request body that can be buffered.

• Buffer size = 16KB.
• Default maxBuffers = 1024.
• Total max size = 16MB.

If the request body exceeds this limit, the handler will throw a RequestTooBigException and return error ERR10068 (PAYLOAD_TOO_LARGE).

Usage

Register com.networknt.handler.RequestInterceptorInjectionHandler in your handler.yml.

handlers:
  - com.networknt.handler.RequestInterceptorInjectionHandler@injection
  # ...

chains:
  default:
    - injection
    # ...

Response Injection Middleware

The Response Injection middleware (ResponseInterceptorInjectionHandler) allows developers to inject ResponseInterceptor implementations into the request/response chain. These interceptors can modify the response headers and body before it is sent back to the client.

Introduction

Modification of the response body in an asynchronous, non-blocking server like Undertow is complex because the response is often streamed. This middleware handles the complexity by injecting a custom SinkConduit (ModifiableContentSinkConduit) when necessary. This allows interceptors to buffer the response, modify it, and then send the modified content to the client.

Configuration

The handler is configured via response-injection.yml.

• appliedBodyInjectionPathPrefixes: Modifying the response body requires buffering the entire response in memory (to replace it). This is an expensive operation and should only be enabled for specific paths.

response-injection.yml Example

enabled: true
appliedBodyInjectionPathPrefixes:
  - /v1/pets
  - /v1/details

ResponseInterceptor Interface

To use this middleware, Implement the ResponseInterceptor interface.

public interface ResponseInterceptor extends Interceptor {
    /**
     * Indicate if the interceptor requires the response body.
     * @return boolean
     */
    boolean isRequiredContent();

    /**
     * Indicate if execution is async.
     * @return boolean (default false)
     */
    default boolean isAsync() { return false; }

    /**
     * Handle the response.
     * @param exchange The HttpExchange
     * @throws Exception Exception
     */
    void handleRequest(HttpServerExchange exchange) throws Exception;
}

• isRequiredContent: Return true if you need to modify the response body. This signals the middleware to inject the ModifiableContentSinkConduit.
• handleRequest: Implement your transformation logic.

Compression Handling

If an interceptor requires response content (isRequiredContent() returns true), the middleware forces the Accept-Encoding header in the request to identity.

• This prevents the upstream handler (or backend service) from compressing the response (e.g., GZIP), ensuring the interceptor receives plain text (JSON/XML) to modify.
• If the response is already compressed (e.g., from a static file handler that ignores Accept-Encoding), the middleware might skip injection to avoid corruption, as indicated by !isCompressed(exchange) check.

Usage

1. Register the handler in handler.yml:

   handlers:
     - com.networknt.handler.ResponseInterceptorInjectionHandler@responseInjection
   

2. Add it to your chain (usually early in the chain, so it wraps subsequent handlers):

   chains:
     default:
       - exception
       - metrics
       - responseInjection
       - ...
   

Logic Flow

1. Request Flow:
   • Handler checks if any registered ResponseInterceptor requires content.
   • If yes, and path matches config, and response is not already compressed:
     • It wraps the response channel with ModifiableContentSinkConduit.
     • It sets request Accept-Encoding to identity.
2. Response Flow:
   • When the downstream handler writes the response, it goes into the ModifiableContentSinkConduit.
   • The conduit buffers the response.
   • Interceptors are invoked to modify the buffered response.
   • The modified response is sent to the original sink.

Prometheus Metrics

The prometheus module in light-4j provides a systems monitoring middleware handler that integrates with Prometheus. Unlike other metrics modules that “push” data to a central server (e.g., InfluxDB), Prometheus adopts a pull-based model where the Prometheus server periodically scrapes metrics from instrumented targets.

Features

• Runtime Collection: Captures HTTP request counts and response times using the Prometheus dimensional data model.
• Dimensionality: Automatically attaches labels like endpoint and clientId to every metric, allowing for granular filtering and aggregation in Grafana.
• JVM Hotspot Monitoring: Optional collection of JVM-level metrics including CPU usage, memory pools, garage collection, and thread states.
• Standardized Scrape Endpoint: Provides a dedicated handler to expose metrics in the standard Prometheus text format.
• Auto-Registration: The module automatically registers its configuration with the ModuleRegistry during initialization.

Configuration (prometheus.yml)

The behavior of the Prometheus middleware is controlled by the prometheus.yml file.

# Prometheus Metrics Configuration
---
# If metrics handler is enabled or not. Default is false.
enabled: ${prometheus.enabled:false}

# If the Prometheus hotspot monitor is enabled or not. 
# includes thread, memory, classloader statistics, etc.
enableHotspot: ${prometheus.enableHotspot:false}

Setup

To enable Prometheus metrics, you need to add two handlers to your handler.yml: one to collect the data and another to expose it as an endpoint for scraping.

1. Add Handlers to handler.yml

handlers:
  # Captures metrics for every request
  - com.networknt.metrics.prometheus.PrometheusHandler@prometheus
  # Exposes the metrics to the Prometheus server
  - com.networknt.metrics.prometheus.PrometheusGetHandler@getprometheus

chains:
  default:
    - exception
    - prometheus  # Place early in the chain
    - correlation
    - ...

2. Define the Scrape Path

You must expose a GET endpoint (typically /v1/prometheus or /metrics) that invoke the getprometheus handler.

paths:
  - path: '/v1/prometheus'
    method: 'get'
    exec:
      - getprometheus

Collected Metrics

The PrometheusHandler defines several core application metrics:

• requests_total: Total number of HTTP requests received.
• success_total: Total number of successful requests (status codes 200-399).
• auth_error_total: Total number of authentication/authorization errors (status codes 401, 403).
• request_error_total: Total number of client-side request errors (status codes 400-499).
• server_error_total: Total number of server-side errors (status codes 500+).
• response_time_seconds: A summary of HTTP response latencies.

JVM Hotspot Monitoring

When enableHotspot is set to true, the module initializes the Prometheus DefaultExports, which captures:

• Process Statistics: process_cpu_seconds_total, process_open_fds.
• Memory Usage: jvm_memory_pool_bytes_used, jvm_memory_pool_bytes_max.
• Thread Areas: jvm_threads_state, jvm_threads_deadlocked.
• Garbage Collection: jvm_gc_collection_seconds.

Scraping with Prometheus

To pull data from your service, configure your Prometheus server’s scrape_configs in its configuration file:

scrape_configs:
  - job_name: 'light-4j-services'
    scrape_interval: 15s
    metrics_path: /v1/prometheus
    static_configs:
      - targets: ['localhost:8080']
    scheme: https
    tls_config:
      insecure_skip_verify: true

Dependency

Add the following to your pom.xml:

<dependency>
    <groupId>com.networknt</groupId>
    <artifactId>prometheus</artifactId>
    <version>${version.light-4j}</version>
</dependency>

Module Registration

The prometheus module registers itself with the ModuleRegistry automatically when PrometheusConfig.load() is called. This registration (along with the current configuration values) can be inspected via the Server Info endpoint.

Sanitizer Handler

The SanitizerHandler is a middleware component in light-4j designed to address Cross-Site Scripting (XSS) concerns by sanitizing request headers and bodies. It leverages the OWASP Java Encoder to perform context-aware encoding of potentially malicious user input.

Features

• Context-Aware Encoding: Uses the owasp-java-encoder to safely encode data.
• Header Sanitization: Encodes request headers to prevent header-based XSS or injection.
• Body Sanitization: Deep-scans JSON request bodies and encodes string values in Maps and Lists.
• Selective Sanitization: Fine-grained control with AttributesToEncode and AttributesToIgnore lists for both headers and bodies.
• Flexible Encoders: Supports multiple encoding formats like javascript-source, javascript-attribute, javascript-block, etc.
• Auto-Registration: Automatically registers with ModuleRegistry during configuration loading for runtime visibility.

Dependency

To use the SanitizerHandler, include the following dependency in your pom.xml:

<dependency>
    <groupId>com.networknt</groupId>
    <artifactId>sanitizer</artifactId>
    <version>${version.light-4j}</version>
</dependency>

Configuration (sanitizer.yml)

The behavior of the handler is managed through sanitizer.yml.

# Sanitizer Configuration
---
# Indicate if sanitizer is enabled or not. Default is false.
enabled: ${sanitizer.enabled:false}

# --- Body Sanitization ---
# If enabled, the request body will be sanitized. 
# Note: Requires BodyHandler to be in the chain before SanitizerHandler.
bodyEnabled: ${sanitizer.bodyEnabled:true}

# The encoder for the body. Default is javascript-source.
bodyEncoder: ${sanitizer.bodyEncoder:javascript-source}

# Optional list of body keys to encode. If specified, ONLY these keys are scanned.
bodyAttributesToEncode: ${sanitizer.bodyAttributesToEncode:}

# Optional list of body keys to ignore. All keys EXCEPT these will be scanned.
bodyAttributesToIgnore: ${sanitizer.bodyAttributesToIgnore:}

# --- Header Sanitization ---
# If enabled, the request headers will be sanitized.
headerEnabled: ${sanitizer.headerEnabled:true}

# The encoder for the headers. Default is javascript-source.
headerEncoder: ${sanitizer.headerEncoder:javascript-source}

# Optional list of header keys to encode. If specified, ONLY these headers are scanned.
headerAttributesToEncode: ${sanitizer.headerAttributesToEncode:}

# Optional list of header keys to ignore. All headers EXCEPT these will be scanned.
headerAttributesToIgnore: ${sanitizer.headerAttributesToIgnore:}

Setup

1. Order in Chain

If bodyEnabled is set to true, the SanitizerHandler must be placed in the handler chain after the BodyHandler. This is because the sanitizer expects the request body to be already parsed into the exchange attachment.

2. Register Handler

In your handler.yml, register the handler and add it to your chain.

handlers:
  - com.networknt.sanitizer.SanitizerHandler@sanitizer

chains:
  default:
    - ...
    - body
    - sanitizer
    - ...

When to use Sanitizer

The SanitizerHandler is primarily intended for applications that collect user input via Web or Mobile UIs and later use that data to generate HTML pages (e.g., forums, blogs, profile pages).

Do not use this handler for:

• Services where input is only processed internally and never rendered back to a browser.
• Encrypted or binary data.
• High-performance logging services where the overhead of string manipulation is unacceptable.

Query Parameters

The SanitizerHandler does not process query parameters. Modern web servers like Undertow already perform robust sanitization and decoding of special characters in query parameters before they reach the handler chain.

Operational Visibility

The sanitizer module utilizes the Singleton pattern for its configuration. During startup, it automatically registers itself with the ModuleRegistry. You can verify the current configuration, including active encoders and ignore/encode lists, at runtime via the Server Info endpoint.

Encode Library

The underlying library used for sanitization is the OWASP Java Encoder. The default encoding level is javascript-source, which provides a high degree of security without corrupting most types of textual data.

Security Handler

The SecurityHandler (implemented as JwtVerifyHandler in REST, GraphQL, etc.) is responsible for verifying the security tokens (JWT or SWT) in the request header. It is a core component of the light-4j framework and provides distributed policy enforcement without relying on a centralized gateway.

Configuration

The security configuration is usually defined in security.yml, but it can be overridden by framework-specific files like openapi-security.yml or graphql-security.yml.

Example security.yml

# Security configuration for security module in light-4j.
---
# Enable JWT verification flag.
enableVerifyJwt: ${security.enableVerifyJwt:true}

# Enable SWT verification flag.
enableVerifySwt: ${security.enableVerifySwt:true}

# swt clientId header name.
swtClientIdHeader: ${security.swtClientIdHeader:swt-client}

# swt clientSecret header name. 
swtClientSecretHeader: ${security.swtClientSecretHeader:swt-secret}

# Extract JWT scope token from the X-Scope-Token header and validate the JWT token
enableExtractScopeToken: ${security.enableExtractScopeToken:true}

# Enable JWT scope verification. Only valid when enableVerifyJwt is true.
enableVerifyScope: ${security.enableVerifyScope:true}

# Skip scope verification if the endpoint specification is missing.
skipVerifyScopeWithoutSpec: ${security.skipVerifyScopeWithoutSpec:false}

# If set true, the JWT verifier handler will pass if the JWT token is expired already.
ignoreJwtExpiry: ${security.ignoreJwtExpiry:false}

# User for test only. should be always be false on official environment.
enableMockJwt: ${security.enableMockJwt:false}

# Enables relaxed verification for jwt. e.g. Disables key length requirements.
enableRelaxedKeyValidation: ${security.enableRelaxedKeyValidation:false}

# JWT signature public certificates. kid and certificate path mappings.
jwt:
  certificate: ${security.certificate:100=primary.crt&101=secondary.crt}
  clockSkewInSeconds: ${security.clockSkewInSeconds:60}
  # Key distribution server standard: JsonWebKeySet for other OAuth 2.0 provider| X509Certificate for light-oauth2
  keyResolver: ${security.keyResolver:JsonWebKeySet}

# Enable or disable JWT token logging for audit.
logJwtToken: ${security.logJwtToken:true}

# Enable or disable client_id, user_id and scope logging.
logClientUserScope: ${security.logClientUserScope:false}

# Enable JWT token cache to speed up verification.
enableJwtCache: ${security.enableJwtCache:true}

# Max size of the JWT cache before a warning is logged.
jwtCacheFullSize: ${security.jwtCacheFullSize:100}

# Retrieve public keys dynamically from the OAuth2 provider.
bootstrapFromKeyService: ${security.bootstrapFromKeyService:false}

# Provider ID for federated deployment.
providerId: ${security.providerId:}

# Define a list of path prefixes to skip security.
skipPathPrefixes: ${security.skipPathPrefixes:}

# Pass specific claims from the token to the backend API via HTTP headers.
passThroughClaims:
  clientId: client_id
  tokenType: token_type

Key Features

JWT Verification

Verification includes signature check, expiration check, and scope verification. The handler uses the JwtVerifier to perform these checks.

SWT Verification

The handler also supports Shared Secret Token (SWT) verification. When enabled, it checks the token using client ID and secret, which can be passed in headers specified by swtClientIdHeader and swtClientSecretHeader.

Scope Verification

If enableVerifyScope is true, the handler compares the scopes in the JWT against the scopes defined in the API specification (e.g., openapi.yaml). If the specification is missing and skipVerifyScopeWithoutSpec is true, scope verification is skipped.

Token Caching

To improve performance, verified tokens can be cached. This avoids expensive signature verification for subsequent requests with the same token. The cache size can be monitored using jwtCacheFullSize.

Dynamic Key Loading

By setting bootstrapFromKeyService to true, the handler can pull public keys (JWK) from the OAuth2 provider’s key service based on the kid (Key ID) in the JWT header.

Path Skipping

You can bypass security for specific endpoints by adding their prefixes to skipPathPrefixes.

Claim Pass-through

Claims from the JWT or SWT can be mapped to HTTP headers and passed to the backend service using passThroughClaims. This is useful for passing user information or client details without the backend having to parse the token again.

Security Attacks Mitigation

alg header attacks

The light-4j implementation is opinionated and primarily uses RS256. It prevents “alg: none” attacks and HMAC-RSA confusion attacks by explicitly specifying the expected algorithm.

kid and Key Rotation

The use of kid allows the framework to support multiple keys simultaneously, which is essential for seamless key rotation.

Unified Security

The UnifiedSecurityHandler is a powerful middleware that consolidates various security mechanisms into a single handler. It is designed to simplify security configuration, especially in complex environments like a shared light-gateway, where different paths might require different authentication methods.

By using the UnifiedSecurityHandler, you avoid chaining multiple security-specific handlers (like BasicAuthHandler, JwtVerifyHandler, ApiKeyHandler) and can manage all security policies in one place.

Configuration

The configuration for this handler is defined in unified-security.yml.

Example unified-security.yml

# Unified security configuration.
---
# Indicate if this handler is enabled.
enabled: ${unified-security.enabled:true}

# Anonymous prefixes configuration. Request paths starting with these prefixes bypass all security.
anonymousPrefixes: ${unified-security.anonymousPrefixes:}

# Path prefix security configuration.
pathPrefixAuths:
  - prefix: /api/v1
    basic: false
    jwt: true
    sjwt: false
    swt: false
    apikey: false
    jwkServiceIds: com.networknt.petstore-1.0.0
  - prefix: /api/v2
    basic: true
    jwt: true
    apikey: true
    jwkServiceIds: service1,service2

Configuration Parameters

Path Prefix Auth Fields

How it works

1. Anonymous Check: The handler first checks if the request path matches any prefix in anonymousPrefixes. If it does, the request proceeds to the next handler immediately.
2. Path Policy Lookup: The handler iterates through pathPrefixAuths to find a match for the request path.
3. Authentication Execution:
   • If Basic, JWT, SJWT, or SWT is enabled, the handler looks for the Authorization header.
   • It identifies the authentication type (Basic vs. Bearer).
   • For Bearer tokens, it determines if the token is a JWT/SJWT or an SWT.
   • If SJWT (Simple JWT) is enabled, it can differentiate between a full JWT (with scopes) and a Simple JWT (without scopes) and invoke the appropriate verification logic.
   • If multiple methods are enabled (e.g., both JWT and Basic), it attempts to verify based on the provided credentials.
   • If ApiKey is enabled, it checks for the API key in the configured headers/parameters.

Benefits

• Centralized Management: Configure all security policies for the entire gateway or service in one file.
• Reduced Performance Overhead: Minimizes the number of handlers in the middleware chain.
• Flexibility: Supports different security requirements for different API versions or functional areas.
• Dynamic Discovery: Supports multiple JWK or introspection services per path prefix, enabling integration with multiple identity providers.

Sidecar Handler

The sidecar module provides a set of middleware handlers designed to operate in a sidecar or gateway environment. It enables the separation of cross-cutting concerns from business logic, allowing the light-sidecar to act as a micro-gateway for both incoming (ingress) and outgoing (egress) traffic.

Overview

In a sidecar deployment (typically within a Kubernetes Pod), the sidecar module manages traffic between the backend service and the external network. It allows the backend service—regardless of the language or framework it’s built with—to benefit from light-4j features like security, discovery, and observability.

Configuration (sidecar.yml)

The behavior of the sidecar handlers is controlled via sidecar.yml. This configuration is mapped to the SidecarConfig class.

Example sidecar.yml

# Light http sidecar configuration
---
# Indicator used to determine the condition for router traffic.
# Valid values: 'header' (default), 'protocol', or other custom indicators.
# If 'header', it looks for service_id or service_url in the request headers.
# If 'protocol', it treats HTTP traffic as egress and HTTPS as ingress (or vice versa depending on setup).
egressIngressIndicator: ${sidecar.egressIngressIndicator:header}

Parameters

Ingress vs. Egress Logic

The SidecarRouterHandler and related middleware use the egressIngressIndicator to decide how to process a request:

1. Ingress (Incoming): Traffic coming from external clients to the backend API. The sidecar applies security (Token validation), logging, and then proxies the request to the backend API.
2. Egress (Outgoing): Traffic initiated by the backend API to call another service. The sidecar handles service discovery, load balancing, and token injection (via SAML or OAuth2) before forwarding the request to the target service.

Sidecar Middleware Handlers

The sidecar module includes specialized versions of standard middleware that are aware of the traffic direction.

SidecarTokenHandler

Validates tokens for Ingress traffic. If the request is identified as Egress (based on the indicator), it skips validation to allow the request to proceed to the external service.

SidecarSAMLTokenHandler

Similar to the Token Handler, it handles SAML token validation for Ingress traffic and skips for Egress.

SidecarServiceDictHandler

Applies service dictionary mapping for Egress traffic, ensuring the outgoing request is correctly mapped to a target service definition.

SidecarPathPrefixServiceHandler

Identifies the target service ID based on the path prefix for Egress traffic. For Ingress traffic, it populates audit attachments and passes the request to the proxy.

Implementation Details

The sidecar module follows the standard light-4j implementation patterns:

• Singleton + Caching: SidecarConfig uses a cached singleton instance for performance, loaded via SidecarConfig.load().
• Hot Reload: Supports runtime configuration updates via the reload() method, which re-registers the module in the ModuleRegistry.
• Module Registry Integration: Automatically registers itself with /server/info to provide visibility into the active sidecar configuration.

SAML Token Handler

The SAMLTokenHandler is a middleware handler designed to support the SAML 2.0 Bearer Assertion Grant type (RFC 7522). It allows a client to exchange a SAML 2.0 assertion (and optionally a JWT client assertion) for an OAuth 2.0 access token (JWT) from an authorization server.

Introduction

In some enterprise environments, legacy identity providers (IdPs) typically issue SAML assertions. However, modern microservices usually require JWTs for authorization. This handler bridges the gap by intercepting requests containing a SAML assertion, exchanging it for a JWT with the authorization server, and then replacing the SAML assertion with the JWT in the Authorization header for downstream services.

Logic Flow

1. Intercept Request: The handler looks for specific headers in the incoming request:
   • assertion: Contains the Base64-encoded SAML 2.0 assertion.
   • client_assertion (Optional): Contains a JWT for client authentication (if required).
2. Exchange Token: It constructs a simplified token request using these assertions and sends it to the configured OAuth 2.0 provider (via OauthHelper).
3. Process Response:
   • Success: If the IdP returns a valid access token (JWT), the handler:
     • Sets the Authorization header to Bearer <access_token>.
     • Removes the assertion and client_assertion headers to clean up the request.
     • Passes the request to the next handler in the chain.
   • Failure: If the exchange fails, it returns an error response (typically with the status code from the IdP) and terminates the request.

Configuration

This handler shares configuration with the TokenHandler and relies on client.yml for OAuth provider details.

token.yml

The handler’s enabled status is controlled by the token.yml configuration (shared with TokenHandler).

# Token Handler Configuration
enabled: true

client.yml

The details for connecting to the OAuth provider are defined in the client.yml file under the oauth section.

oauth:
  token:
    # URL of the OAuth 2.0 token endpoint
    authorization_code_url: https://localhost:6882/oauth/token
    # Other standard client config...

security.yml

HTTP/2 support for the connection to the OAuth provider can be toggled in security.yml.

# Enable HTTP/2 for OAuth Token Request
oauthHttp2Support: true

Usage

To use this handler, register it in handler.yml and place it before any handlers that require a valid JWT (like JwtVerifyHandler) and usually before the proxy/router handler.

handler.yml

handlers:
  - com.networknt.router.middleware.SAMLTokenHandler@saml
  # ... other handlers

chains:
  default:
    - correlation
    - saml   # Exchanges SAML for JWT here
    - router # Forwards request with JWT

Headers

Token Handler

The TokenHandler is a middleware handler used in the Light-Gateway or http-sidecar. Its primary responsibility is to fetch an OAuth 2.0 access token (Client Credentials Grant) from an authorization server on behalf of the client and inject it into the request header.

Introduction

In a microservices environment, a service often needs to call other services securely. Instead of implementing token retrieval logic in every service (or when using a language without a mature OAuth client), you can offload this responsibility to the gateway or sidecar.

The TokenHandler intercepts outgoing requests, checks if a valid token is cached, and if not, retrieves a new one from the configured OAuth provider derived from client.yml. It then attaches this token to the request so the downstream service can authenticate the call.

Logic Flow

1. Resolve Service ID: The handler first looks for the service_id header. This header is typically populated by upstream handlers like:
   • PathPrefixServiceHandler
   • ServiceDictHandler
   • PathServiceHandler
2. Filter by Path: It checks if the request path matches any of the configured appliedPathPrefixes. If not, it skips execution.
3. Fetch Token:
   • It checks an internal cache for a valid JWT associated with the serviceId.
   • If missing or expired, it initiates a Client Credentials Grant request to the OAuth provider defined in client.yml.
4. Inject Token:
   • Standard: If the Authorization header is empty, it sets Authorization: Bearer <jwt>.
   • On-Behalf-Of: If the Authorization header already contains a token (e.g., the original user’s token), it preserves it and puts the new “scope token” (for service-to-service calls) in the X-Scope-Token header.

Configuration

The handler is configured via token.yml for enabling/filtering and client.yml for OAuth provider details.

token.yml

# Token Handler Configuration
enabled: true

# List of path prefixes where this handler should be active.
# This allows you to selectively apply token injection only for specific routes.
appliedPathPrefixes:
  - /v1/pets
  - /v2/orders

client.yml

The OAuth 2.0 provider configuration is managed by the client module.

oauth:
  # Enable multiple auth servers if you need different credentials for different services
  multipleAuthServers: true
  token:
    # Default Client Credentials Config
    client_credentials:
      clientId: default_client
      clientSecret: default_secret
      uri: /oauth2/token
    # Service specific overrides (mapped by serviceId)
    serviceIdAuthServers:
      petstore-service:
        clientId: petstore_client
        clientSecret: petstore_secret
        uri: /oauth2/token

Usage

To use this handler, register it in handler.yml. It must be placed after any handler that resolves the serviceId (like PathPrefixServiceHandler) and before the RouterHandler.

handler.yml

handlers:
  - com.networknt.router.middleware.TokenHandler@token
  # ... other handlers

chains:
  default:
    - correlation
    - prefix   # Resolves service_id from path
    - token    # Uses service_id to get JWT
    - router   # Forwards request to downstream

Error Handling

If the service_id cannot be resolved (because the upstream handler is missing or failed), the TokenHandler will return an error ERR10074 indicating a dependency error.

SSE Handler

The SSE (Server-Sent Events) Handler in light-4j allows the server to push real-time updates to the client over a standard HTTP connection. This handler manages the connection lifecycle and keep-alive messages.

Configuration

The configuration for the SSE Handler is defined in sse.yml (or sse.json/yaml). It corresponds to the SseConfig.java class.

Configuration Properties

Configuration Example (sse.yml)

# Enable SSE Handler
enabled: true

# Default SSE Endpoint Path
path: /sse

# Default keep-alive interval in milliseconds
keepAliveInterval: 10000

# Define path prefix related configuration properties as a list of key/value pairs.
# If request path cannot match to one of the pathPrefixes or the default path, the request will be skipped.
pathPrefixes:
  - pathPrefix: /sse/abc
    keepAliveInterval: 20000
  - pathPrefix: /sse/def
    keepAliveInterval: 40000

# Metrics injection (optional, for gateway/sidecar usage)
metricsInjection: false
metricsName: router-response

Usage

Register the SseHandler in your handler.yml chain. When a client connects to the configured path (e.g., /sse), the handler will establish a persistent connection and manage keep-alive signals based on the keepAliveInterval.

Path Matching

1. Prefix Matching: If pathPrefixes are configured, the handler checks if the request path starts with any of the defined prefixes. If a match is found, the specific keepAliveInterval for that prefix is used.
2. Exact Matching: If no prefix matches, the handler checks if the request path exactly matches the default path (/sse).
3. Passthrough: If neither matches, the request is passed to the next handler in the chain.

Traceability

The handler supports traceability via the X-Traceability-Id header or an id query parameter to track connections in the SseConnectionRegistry.

Service Dict Handler

The ServiceDictHandler is a middleware handler that provides a flexible way to map request paths and methods to serviceIds. It bridges the gap between PathPrefixServiceHandler (simple prefix mapping) and PathServiceHandler (exact Swagger endpoint mapping).

Introduction

In a microservices gateway, routing requests purely based on path prefixes might be insufficient, and requiring a full OpenAPI specification for PathServiceHandler might be overkill or not feasible for some legacy services.

ServiceDictHandler offers a middle ground. It allows you to define mappings using a combination of Path Prefix and HTTP Method. This enables you to route requests to different services based on the operation (e.g., READs go to one service, WRITEs to another) without needing a full API definition.

Logic Flow

1. Intercept Request: The handler captures the incoming request’s URI and HTTP Method.
2. Lookup Mapping: It normalizes the request to key format pathPrefix@method and searches for a match in the configuration.
3. Inject Header:
   • If a match is found, it injects the corresponding serviceId into the service_id header.
   • It also populates the audit attachment with the matched endpoint Pattern.
4. No Match: If no mapping is found, it marks the endpoint as Unknown in the audit logs.

Configuration

The handler is configured via the serviceDict.yml file.

Configuration Format

The key format in the mapping is [PathPrefix]@[Method]. The method should be lowercase.

Example serviceDict.yml

# Service Dictionary Configuration
enabled: true

# Mapping: PathPrefix@Method -> Service ID
mapping:
  # Route all GET requests under /v1/address to address-read service
  /v1/address@get: party.address-read-1.0.0
  
  # Route all POST requests under /v1/address to address-write service
  /v1/address@post: party.address-write-1.0.0
  
  # Route all requests (any method) under /v2/address to address-v2 service
  # Note: logic depends on specific implementation match order if keys overlap
  /v2/address@get: party.address-v2-1.0.0

Supported Formats

Like other handlers, the mapping can be provided as:

1. YAML Map: Standard key: value structure.
2. JSON String: {"/v1/data@get": "service-1"}.
3. Java Map String: /v1/data@get=service-1&/v1/data@post=service-2.

Usage

To use this handler, register it in handler.yml and place it before the RouterHandler.

handler.yml

handlers:
  - com.networknt.router.middleware.ServiceDictHandler@dict
  # ... other handlers

chains:
  default:
    - correlation
    - dict   # Perform lookup here
    - router

Comparison

MCP Router

The MCP (Model Context Protocol) Router in light-4j allows binding LLM tools to HTTP endpoints, enabling an AI model to interact with your services via the MCP standard. It supports both Server-Sent Events (SSE) for connection establishment and HTTP POST for JSON-RPC messages.

Configuration

The configuration for the MCP Router is defined in mcp-router.yml (or mcp-router.json/yaml). It corresponds to the McpConfig.java class.

Configuration Properties

Configuration Example (mcp-router.yml)

# Enable MCP Router Handler
enabled: true

# Path for MCP endpoint (Streamable HTTP)
path: /mcp

# Define tools exposed by this router
tools:
  - name: getWeather
    description: Get the weather for a location
    host: https://api.weather.com
    path: /v1/current
    method: GET
    # inputSchema is optional; default is type: object

Architecture

The MCP Router implements the MCP HTTP transport specification using a Single Path architecture:

1. Connection: Clients connect to the configured path (default /mcp) via GET to establish an SSE session. The server generates a unique session ID and responds with an endpoint event containing the URI for message submission (e.g., /mcp?sessionId=uuid).
2. Messaging: Clients send JSON-RPC 2.0 messages to the same path (default /mcp) via POST. The session context is maintained via the sessionId query parameter or header.
3. Tool Execution: The router translates tools/call requests into HTTP requests to the configured downstream services (host + path).

Supported Methods

The router supports the following MCP JSON-RPC methods:

• initialize: Returns server capabilities (tools list changed notification) and server info.
• notifications/initialized: Acknowledgment from the client.
• tools/list: Lists all configured tools with their names, descriptions, and input schemas.
• tools/call: Executes a specific tool by name. The arguments in the request are passed to the downstream service.

Usage

Register the McpHandler in your handler.yml chain.

Example:

paths:
  - path: '/mcp'
    method: 'GET'
    exec:
      - mcp-router
  - path: '/mcp'
    method: 'POST'
    exec:
      - mcp-router

Verification with verify_mcp.py

To assist with testing and verification, a Python script verify_mcp.py is available in the light-gateway root directory (and potentially other project roots). This script simulates an MCP client to ensure the router is correctly configured and routing requests.

Prerequisite

Install the required Python packages:

pip install requests sseclient-py

Running the Script

Run the script against your running gateway instance (default https://localhost:8443):

python3 verify_mcp.py

What it does

1. Connects to SSE: Initiates a GET request to /mcp and listens for server sent events. It confirms receipt of the endpoint event.
2. Initializes: Sends a POST request with the initialize JSON-RPC method to verify protocol negotiation.
3. Lists Tools: Sends a tools/list request to verify that configured tools are being correctly exposed by the router.

If successful, you will see output confirming the SSE connection, the endpoint event, and the JSON responses for initialization and tool listing.

Token Exchange Handler

The Token Exchange Handler implements RFC 8693 OAuth 2.0 Token Exchange. It allows a gateway or sidecar to exchange an incoming token (e.g., an external Client Credentials token) for an internal token (e.g., an Authorization Code token with a user identity) by calling an OAuth 2.0 provider.

Introduction

In a microservices architecture, especially when integrating with external partners, the incoming request often carries a token issued by an external Identity Provider (IdP). This token might lack the necessary context (like internal user ID, roles, or custom claims) required by internal services.

The Token Exchange Handler intercepts the request, validates the incoming token (optional), exchanges it for an internal token via the urn:ietf:params:oauth:grant-type:token-exchange grant type, and replaces the Authorization header with the new token.

Configuration

The configuration is managed via token-exchange.yml.

Example token-exchange.yml

# Enable or disable the handler
enabled: true

# The path to the token exchange endpoint on OAuth 2.0 provider
tokenExUri: https://localhost:6882/oauth2/token

# The client ID for the token exchange authentication
tokenExClientId: portal-client

# The client secret for the token exchange authentication
tokenExClientSecret: portal-secret

# The scope of the returned token
tokenExScope:
  - portal.w
  - portal.r

# The subject token type. Default is urn:ietf:params:oauth:token-type:jwt
subjectTokenType: urn:ietf:params:oauth:token-type:jwt

# The requested token type. Default is urn:ietf:params:oauth:token-type:jwt
requestedTokenType: urn:ietf:params:oauth:token-type:jwt

Usage

1. Add Dependency

Add the token-exchange module to your pom.xml.

<dependency>
    <groupId>com.networknt</groupId>
    <artifactId>token-exchange</artifactId>
    <version>${version.light-4j}</version>
</dependency>

2. Register Handler

In your handler.yml, add com.networknt.token.exchange.TokenExchangeHandler to the desired chain. It should typically be placed after the TraceabilityHandler or CorrelationHandler and before the security handler if you want to validate the new token, or before the security handler if you want to validate the original token.

Note: If you place it before the AuditHandler, the audit log will reflect the exchanged token.

handlers:
  - com.networknt.token.exchange.TokenExchangeHandler@token
  # ... other handlers ...

chains:
  default:
    - exception
    - metrics
    - traceability
    - correlation
    - token 
    - specification
    - security
    - body
    - validator
    # ...

Mapping Strategies

To map an external Client ID (from the subject_token) to an internal User ID:

1. Database (Recommended): Use a “Shadow Client” in light-oauth2. Create a client with the same ID as the external client and populate custom_claims with the internal userId or functionId. light-oauth2 will automatically inject these claims into the minted token.
2. Config: (Future support) Map IDs directly in token-exchange.yml.

Interceptor

Request Transformer Interceptor

The request-transformer is a powerful middleware interceptor that allows for dynamic modification of incoming requests using the light-4j rule engine. It provides a highly flexible way to manipulate request metadata (headers, query parameters, path) and the request body based on custom business logic defined in rules.

Features

• Dynamic Transformation: Modify request elements at runtime without changing application code.
• Rule-Based: Leverage the light-4j rule engine to define complex transformation logic.
• Path-Based Activation: Target specific request paths using the appliedPathPrefixes configuration.
• Metadata Manipulation: Add, update, or remove request headers, query parameters, path, and URI.
• Body Transformation: Overwrite the request body (supports JSON, XML, and other text-based formats).
• Short-Circuiting: Generate an immediate response body or validation error to return to the caller, stopping the handler chain.
• Encoding Support: Configurable body encoding per path prefix for legacy API compatibility.
• Auto-Registration: Automatically registers with the ModuleRegistry for administrative visibility.

Configuration (request-transformer.yml)

The interceptor is configured via request-transformer.yml.

# Request Transformer Configuration
---
# Indicate if this interceptor is enabled or not. Default is true.
enabled: ${request-transformer.enabled:true}

# Indicate if the transform interceptor needs to change the request body. Default is true.
requiredContent: ${request-transformer.requiredContent:true}

# Default body encoding for the request body. Default is UTF-8.
defaultBodyEncoding: ${request-transformer.defaultBodyEncoding:UTF-8}

# A list of applied request path prefixes. Only requests matching these prefixes will be processed.
# This can be a single string or a list of strings.
appliedPathPrefixes: ${request-transformer.appliedPathPrefixes:}

# Customized encoding for specific path prefixes.
# This is useful for legacy APIs that require non-UTF-8 encoding (e.g., ISO-8859-1).
# pathPrefixEncoding:
#   /v1/pets: ISO-8859-1
#   /v1/party/info: ISO-8859-1

Setup

1. Add Dependency

Add the following dependency to your pom.xml:

<dependency>
    <groupId>com.networknt</groupId>
    <artifactId>request-transformer</artifactId>
    <version>${version.light-4j}</version>
</dependency>

2. Register Interceptor

In your handler.yml, add the RequestTransformerInterceptor to the interceptors list and include it in the appropriate handler chains.

handlers:
  - com.networknt.reqtrans.RequestTransformerInterceptor@requestTransformer

chains:
  default:
    - ...
    - requestTransformer
    - ...

Transformation Rules

The interceptor passes a context map (objMap) to the rule engine. Your rules can inspect these values and return a result map to trigger specific modifications.

Rule Input Map (objMap)

• auditInfo: Map of audit information (e.g., clientId, endpoint).
• requestHeaders: Map of current request headers.
• queryParameters: Map of current query parameters.
• pathParameters: Map of current path parameters.
• method: HTTP method (e.g., “POST”, “GET”).
• requestURL: The full request URL.
• requestURI: The request URI.
• requestPath: The request path.
• requestBody: The request body string (only if requiredContent is true).

Rule Output Result Map

To perform a transformation, your rule must return a map containing one or more of the following keys:

• requestPath: String - Overwrites the exchange request path.
• requestURI: String - Overwrites the exchange request URI.
• queryString: String - Overwrites the exchange query string.
• requestHeaders: Map - Contains two sub-keys:
  • remove: List<String> - List of header names to remove.
  • update: Map<String, String> - Map of header names and values to add or update.
• requestBody: String - Overwrites the request body buffer with the provided string.
• responseBody: String - Immediately returns this string as the response body, bypassing the rest of the chain.
  • Also supports statusCode (Integer) and contentType (String) in the result map.
• validationError: Map - Short-circuits the request with a validation error.
  • Requires errorMessage (String), contentType (String), and statusCode (Integer).

Example Transformation Logic

If a rule decides to update a header and the request path, the result map returned from the rule engine might look like this:

{
  "result": true,
  "requestPath": "/v2/new-endpoint",
  "requestHeaders": {
    "update": {
      "X-Transformation-Status": "transformed"
    },
    "remove": ["X-Old-Header"]
  }
}

Operational Visibility

The request-transformer module automatically registers itself with the ModuleRegistry during configuration loading. You can inspect its current status and configuration parameters at runtime via the Server Info endpoint.

Response Transformer Interceptor

The response-transformer is a powerful middleware interceptor that allows for dynamic modification of outgoing responses using the light-4j rule engine. It provides a highly flexible way to manipulate response headers and the response body based on custom business logic defined in rules.

Features

• Dynamic Transformation: Modify response elements at runtime without changing application code.
• Rule-Based: Leverage the light-4j rule engine to define complex transformation logic.
• Path-Based Activation: Target specific request paths using the appliedPathPrefixes configuration.
• Header Manipulation: Add, update, or remove response headers.
• Body Transformation: Overwrite the response body (supports JSON, XML, and other text-based formats).
• Encoding Support: Configurable body encoding per path prefix for legacy API compatibility.
• Auto-Registration: Automatically registers with the ModuleRegistry during configuration loading for administrative visibility.

Configuration (response-transformer.yml)

The interceptor is configured via response-transformer.yml.

# Response Transformer Configuration
---
# Indicate if this interceptor is enabled or not. Default is true.
enabled: ${response-transformer.enabled:true}

# Indicate if the transform interceptor needs to change the response body. Default is true.
requiredContent: ${response-transformer.requiredContent:true}

# Default body encoding for the response body. Default is UTF-8.
defaultBodyEncoding: ${response-transformer.defaultBodyEncoding:UTF-8}

# A list of applied request path prefixes. Only requests matching these prefixes will be processed.
appliedPathPrefixes: ${response-transformer.appliedPathPrefixes:}

# For certain path prefixes that are not using the defaultBodyEncoding UTF-8, you can define the customized
# encoding like ISO-8859-1 for the path prefixes here.
# pathPrefixEncoding:
#   /v1/pets: ISO-8859-1
#   /v1/party/info: ISO-8859-1

Setup

1. Add Dependency

Include the following dependency in your pom.xml:

<dependency>
    <groupId>com.networknt</groupId>
    <artifactId>response-transformer</artifactId>
    <version>${version.light-4j}</version>
</dependency>

2. Register Interceptor

In your handler.yml, add the ResponseTransformerInterceptor to the interceptors list and include it in the appropriate handler chains.

handlers:
  - com.networknt.restrans.ResponseTransformerInterceptor@responseTransformer

chains:
  default:
    - ...
    - responseTransformer
    - ...

Transformation Rules

The interceptor passes a context map (objMap) to the rule engine. Your rules can inspect these values and return a result map to trigger specific modifications.

Rule Input Map (objMap)

• auditInfo: Map of audit information (e.g., clientId, endpoint).
• requestHeaders: Map of current request headers.
• responseHeaders: Map of current response headers.
• queryParameters: Map of current query parameters.
• pathParameters: Map of current path parameters.
• method: HTTP method (e.g., “POST”, “GET”).
• requestURL: The full request URL.
• requestURI: The request URI.
• requestPath: The request path.
• requestBody: The request body (if available in the exchange attachment).
• responseBody: The original response body string (only if requiredContent is true).
• statusCode: The HTTP status code of the response.

Rule Output Result Map

To perform a transformation, your rule must return a map containing one or more of the following keys:

• responseHeaders: Map - Contains two sub-keys:
  • remove: List<String> - List of header names to remove.
  • update: Map<String, String> - Map of header names and values to add or update.
• responseBody: String - Overwrites the response body buffer with the provided string.

Example Transformation Logic

If a rule decides to update a header and the response body, the result map returned from the rule engine might look like this:

{
  "result": true,
  "responseHeaders": {
    "update": {
      "X-Transformation-Version": "2.0"
    },
    "remove": ["Server"]
  },
  "responseBody": "{\"status\":\"success\", \"data\": \"transformed content\"}"
}

Operational Visibility

The response-transformer module automatically registers itself with the ModuleRegistry. This allows administrators to inspect the active configuration and status of the interceptor at runtime via the Server Info endpoint.

Admin Endpoint

Cache Explorer

The CacheExplorerHandler is an admin endpoint that allows developers and operators to inspect the contents of the application’s internal caches. It is particularly useful for debugging and verifying that data (such as JWKs, ensuring config reloading, etc.) is correctly cached.

Overview

• Type: Admin Handler
• Class: com.networknt.cache.CacheExplorerHandler
• Source: light-4j/cache-explorer

Usage

The handler operates by retrieving a specific cache by name via the CacheManager singleton.

Endpoint

The endpoint is typically accessible via the admin port (default 8473) if configured using the admin-endpoint module.

Method: GET Path: /adm/cache (or as configured in handler.yml)

Parameters

Examples

Inspect JWK Cache

The jwk cache has special handling to format the output as a simple JSON map of strings.

Request:

GET /adm/cache?name=jwk

Response:

{
  "key1": "value1",
  "key2": "value2"
}

Inspect Generic Cache

For other caches, it returns the JSON representation of the cache’s key-value pairs.

Request:

GET /adm/cache?name=myCache

Response:

{
  "user123": {
    "name": "John",
    "role": "admin"
  }
}

Configuration

This module does not have a specific configuration file (e.g., cache-explorer.yml). It relies on the CacheManager being available and populated.

To use it, you must register the handler in handler.yml and add it to the admin chain (or another chain).

handler.yml Registration

handler.handlers:
  # ... other handlers ...
  - com.networknt.cache.CacheExplorerHandler@cache_explorer

handler.chains.default:
  # ...
  - cache_explorer

Note: In most standard light-4j templates, if you are using the unified admin-endpoint, this handler might not be wired by default and needs explicit registration if you want it exposed on the main port, or it might be auto-wired in the admin chain if the dependency is present (depending on the chassis version).

Config Reload

The config-reload module provides admin endpoints to reload configuration files for specific modules or plugins at runtime without restarting the server. This is particularly useful in shared environments like a gateway where restarting would impact multiple services, or for dynamic updates to policies and rules.

Endpoints

The module typically exposes two operations on the same path (e.g., /adm/modules).

Get Registered Modules

• Path: /adm/modules
• Method: GET
• Description: Returns a list of all registered modules and plugins capable of being reloaded.
• Response: A JSON array of class names (Strings).

Trigger Config Reload

• Path: /adm/modules
• Method: POST
• Description: Triggers a config reload for the specified list of modules or plugins.
• Request Body: A JSON array of class names (Strings) to reload. If the list is empty or contains “ALL”, it attempts to reload all registered modules.
• Response: A JSON array of the class names that were successfully reloaded.

Configuration

Module Configuration (configreload.yml)

The module itself is configured via configReload.yml (or configReload.yaml/json).

Handler Configuration (handler.yml)

To expose the endpoints, you need to configure them in your handler.yml.

paths:
  - path: '/adm/modules'
    method: 'get'
    exec:
      - admin
      - modules
  - path: '/adm/modules'
    method: 'post'
    exec:
      - admin
      - configReload

And in values.yml (if using aliases):

handler.handlers:
  - com.networknt.config.reload.handler.ModuleRegistryGetHandler@modules
  - com.networknt.config.reload.handler.ConfigReloadHandler@configReload

Security

These are sensitive admin endpoints. They should be protected by OAuth 2.0 (requiring a specific scope like admin or ${serviceId}/admin) or restricted by IP whitelist in a sidecar/gateway deployment.

Dependency

Add the following dependency to your pom.xml:

<dependency>
  <groupId>com.networknt</groupId>
  <artifactId>config-reload</artifactId>
  <version>${version.light-4j}</version>
</dependency>

Usage Examples

List Modules

curl -k https://localhost:8443/adm/modules

Response:

[
    "com.networknt.limit.LimitHandler",
    "com.networknt.audit.AuditHandler",
    "com.networknt.router.middleware.PathPrefixServiceHandler",
    ...
]

Reload Config

Reload configuration for the Limit Handler and Path Prefix Handler.

curl -k -X POST https://localhost:8443/adm/modules \
  -H 'Content-Type: application/json' \
  -d '[
    "com.networknt.limit.LimitHandler",
    "com.networknt.router.middleware.PathPrefixServiceHandler"
  ]'

Response:

[
    "com.networknt.limit.LimitHandler",
    "com.networknt.router.middleware.PathPrefixServiceHandler"
]

Health Check Handler

The Health Check Handler (HealthGetHandler) manages the server’s health endpoint. It is critical for load balancers (like F5, AWS ALB), container orchestrators (Kubernetes Liveness/Readiness probes), and the Light Control Plane to verify that the service is running and capable of handling requests.

Introduction

A running process does not always mean a functioning service. The HealthGetHandler provides a lightweight endpoint to confirm application status. It can return a simple “OK” string or a JSON object, and optionally verify downstream dependencies before reporting healthy.

Configuration

The handler is configured via health.yml.

health.yml Example

enabled: true
useJson: true
timeout: 500
# For services needing to verify a sidecar (e.g. light-proxy, kafka-sidecar)
downstreamEnabled: false
downstreamHost: http://localhost:8081
downstreamPath: /health

Usage

To use the health handler, register it in your handler.yml and map it to a path (typically /health).

handler.yml Configuration

1. Register the Handler:

   handlers:
     - com.networknt.health.HealthGetHandler@health
   

2. Define the Path:

   paths:
     - path: '/health'
       method: 'get'
       exec:
         - health
     # Secure endpoint for Control Plane (optional)
     - path: '/adm/health/${server.serviceId}'
       method: 'get'
       exec:
         - security
         - health
   

Scenarios

1. Kubernetes Probes: Configure livenessProbe and readinessProbe in your deployment.yaml to hit /health.
2. Load Balancers: Configure the health monitor to check /health and expect “OK” (or JSON if configured).
3. Control Plane: The Control Plane uses a secure path (e.g., /adm/health/...) with service discovery to monitor specific instances.

Downstream Check

If your service relies heavily on a sidecar (like http-sidecar or kafka-sidecar) or a specific downstream API, you can enable downstreamEnabled.

• The handler will make a request to downstreamHost + downstreamPath.
• If the downstream call fails or times out, the health check returns an error, preventing traffic from being routed to this broken instance.

Note: For complex dependency checking (Database, Redis, etc.), it is recommended to extend this handler or implement a custom HealthCheck interface if using a specific framework, though HealthGetHandler is the standard entry point for Light-4j.

Server Info

Introduction

The ServerInfoGetHandler is a middleware component in the light-4j framework that provides a comprehensive overview of the server’s runtime state. This includes:

• Environment Info: Host IP, hostname, DNS, and runtime metrics (processors, memory).
• System Properties: Java version, OS details, timezone.
• Component Configuration: Configuration details for all registered modules.
• Specification: The API specification (OpenAPI, etc.) if applicable.

This handler is crucial for monitoring, debugging, and integration with the light-controller and light-portal for runtime dashboards.

Configuration

The handler is configured via info.yml.

# Server info endpoint that can output environment and component along with configuration

# Indicate if the server info is enabled or not.
enableServerInfo: ${info.enableServerInfo:true}

# String list keys that should not be sorted in the normalized info output.
keysToNotSort: ${info.keysToNotSort:["admin", "default", "defaultHandlers", "request", "response"]}

# Downstream configuration (for gateways/sidecars)
# Indicate if the server info needs to invoke downstream APIs.
downstreamEnabled: ${info.downstreamEnabled:false}
# Downstream API host.
downstreamHost: ${info.downstreamHost:http://localhost:8081}
# Downstream API server info path.
downstreamPath: ${info.downstreamPath:/adm/server/info}

If enableServerInfo is false, the endpoint will return an error ERR10013 - SERVER_INFO_DISABLED.

Handler Registration

Unlike most middleware handlers, ServerInfoGetHandler is typically configured as a normal handler in handler.yml but mapped to a specific path like /server/info.

handlers:
  - com.networknt.info.ServerInfoGetHandler@info

paths:
  - path: '/server/info'
    method: 'get'
    exec:
      - security
      - info

Security Note: The /server/info endpoint exposes sensitive configuration and system details. It must be protected by security handlers. In a typical light-4j deployment, this endpoint is secured requiring a special bootstrap token (e.g., from light-controller).

Module Registration

Modules in light-4j can register themselves to be included in the server info output. This allows the info endpoint to display configuration for custom or contributed modules.

To register a module:

import com.networknt.utility.ModuleRegistry;
import com.networknt.config.Config;

// Registering a module
ModuleRegistry.registerModule(
    MyHandler.class.getName(), 
    Config.getInstance().getJsonMapConfigNoCache(MyHandler.CONFIG_NAME), 
    null
);

Masking Sensitive Data

Configuration files often contain sensitive data (passwords, secrets). When registering a module, you can provide a list of keys to mask in the output.

List<String> masks = new ArrayList<>();
masks.add("trustPass");
masks.add("keyPass");
masks.add("clientSecret");

ModuleRegistry.registerModule(
    Client.class.getName(), 
    Config.getInstance().getJsonMapConfigNoCache(clientConfigName), 
    masks
);

Downstream Info Aggregation

For components like light-gateway, http-sidecar, or light-proxy, the server info endpoint can be configured to aggregate information from a downstream service.

• downstreamEnabled: When set to true, the handler will attempt to fetch server info from the configured downstream service.
• downstreamHost: The base URL of the downstream service.
• downstreamPath: The path to the server info endpoint on the downstream service.

If the downstream service does not implement the info endpoint, the handler usually fails gracefully or returns its own info depending on the implementation specifics (e.g., in light-proxy scenarios).

Sample Output

The output is a JSON object structured as follows:

{
  "environment": {
    "host": {
      "ip": "10.0.0.5",
      "hostname": "service-pod-1",
      "dns": "localhost"
    },
    "runtime": {
      "availableProcessors": 4,
      "freeMemory": 12345678,
      "totalMemory": 23456789,
      "maxMemory": 1234567890
    },
    "system": {
      "javaVendor": "Oracle Corporation",
      "javaVersion": "17.0.1",
      "osName": "Linux",
      "osVersion": "5.4.0",
      "userTimezone": "UTC"
    }
  },
  "specification": { ... }, 
  "component": {
      "com.networknt.info.ServerInfoConfig": {
          "enableServerInfo": true,
          "downstreamEnabled": false,
          ...
      },
      ... other modules ...
  }
}

Logger Handler

The logger-handler module provides a suite of administrative endpoints to manage logger levels and retrieve log contents at runtime. This allows developers and operators to troubleshoot issues in a running system by adjusting verbosity or inspecting logs without requiring a restart.

Note: These administrative features are powerful. In production environments, they must be protected by security handlers (e.g., OAuth 2.0 JWT verification with specific scopes) to prevent unauthorized access. It is highly recommended to use the Light Controller or Light Portal for centralized management.

Features

• Runtime Level Adjustment: Change logging levels for existing loggers or create new ones for specific packages/classes.
• Log Content Inspection: Retrieve log entries directly from service instances for UI display or analysis.
• Pass-Through Support: In gateway or sidecar deployments, these handlers can pass requests through to backend services (even if they use different frameworks like Spring Boot).
• Auto-Registration: The module automatically registers its configuration with the ModuleRegistry for visibility.

Configuration (logging.yml)

The behavior of the logging handlers is controlled via logging.yml.

# Logging endpoint configuration
---
# Indicate if the logging info is enabled or not.
enabled: ${logging.enabled:true}

# Default time period backward in milliseconds for log content retrieval.
# Default is 10 minutes (600,000 ms).
logStart: ${logging.logStart:600000}

# Downstream configuration (useful in sidecar/gateway scenarios)
downstreamEnabled: ${logging.downstreamEnabled:false}

# Downstream API host (e.g., the backend service IP)
downstreamHost: ${logging.downstreamHost:http://localhost:8081}

# Framework of the downstream service (Light4j, SpringBoot, etc.)
downstreamFramework: ${logging.downstreamFramework:Light4j}

Set Up

To enable the logging endpoints, add the following to your handler.yml:

handlers:
  - com.networknt.logging.handler.LoggerGetHandler@getLogger
  - com.networknt.logging.handler.LoggerPostHandler@postLogger
  - com.networknt.logging.handler.LoggerGetNameHandler@getLoggerName
  - com.networknt.logging.handler.LoggerGetLogContentsHandler@getLogContents

paths:
  - path: '/adm/logger'
    method: 'GET'
    exec:
      - admin  # Security handler
      - getLogger
  - path: '/adm/logger'
    method: 'POST'
    exec:
      - admin
      - postLogger
  - path: '/adm/logger/{loggerName}'
    method: 'GET'
    exec:
      - getLoggerName
  - path: '/adm/logger/content'
    method: 'GET'
    exec:
      - admin
      - getLogContents

Usage

1. Get Logger Levels

Retrieves all configured loggers and their current levels.

Endpoint: GET /adm/logger

curl -k https://localhost:8443/adm/logger

Example Response:

[
  {"name": "ROOT", "level": "ERROR"},
  {"name": "com.networknt", "level": "TRACE"}
]

2. Update Logger Levels

Changes the logging level for specific loggers.

Endpoint: POST /adm/logger

curl -k -H "Content-Type: application/json" -X POST \
  -d '[{"name": "com.networknt.handler", "level": "DEBUG"}]' \
  https://localhost:8443/adm/logger

3. Get Log Contents

Retrieves actual log messages from the application’s file appenders.

Endpoint: GET /adm/logger/content Query Parameters:

• loggerName: Filter by logger name.
• loggerLevel: Filter by minimal level (e.g., TRACE).
• startTime / endTime: Epoch milliseconds.
• limit / offset: Pagination controls (default 100/0).

curl -k 'https://localhost:8443/adm/logger/content?loggerLevel=DEBUG&limit=10'

Pass-Through (Sidecars/Gateways)

When using http-sidecar or light-gateway, you can target the backend service’s loggers by adding the X-Adm-PassThrough: true header to your request.

• If the backend is Light4j, the request is passed as-is.
• If the backend is SpringBoot, the sidecar transforms the request to target Spring Boot Actuator endpoints (e.g., /actuator/loggers).

Example for Spring Boot Backend:

logging.downstreamEnabled: true
logging.downstreamHost: http://localhost:8080
logging.downstreamFramework: SpringBoot

curl -H "X-Adm-PassThrough: true" https://localhost:9445/adm/logger

Dependency

Add the following to your pom.xml:

<dependency>
    <groupId>com.networknt</groupId>
    <artifactId>logger-handler</artifactId>
    <version>${version.light-4j}</version>
</dependency>

Utility

LDAP Utility

The ldap-util is a utility module in the light-4j framework that provides a helper class LdapUtil to interact with LDAP servers for authentication and authorization. It simplifies common tasks such as binding (verifying credentials) and searching for user attributes (like memberOf for group-based authorization).

Features

• Authentication: Authenticates users against an LDAP server using simple binding.
• Authorization: Retrieves user attributes (specifically memberOf) from the LDAP server to support group-based access control.
• Secure Connection: Supports both standard LDAP and LDAPS (LDAP over SSL) using a custom socket factory.
• Configuration Driven: Behavior is controlled via externalized configuration (ldap.yml).

Configuration

The ldap-util module uses a configuration file named ldap.yml.

Configuration Options

The following properties can be defined in ldap.yml:

• uri: The LDAP server URI (e.g., ldap://localhost:389 or ldaps://localhost:636).
• domain: The LDAP domain name.
• principal: The user principal (DN or username) used for binding to perform searches.
• credential: The password for the binding principal.
• searchFilter: The search filter used to find user entries (e.g., (&(objectClass=user)(sAMAccountName={0}))). The {0} placeholder is replaced by the username during runtime.
• searchBase: The base DN (Distinguished Name) where searches will start.

Example ldap.yml

# LDAP server connection and search settings.
---
# The LDAP server uri.
uri: ${ldap.uri:ldap://localhost:389}
# The LDAP domain name.
domain: ${ldap.domain:}
# The user principal for binding (authentication).
principal: ${ldap.principal:cn=admin,dc=example,dc=org}
# The user credential (password) for binding.
credential: ${ldap.credential:admin}
# The search filter (e.g., (&(objectClass=user)(sAMAccountName={0}))).
searchFilter: ${ldap.searchFilter:(&(objectClass=person)(uid={0}))}
# The search base DN (Distinguished Name).
searchBase: ${ldap.searchBase:dc=example,dc=org}

Usage

The primary interface is the com.networknt.ldap.LdapUtil class.

Authentication

To verify a user’s password:

boolean isAuthenticated = LdapUtil.authenticate("username", "password");

This method first searches for the user’s DN using the configured principal, and then attempts to bind with the found DN and the provided password.

Authorization

To retrieve a user’s groups (memberOf attribute):

Set<String> groups = LdapUtil.authorize("username");

This method uses the configured principal to search for the user and extracts all values from the memberOf attribute.

Integration

The ldap-util is used by other modules in the light platform that require LDAP integration, such as the ldap-security handler for protecting endpoints with LDAP credentials.

Whenever LdapConfig.load() is called, the module automatically registers itself with the ModuleRegistry so that its configuration can be viewed via the Server Info endpoint.

Module

Load Balance

The light-4j platform encourages client-side discovery to avoid proxies in front of multiple service instances. This architecture reduces network hops and latency. The client-side load balancer is responsible for selecting a single available service instance from a list of discovered URLs for a downstream request.

LoadBalance Interface

All load balancing strategies enforce the LoadBalance interface.

public interface LoadBalance {
    /**
     * Select one url from a list of url with requestKey as optional.
     *
     * @param urls List of available URLs
     * @param serviceId Service ID
     * @param tag Service tag (optional)
     * @param requestKey Optional key for hashing (e.g., userId, clientId)
     * @return Selected URL
     */
    URL select(List<URL> urls, String serviceId, String tag, String requestKey);

    default int getPositive(int originValue){
        return 0x7fffffff & originValue;
    }
}

Strategies

Round Robin

The RoundRobinLoadBalance strategy picks a URL from the list sequentially. It distributes the load equally across all available instances.

• Assumption: All service instances have similar resource configurations and capabilities.
• Mechanism: Uses an internal AtomicInteger index per service to cycle through the list.
• Parameters: The requestKey is ignored.

Local First

The LocalFirstLoadBalance strategy prioritizes service instances running on the same host (same IP address) to minimize network latency.

• Mechanism:
  1. It identifies the local IP address.
  2. It filters the list of available URLs for those matching the local IP.
  3. If local instances are found, it performs Round Robin among them.
  4. If no local instances are found, it falls back to Round Robin across all available URLs.
• Use Case: Ideal for deployments where multiple services run on the same physical or virtual machine (e.g., standalone Java processes, light-hybrid-4j services).

Consistent Hash

The ConsistentHashLoadBalance strategy ensures that requests with the same requestKey (e.g., client_id, user_id) are routed to the same service instance. This is useful for caching or stateful services (Data Sharding / Z-Axis Scaling).

• Mechanism: Uses the hash code of the requestKey to select a specific instance.
• Current Status: Basic implementation using modulo hashing. A more advanced consistent hashing ring implementation is planned.

Configuration

The load balancer implementation is typically configured as a singleton in service.yml. Only one implementation should be active per client instance during runtime.

Example: Round Robin Configuration

In service.yml:

singletons:
  - com.networknt.balance.LoadBalance:
    - com.networknt.balance.RoundRobinLoadBalance

Example: Local First Configuration

In service.yml:

singletons:
  - com.networknt.balance.LoadBalance:
    - com.networknt.balance.LocalFirstLoadBalance

Usage

The load balancer is used internally by Cluster implementations or directly by clients (like light-consumer-4j) after service discovery returns a list of healthy nodes.

// Example usage pattern
List<URL> urls = registry.discover(serviceId, tag);
URL selectedUrl = loadBalance.select(urls, serviceId, tag, requestKey);

Cluster

The Cluster module acts as the orchestrator for client-side service discovery. It integrates the Service Registry (to find service instances) and Load Balance (to select the best instance) modules, providing a simple interface for clients to resolve a service ID to a concrete URL for invocation.

It maintains a local cache of discovered services to minimize calls to the underlying registry (Consul, ZooKeeper, or Direct) and handles notifications when service instances change.

Cluster Interface

The module’s core is the Cluster interface, which provides methods to resolve service addresses.

public interface Cluster {
    // Resolve a serviceId to a single URL string (e.g., "https://192.168.1.10:8443")
    String serviceToUrl(String protocol, String serviceId, String tag, String requestKey);

    // Get a list of all available service instances as URIs
    List<URI> services(String protocol, String serviceId, String tag);
}

Parameters

• protocol: http or https.
• serviceId: The unique identifier of the service (e.g., com.networknt.petstore-1.0.0).
• tag: An optional environment tag (e.g., dev, prod). If used, discovery will be filtered by this tag.
• requestKey: An optional key used for load balancing (e.g., for Consistent Hash). Can be null for policies like Round Robin.

Default Implementation: LightCluster

LightCluster is the default implementation provided by light-4j.

Workflow

1. Check Cache: It first checks its internal in-memory cache (serviceMap) for the list of URLs associated with the serviceId and tag.
2. Discovery (if cache miss):
   • It creates a “subscribe URL” representing the service.
   • It subscribes to the Registry (Direct, Consul, etc.) to receive updates (notifications) about this service.
   • It performs an initial discovery lookup to get the current list of instances.
   • It populates the cache with the results.
3. Load Balance:
   • Once the list of URLs is retrieved (from cache or discovery), LightCluster delegates to the LoadBalance module.
   • The load balancer selects a single URL based on the configured strategy (Round Robin, Local First, etc.) and the requestKey.
4. Return: The selected URL is returned as a string.

Caching and Updates

LightCluster registers a ClusterNotifyListener with the registry. When service instances change (e.g., a node goes down or a new one comes up), the registry notifies this listener, ensuring the serviceMap cache is updated in near real-time.

Configuration

The cluster module does not have its own configuration file (e.g., cluster.yml). Instead, it relies on dependency injection via service.yml to wire up the Cluster, Registry, and LoadBalance implementations.

configuring service.yml

To use LightCluster, ensure your service.yml typically looks like this (it may vary depending on your specific registry choice):

singletons:
  # Registry configuration (e.g., DirectRegistry for local testing)
  - com.networknt.registry.URL:
      - com.networknt.registry.URLImpl:
          protocol: https
          host: localhost
          port: 8080
          path: direct
          parameters:
            com.networknt.petstore-1.0.0: https://localhost:8443,https://localhost:8444
  - com.networknt.registry.Registry:
      - com.networknt.registry.support.DirectRegistry

  # Load Balancer configuration
  - com.networknt.balance.LoadBalance:
      - com.networknt.balance.RoundRobinLoadBalance

  # Cluster configuration
  - com.networknt.cluster.Cluster:
      - com.networknt.cluster.LightCluster

In a production environment using Consul, the service.yml would specify ConsulRegistry instead.

Usage

You primarily use the Cluster instance via the Http2Client or Client methods that abstract this lookup, but you can also use it directly if needed.

Cluster cluster = SingletonServiceFactory.getBean(Cluster.class);

// Find a healthy instance for Petstore service
String url = cluster.serviceToUrl("https", "com.networknt.petstore-1.0.0", "dev", null);

if (url != null) {
    // Make call to url
}

Switch

The Switch module (source package com.networknt.switcher) provides a mechanism to manage runtime feature flags or service availability switches. It allows parts of the system to turn logic on or off dynamically without restarting the server.

A common use case is during the server shutdown process: a “service available” switch can be turned off to stop accepting new requests while allowing existing requests to complete.

Components

• Switcher: A simple entity class representing a named switch with a boolean state (on or off).
• SwitcherService: An interface defining operations to manage switchers (init, get, set, listener).
• LocalSwitcherService: The default in-memory implementation of SwitcherService.
• SwitcherUtil: A static utility class that provides global access to the SwitcherService.

Usage

The module is primarily used via the SwitcherUtil class.

Initializing a Switcher

You can initialize a switcher with a name and a default state.

import com.networknt.switcher.SwitcherUtil;

// Initialize a switcher named "maintenanceMode" to false (off)
SwitcherUtil.initSwitcher("maintenanceMode", false);

Checking Switch State

Check if a switch is currently enabling.

if (SwitcherUtil.isOpen("maintenanceMode")) {
    // Logic when maintenance mode is ON
} else {
    // Normal operation logic
}

You can also check with a default value if the switcher might not have been initialized yet.

// Returns true if "newFeature" is open, otherwise returns the default (false)
boolean isFeatureEnabled = SwitcherUtil.switcherIsOpenWithDefault("newFeature", false);

Setting Switch State

Change the state of a switch programmatically.

// Turn on maintenance mode
SwitcherUtil.setSwitcherValue("maintenanceMode", true);

Listeners

You can register a SwitcherListener to be notified when a switcher’s value changes.

SwitcherUtil.registerSwitcherListener("maintenanceMode", new SwitcherListener() {
    @Override
    public void onSwitcherChanged(String key, boolean value) {
        System.out.println("Switcher " + key + " changed to " + value);
    }
});

Example: Graceful Shutdown

In the light-4j server shutdown hook, the framework might use a switcher to signal that the server is shutting down.

1. Server startup: SwitcherUtil.initSwitcher(Constants.SERVER_STATUS_SWITCHER, true);
2. Request Handler: Checks SwitcherUtil.isOpen(Constants.SERVER_STATUS_SWITCHER). If false, returns 503 Service Unavailable.
3. Shutdown Hook: SwitcherUtil.setSwitcherValue(Constants.SERVER_STATUS_SWITCHER, false);

This allows the registry to be notified and the load balancer to stop sending traffic, while the server wraps up in-flight requests.

Consul Registry

The consul module integrates with HashiCorp Consul to provide service registry and discovery for the light-4j framework. It allows services to automatically register themselves upon startup and discovering other services using client-side load balancing.

Overview

• Registry: When a service starts, it registers itself with the Consul agent. It deregisters on graceful shutdown.
• Discovery: Clients using Cluster and LoadBalance modules can discover available service instances. The ConsulRegistry implementation uses blocking queries (long polling) to receive real-time updates from Consul about service health and availability.
• Health Checks: Supports TCP, HTTP, and TTL health checks to ensure only healthy instances are returned to clients.

Configuration

The module requires configuration in multiple files.

1. service.yml

You must configure the Registry singleton to use ConsulRegistry.

singletons:
  - com.networknt.registry.URL:
      - com.networknt.registry.URLImpl:
          protocol: light
          host: localhost
          port: 8080
          path: consul
          parameters:
            registryRetryPeriod: '30000'
  - com.networknt.consul.client.ConsulClient:
      - com.networknt.consul.client.ConsulClientImpl
  - com.networknt.registry.Registry:
      - com.networknt.consul.ConsulRegistry

2. consul.yml

This file controls the connection to Consul and the health check behavior.

Health Check Options:

• TCP: Consul pings the service IP/Port. Simple, but less reliable in containerized environments.
• HTTP: Consul calls /health/{serviceId}. Recommended for most services as it proves the application is responsive.
• TTL: The service actively sends heartbeats to Consul. Useful if the service is behind a NAT or cannot be reached by Consul.

3. server.yml

The service must be configured to enable the registry and usually dynamic ports (especially for Kubernetes).

serviceId: com.networknt.petstore-1.0.0
enableRegistry: true
dynamicPort: true
minPort: 2400
maxPort: 2500

4. secret.yml

For security, the Consul token should be managed in secret.yml.

consulToken: your-secure-jwt-or-uuid-token

Deployment

When deploying to Kubernetes:

1. Host Network: Services should use hostNetwork: true so they register with the Node’s IP address.
2. Downward API: Pass the host IP to the container so the service knows what IP to register.

    spec:
      hostNetwork: true
      containers:
      - env:
        - name: STATUS_HOST_IP
          valueFrom:
            fieldRef:
              fieldPath: status.hostIP

Blocking Queries

ConsulRegistry uses Consul’s blocking query feature. Instead of polling every X seconds, it opens a connection that Consul holds open for up to wait time (default 10 minutes). If the service catalog changes (new instance, health failure), Consul returns the result immediately. This provides near-real-time updates with minimal overhead.

Usage

You rarely interact with ConsulRegistry directly. Instead, you use the Cluster or Http2Client which uses the registry behind the scenes.

// The cluster lookups will automatically use ConsulRegistry to find instances
Cluster cluster = SingletonServiceFactory.getBean(Cluster.class);
String url = cluster.serviceToUrl("https", "com.networknt.petstore-1.0.0", "dev", null);

Common Module

The common module provides shared utilities, constants, and enumerations that are used across both the client and server components of the Light-4j framework.

Components

ContentType

ContentType is an enumeration that defines standard HTTP Content-Type header values. It simplifies code maintenance by replacing hardcoded strings with strongly typed constants.

Supported Types:

• application/json
• text/xml
• application/xml
• application/yaml
• application/x-www-form-urlencoded
• multipart/form-data
• … and more.

It also includes a helper method toContentType(String value) to convert a string header value into the corresponding enum constant.

SecretConstants

This class defines standard keys for sensitive values found in secret.yml or other configuration files. These constants ensure consistency when accessing secrets across different modules.

Common Constants:

• Certificates: serverKeystorePass, serverTruststorePass, clientKeystorePass, etc.
• OAuth2: authorizationCodeClientSecret, clientCredentialsClientSecret, etc.
• Infrastructure: consulToken, emailPassword.

DecryptUtil

DecryptUtil is a critical utility for handling encrypted sensitive values within configuration files (like secret.yml). It enables the “Encrypt-Once, Deploy-Everywhere” pattern where secrets are stored in version control in an encrypted format and decrypted only at runtime.

How it works

1. Iterative Decryption: The decryptMap method recursively iterates through a configuration map (nested maps and lists).
2. Detection: It identifies values that start with the CRYPT prefix.
3. Decryption: When an encrypted value is found, it uses the configured Decryptor implementation to decrypt it.

Usage

The framework typically handles this automatically when loading secret.yml. However, if you are loading custom configuration files with encrypted values, you can use it directly:

Map<String, Object> myConfig = Config.getInstance().getJsonMapConfig("my-config");
if(myConfig != null) {
    // recursively decrypt all values in the map
    DecryptUtil.decryptMap(myConfig);
}

Decryptor Configuration

DecryptUtil relies on a Decryptor implementation being registered in service.yml. The default implementation typically uses AES encryption.

For example, in service.yml:

singletons:
  - com.networknt.decrypt.Decryptor:
      - com.networknt.decrypt.AESDecryptor

For more details on generating keys and encrypting values, refer to the Decryptor Module.

Config

The Config module is the backbone of the light-4j framework, handling the loading and management of configuration files for all modules and the application itself. It follows a “configuration over code” philosophy and supports externalized configuration for containerized deployments.

Singleton and Instance

The Config class is a singleton. It caches loaded configurations to ensure high performance.

// Get the singleton instance
Config config = Config.getInstance();

Formats and Priority

Supported formats are yml, yaml, and json. The loading priority for a given configuration name (e.g., server) is:

1. server.yml
2. server.yaml
3. server.json

YAML is highly recommended due to its readability and support for comments.

Loading Sequence

The Config module loads configuration files in a specific order, allowing for hierarchical overrides. This is critical for managing configurations across different environments (Dev, Test, Prod) without rebuilding the application.

1. Externalized Directory: Specified by the system property -Dlight-4j-config-dir=/config. This is the highest priority and is typically used in Docker/Kubernetes to map a volume containing environment-specific configs.
2. Application Resources: Files found in src/main/resources/config of your application.
3. Module Resources: Default configuration files packaged within the jar of each module (e.g., light-4j core modules).

Usage

Loading as Map

Useful for dynamic configurations.

Map<String, Object> serverConfig = Config.getInstance().getJsonMapConfig("server");
int port = (int) serverConfig.get("httpsPort");

Loading as Object (POJO)

Encouraged for type safety. The POJO must map to the JSON/YAML structure.

ServerConfig serverConfig = (ServerConfig) Config.getInstance().getJsonObjectConfig("server", ServerConfig.class);

Loading Strings or Streams

For loading raw files like certificates or text files.

String certContent = Config.getInstance().getStringFromFile("primary.crt");
InputStream is = Config.getInstance().getInputStreamFromFile("primary.crt");

Configuration Injection

Starting from 1.5.26, you can inject values into your yml files using the syntax ${VAR:default}. The values are resolved from:

1. values.yml: A global values file in the config path.
2. Environment Variables: System environment variables.

Syntax

• ${VAR}: value of VAR. Throws exception if missing.
• ${VAR:default}: value of VAR. Uses default if missing.
• ${VAR:?error}: value of VAR. Throws custom error message if missing.
• ${VAR:$}: Escapes the injection (keeps ${VAR}).

Example

server.yml:

serviceId: ${SERVER_SERVICEID:com.networknt.petstore-1.0.0}

values.yml:

SERVER_SERVICEID: com.networknt.petstore-2.0.0

Injecting Environment Variables into values.yml

You can also inject environment variables directly into values.yml. This allows for a two-stage injection where environment variables populate values.yml, and values.yml populates other configuration files. This is particularly useful when using secret management tools (like HashiCorp Vault) that populate environment variables, allowing you to manage sensitive data without committing it to the code repository.

values.yml:

router.hostWhitelist:
  - 192.168.0.*
  - localhost
  - ${DOCKER_HOST_IP}

If the environment variable DOCKER_HOST_IP is set to 192.168.5.10, the final loaded configuration for router.hostWhitelist will include 192.168.5.10.

Benefits:

• Security: Sensitive information (IPs, credentials) remains in environment variables/secrets engine.
• Flexibility: You can use the same values.yml structure across environments, populated by environment-specific variables.

Handling Secrets

Release 1.6.0+ supports transparent decryption of sensitive values.

Setup

In your config.yml (or service.yml in newer versions):

decryptorClass: com.networknt.decrypt.AESDecryptor

Decryptors

• AESDecryptor: Uses default password light.
• AutoAESDecryptor: Reads password from environment variable light-4j-config-password.
• ManualAESDecryptor: Prompts for password in the console/terminal at startup.

If a value in your config file starts with CRYPT: (e.g., CRYPT:adfasdfadf...), the Config module will automatically attempts to decrypt it when loading.

Best Practices

1. Externalize Environments: Use -Dlight-4j-config-dir or mapped volumes in Kubernetes for strictly environment-specific configs (like DB connections, secrets).
2. App Level Configs: Put common configurations that differ from defaults in src/main/resources/config.
3. Values.yml: Use values.yml to centrally manage variables that are injected into multiple other config files.
4. Secrets: Never commit plain text passwords. Use the encryption feature or Kubernetes Secrets mapped to files in the config directory.

Cache Manager

The CacheManager module provides a unified interface for managing in-memory caches within the light-4j application. It abstracts the underlying caching implementation, allowing developers to configure and use caches without tying their code to a specific library.

The default implementation provided by light-4j is based on Caffeine, a high-performance Java caching library.

CacheManager Interface

The CacheManager interface defines standard operations for interacting with caches:

public interface CacheManager {
    void addCache(String cacheName, long maxSize, long expiryInMinutes);
    Map<Object, Object> getCache(String cacheName);
    void put(String cacheName, String key, Object value);
    Object get(String cacheName, String key);
    void delete(String cacheName, String key);
    void removeCache(String cacheName);
    int getSize(String cacheName);
}

Configuration

The configuration is managed by CacheConfig and corresponds to cache.yml (or cache.json/cache.properties). The configuration allows defining multiple named caches, each with its own size limit and expiration policy.

Configuration Properties

Cache Item Properties

Each item in the caches list contains:

Configuration Example (cache.yml)

# Cache Configuration
caches:
  - cacheName: jwt
    expiryInMinutes: 10
    maxSize: 10000
  - cacheName: jwk
    expiryInMinutes: 1440
    maxSize: 100

Values Example (values.yml)

You can override the cache configuration in values.yml. Note that since caches is a list, you typically override the entire list or use stringified JSON if supported by the config loader, but YAML list format is standard.

cache.caches:
  - cacheName: userCache
    expiryInMinutes: 60
    maxSize: 500
  - cacheName: tokenCache
    expiryInMinutes: 15
    maxSize: 2000

Usage

To use the cache manager, you can retrieve the singleton instance and interact with your named caches.

Retrieving the Instance

The CacheManager follows a singleton pattern (or can be injected via dependency injection if using a DI framework).

CacheManager cacheManager = CacheManager.getInstance();

Basic Operations

// Put a value into the cache
cacheManager.put("userCache", "userId123", userObject);

// Get a value from the cache
Object value = cacheManager.get("userCache", "userId123");
if (value != null) {
    // Cache hit
}

// Delete a specific key
cacheManager.delete("userCache", "userId123");

// Remove an entire cache
cacheManager.removeCache("userCache");

Implementation

The default implementation CaffeineCacheManager uses Caffeine’s Caffeine.newBuilder() to create caches with:

• maximumSize: Limits the cache size.
• expireAfterWrite: Expires entries after the specified duration.

This implementation is registered automatically via SingletonServiceFactory or module registration when CacheConfig loads.

Data Source

The data-source module provides a flexible way to configure and manage JDBC data sources in light-4j applications. It wraps HikariCP to provide high-performance connection pooling.

Overview

Unlike the simple javax.sql.DataSource configuration in service.yml, the data-source module offers:

• Support for multiple data sources.
• Detailed HikariCP configuration.
• Encrypted password support.
• Separation of concerns (DataSource config vs. Service config).

Dependency

Add the following dependency to your pom.xml:

<dependency>
    <groupId>com.networknt</groupId>
    <artifactId>data-source</artifactId>
    <version>${version.light-4j}</version>
</dependency>
<dependency>
    <groupId>com.zaxxer</groupId>
    <artifactId>HikariCP</artifactId>
</dependency>
<!-- Add your JDBC driver dependency here (e.g., mysql-connector-java, postgresql) -->

Configuration

Configuration is primarily handled in datasource.yml.

datasource.yml

You can define multiple data sources by giving them unique names (e.g., PostgresDataSource, H2DataSource).

# Example Configuration
PostgresDataSource:
  DriverClassName: org.postgresql.ds.PGSimpleDataSource
  jdbcUrl: jdbc:postgresql://postgresdb:5432/mydb
  username: postgres
  # Password can be encrypted using the light-4j decryptor
  password: CRYPT:747372737...
  maximumPoolSize: 10
  connectionTimeout: 30000
  # Optional HikariCP settings
  settings:
    autoCommit: true
  # Optional driver parameters
  parameters:
    ssl: 'false'

H2DataSource:
  DriverClassName: org.h2.jdbcx.JdbcDataSource
  jdbcUrl: jdbc:h2:mem:testdb;DB_CLOSE_DELAY=-1
  username: sa
  password: sa

service.yml

You need to register the data sources in service.yml so they can be injected or retrieved via SingletonServiceFactory.

singletons:
  # Register the Decryptor for encrypted passwords
  - com.networknt.utility.Decryptor:
      - com.networknt.decrypt.AESDecryptor

  # Register specific Data Sources
  - com.networknt.db.GenericDataSource:
      - com.networknt.db.GenericDataSource:
          - java.lang.String: PostgresDataSource
      - com.networknt.db.GenericDataSource:
          - java.lang.String: H2DataSource

  # Or specific implementations if available
  - javax.sql.DataSource:
      - com.networknt.db.GenericDataSource::getDataSource:
          - java.lang.String: PostgresDataSource

Usage

Getting a DataSource

You can retrieve the DataSource instance using SingletonServiceFactory.

import com.networknt.service.SingletonServiceFactory;
import com.networknt.db.GenericDataSource;
import javax.sql.DataSource;

public class MyDao {
    public void query() {
        // Option 1: Get the GenericDataSource wrapper
        GenericDataSource genericDs = SingletonServiceFactory.getBean(GenericDataSource.class); 
        // Note: If you have multiple, you might need getBean(Class, name) or check how it's registered.
        
        // Option 2: If registered as javax.sql.DataSource
        DataSource ds = SingletonServiceFactory.getBean(DataSource.class);
        
        try (Connection conn = ds.getConnection()) {
            // ... use connection
        }
    }
}

Multiple Data Sources

If you have multiple data sources, it is best to register them by name in service.yml and retrieve them by name or class if you created specific subclasses.

// Retrieving by name if registered as such (requires custom registration logic or specific class beans)
GenericDataSource postgres = (GenericDataSource) SingletonServiceFactory.getBean("PostgresDataSource");

Features

Encrypted Passwords

You can use the light-4j encryption tool to encrypt database passwords. Prefix the encrypted string with CRYPT: in datasource.yml or secret.yml.

Setting Configuration

The settings map in datasource.yml matches the setXxx methods of HikariDataSource. For example:

settings:
  autoCommit: false
  idleTimeout: 60000

DB Provider

The db-provider module is a lightweight database provider designed to simplify the initialization of a generic SQL/JDBC data source, typically for internal services or control plane components (like light-portal or light-config-server) that need a single, primary database connection.

Overview

Unlike the more flexible data-source module which is designed for multiple, named data sources, db-provider focuses on providing a single, global HikariDataSource initialized at startup. It is often used in conjunction with a StartupHookProvider to ensure the database connection is ready before the server starts handling requests.

Configuration

The module is configured via db-provider.yml (or yaml/json).

Example db-provider.yml:

driverClassName: org.h2.Driver
jdbcUrl: jdbc:h2:mem:testdb
username: sa
password: sa
maximumPoolSize: 5

Startup Hook

To use this module, you typically register the SqlDbStartupHook in your service.yml. This hook reads the configuration, creates the HikariDataSource, and initializes the CacheManager.

service.yml:

startupHooks:
  - com.networknt.db.provider.SqlDbStartupHook

Usage

Once initialized by the startup hook, the data source is available via the static field in SqlDbStartupHook.

import com.networknt.db.provider.SqlDbStartupHook;
import javax.sql.DataSource;
import java.sql.Connection;

public class MyService {
    public void doSomething() {
        DataSource ds = SqlDbStartupHook.ds;
        try (Connection conn = ds.getConnection()) {
            // ... perform DB operations
        } catch (SQLException e) {
            // handle exception
        }
    }
}

Key Classes

• DbProviderConfig: Loads configuration from db-provider.yml.
• SqlDbStartupHook: Initializes the global HikariDataSource and CacheManager.
• DbProvider: Interface for provider extensions (defaulting to DbProviderImpl).

When to use

• Use db-provider if you are building a simple service or a light-4j platform component (like Config Server) that needs a single, standard SQL connection setup globally at startup.
• Use data-source if you need multiple data sources, named data sources, or more complex configuration that doesn’t rely on a specific startup hook.

Decryptor

The decryptor module provides a mechanism to decrypt sensitive configuration values (like passwords, tokens, and keys) at runtime. This allows you to check in your configuration files with encrypted values, enhancing security.

Configuration

The decryptor is configured in config.yml.

Note: The default class name in newer versions might be AutoAESSaltDecryptor. The Config module loads this class to handle any value starting with CRYPT:.

Implementations

The module provides several implementations of the Decryptor interface.

AutoAESSaltDecryptor

This is the default and recommended implementation. It uses AES encryption with a salt.

• Key Source: Checks the environment variable LIGHT_4J_CONFIG_PASSWORD.
• Fallback: If the environment variable is not set, it defaults to the password light (useful for testing).
• Usage: Locate the sensitive value in your config file (e.g., values.yml or secret.yml) and replace the plain text with CRYPT:encoded_string.

ManualAESSaltDecryptor

This implementation prompts the user to enter the password in the console/terminal during server startup. This is useful for local development or environments where environment variables cannot be securely set.

Encryption Utility

To encrypt your secrets, you can use the light-encryptor utility tool.

1. Clone https://github.com/networknt/light-encryptor.
2. Run the utility (Java jar) with your master password and the clear text string.
3. The tool will output the encrypted string (e.g., CRYPT:fa343a...).
4. Copy this string into your configuration file.

Deployment

1. Generate Encrypted Values: Use light-encryptor to generate CRYPT:... strings.
2. Update Config: Put these strings in secret.yml or other config files.
3. Set Master Password: In your deployment environment (Kubernetes, VM), set the LIGHT_4J_CONFIG_PASSWORD environment variable to your master password.
   • In Kubernetes, use a Secret mapped to an environment variable.

env:
  - name: LIGHT_4J_CONFIG_PASSWORD
    valueFrom:
      secretKeyRef:
        name: my-app-secret
        key: master-password

Creating a Review Decryptor

If you need a custom encryption algorithm (e.g., integrating with a cloud KMS or HashiCorp Vault), you can implement the com.networknt.decrypt.Decryptor interface and update config.yml to point to your class.

Email Sender Module

The email-sender module provides a simple utility to send emails using Jakarta Mail (formerly JavaMail). It is designed to be used within the Light-4j framework and integrates with the configuration system.

Introduction

This module simplifies the process of sending emails from your application. It supports:

• Sending both plain text and HTML emails.
• Sending emails with attachments.
• Template variable replacement for dynamic email bodies.
• Configuration via email.yml (localized or centralized).

Configuration

The module is configured using the email.yml file.

Example email.yml

host: smtp.gmail.com
port: 587
user: [email protected]
pass: your_app_password
debug: false
auth: true

Security Note: Never commit plain text passwords to version control. Use Decryptor Module to encrypt sensitive values in configuration files or use environment variables.

Usage

dependency

Add the following dependency to your pom.xml.

<dependency>
    <groupId>com.networknt</groupId>
    <artifactId>email-sender</artifactId>
    <version>${version.light-4j}</version>
</dependency>

Sending a Simple Email

import com.networknt.email.EmailSender;

public void send() {
    EmailSender sender = new EmailSender();
    try {
        sender.sendMail("[email protected]", "Subject Line", "<h1>Hello World</h1>");
    } catch (MessagingException e) {
        logger.error("Failed to send email", e);
    }
}

Sending an Email with Attachment

public void sendWithAttachment() {
    EmailSender sender = new EmailSender();
    try {
        sender.sendMailWithAttachment(
            "[email protected]", 
            "Report", 
            "Please find the attached report.", 
            "/tmp/report.pdf"
        );
    } catch (MessagingException e) {
        logger.error("Failed to send email", e);
    }
}

Template Replacement

The EmailSender provides a utility to replace variables in a template string. Variables are defined as [key].

Map<String, String> replacements = new HashMap<>();
replacements.put("name", "John Doe");
replacements.put("link", "https://example.com/activate");

String template = "<p>Hi [name],</p><p>Click <a href='[link]'>here</a> to activate.</p>";
String content = EmailSender.replaceTokens(template, replacements);

sender.sendMail("[email protected]", "Activation", content);

Handler Module

The handler module is the core component of the Light-4j framework responsible for managing the request/response lifecycle. It implements the Chain of Responsibility pattern, allowing developers to compose complex processing flows using small, reusable middleware components.

core Concepts

MiddlewareHandler

The MiddlewareHandler interface is the contract for all middleware plugins. It allows handlers to be chained together.

public interface MiddlewareHandler extends LightHttpHandler {
    HttpHandler getNext();
    MiddlewareHandler setNext(final HttpHandler next);
    boolean isEnabled();
}

• Chaining: Each handler holds a reference to the next handler in the chain.
• Enabling: Handlers can be enabled or disabled via configuration.
• Execution: When handleRequest is called, the handler performs its logic and then calls Handler.next(exchange, next) to pass control to the next handler.

LightHttpHandler

The LightHttpHandler interface extends Undertow’s HttpHandler. It is recommended for all business handlers to implement this interface. It provides helper methods for:

• Standardized Error Responses: setExchangeStatus helps return consistent JSON error responses based on status.yml.
• Audit Logging: Automatically integrates with the Audit module to log errors and stack traces if configured.

HandlerProvider

The HandlerProvider interface is used by Service modules to expose a bundle of handlers.

public interface HandlerProvider {
    HttpHandler getHandler();
}

This is often used when a library provides a set of API endpoints that need to be injected into the main application (e.g., ServerInfoGetHandler or HealthGetHandler).

Handler Orchestration

The Handler class is the main orchestrator. It loads the configuration from handler.yml and initializes:

1. Handlers: Instantiates handler classes.
2. Chains: Composes lists of handlers into named execution chains.
3. Paths: Maps HTTP methods and path templates to specific executor chains.

When a request arrives, Handler.start(exchange) matches the request path against the configured paths.

• If a match is found, the corresponding chain ID is attached to the exchange, and execution begins.
• If no path matches, it attempts to execute defaultHandlers if configured.
• If no match and no default handlers, the request processing stops (or falls through to the next external handler).

Configuration (handler.yml)

The configuration file handler.yml is central to defining how requests are processed.

1. Handlers

Defines all the handler classes used in the application. You can provide an alias using @ to reference them easily in chains.

handlers:
  # Format: com.package.ClassName@alias
  - com.networknt.exception.ExceptionHandler@exception
  - com.networknt.metrics.MetricsHandler@metrics
  - com.networknt.validator.ValidatorHandler@validator
  - com.example.MyBusinessHandler@myHandler

2. Chains

Defines reusable sequences of handlers. The default chain is commonly used for shared middleware.

chains:
  default:
    - exception
    - metrics
    - validator
  secured:
    - exception
    - metrics
    - security
    - validator

3. Paths

Maps specific API endpoints to execution chains.

paths:
  - path: '/v1/address'
    method: 'get'
    exec:
      - default
      - myHandler
  - path: '/v1/admin'
    method: 'post'
    exec:
      - secured
      - adminHandler

• path: The URI template (e.g., /v1/pets/{petId}).
• method: HTTP method (GET, POST, etc.).
• exec: A list of chains or handlers to execute in order.

4. Default Handlers

A fallback chain executed if no path matches. Useful for 404 handling or SPA routing.

defaultHandlers:
  - exception
  - cors
  - fileHandler

Best Practices

1. Use Aliases: Always use aliases (e.g., @exception) in handlers lists. It makes chains and paths configuration much more readable.
2. Granular Chains: Define common middleware stacks (like default, middleware, security) in chains and reuse them in paths. This reduces duplication.
3. Order Matters: Place ExceptionHandler at the very beginning of the chain to catch errors from all subsequent handlers. Place MetricsHandler early to capture accurate timing.

Http2 Client

The Http2Client module provided by light-4j is a high-performance, asynchronous HTTP client that supports both HTTP/1.1 and HTTP/2. It is built on top of Undertow’s client capabilities and is designed to be efficient, lightweight, and easy to use in microservices architectures.

Features

• HTTP/1.1 and HTTP/2 Support: Transparently handles both protocol versions.
• Asynchronous & Non-blocking: Uses callbacks and futures to handle requests without blocking threads.
• Connection Pooling: Built-in connection pooling for efficient resource management, especially for HTTP/1.1.
• OAuth 2.0 Integration: Built-in support for handling OAuth 2.0 tokens (client credentials, authorization code, etc.).
• Service Discovery: Integrates with light-4j service discovery to resolve service IDs to URLs.
• TLS/SSL Support: Comprehensive TLS configuration including two-way SSL.

Important

Migration Notice: The borrowConnection/returnConnection methods are deprecated. Migrate to the new borrow()/restore() SimplePool API for better metrics, health checks, and pool management. See the SimplePool Migration Guide for details.

Configuration

The client is configured via client.yml.

Key Configuration Sections

TLS (tls)

Configures Transport Layer Security settings.

Request (request)

Configures request behavior and connection pooling.

OAuth (oauth)

Configures interactions with OAuth 2.0 providers for token management. Includes sections for token (acquiring tokens), sign (signing requests), and key (fetching JWKs).

Example client.yml

tls:
  verifyHostname: true
  loadTrustStore: true
  trustStore: client.truststore
  trustStorePass: password
request:
  timeout: 3000
  enableHttp2: true
  connectionPoolSize: 100
  poolMetricsEnabled: true
  healthCheckEnabled: true
oauth:
  token:
    server_url: https://localhost:6882
    client_credentials:
      client_id: my-client
      client_secret: my-secret

Usage

Getting the Client Instance

The Http2Client is a singleton.

import com.networknt.client.Http2Client;

Http2Client client = Http2Client.getInstance();

Making a Request (Recommended - SimplePool)

Use borrow() to get a connection token and restore() to return it.

import com.networknt.client.simplepool.SimpleConnectionHolder.ConnectionToken;
import io.undertow.client.ClientConnection;
import io.undertow.client.ClientRequest;
import io.undertow.util.Methods;
import java.net.URI;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.atomic.AtomicReference;

// ...

ConnectionToken token = null;
try {
    // Borrow a connection token
    token = client.borrow(
        new URI("https://example.com"),
        Http2Client.WORKER,
        Http2Client.BUFFER_POOL,
        true  // HTTP/2 enabled
    );
    
    ClientConnection connection = token.connection().getRawConnection();

    // Create the request
    ClientRequest request = new ClientRequest().setMethod(Methods.GET).setPath("/api/v1/data");
    request.getRequestHeaders().put(Headers.HOST, "example.com");

    // Latch to wait for response
    final CountDownLatch latch = new CountDownLatch(1);
    final AtomicReference<ClientResponse> reference = new AtomicReference<>();

    // Send the request
    connection.sendRequest(request, client.createClientCallback(reference, latch));

    // Wait for the response
    latch.await();

    // Process response
    ClientResponse response = reference.get();
    int statusCode = response.getResponseCode();
    String body = response.getAttachment(Http2Client.RESPONSE_BODY);

    System.out.println("Status: " + statusCode);
    System.out.println("Body: " + body);

} catch (Exception e) {
    e.printStackTrace();
} finally {
    // Always restore the token
    if (token != null) {
        client.restore(token);
    }
}

Handling Token Injection

The client can automatically inject OAuth tokens.

// Inject Client Credentials token
client.addCcToken(request);

// Inject Authorization Code token (Bearer token passed from caller)
client.addAuthToken(request, "bearer_token_string");

Propagating Headers

For service-to-service calls, you often need to propagate headers like correlation ID and traceability ID.

// Propagate headers from an existing HttpServerExchange
client.propagateHeaders(request, exchange);

Pool Warm-Up

Pre-establish connections to reduce first-request latency:

// Warm up connections at application startup
client.warmUpPool(URI.create("https://api.example.com:443"));

Pool Metrics

Access connection pool statistics:

SimplePoolMetrics metrics = client.getPoolMetrics();
if (metrics != null) {
    logger.info(metrics.getSummary());
}

Shutdown

Stop background threads during application shutdown:

client.shutdown();

Best Practices

1. Use SimplePool API: Prefer borrow()/restore() over deprecated borrowConnection()/returnConnection().
2. Release Connections: Always restore tokens in a finally block to avoid leaking connections.
3. Timeouts: Always set timeouts for your requests to prevent indefinite hanging.
4. HTTP/2: Enable HTTP/2 for better performance (multiplexing) if your infrastructure supports it.
5. Singleton: Use the singleton Http2Client.getInstance() rather than creating new instances.
6. Shutdown: Call client.shutdown() during application shutdown to stop health check threads.

Related Documentation

• SimplePool Migration Guide
• SimplePool Design

SimplePool Migration Guide

This guide helps migrate from deprecated Http2Client connection pooling methods to the new SimplePool API.

Important

The old borrowConnection/returnConnection methods are deprecated for removal. Migrate to borrow()/restore() for better pool management, metrics, and health checks.

Quick Reference

Key Difference: Token-Based Pool Management

The SimplePool uses a token pattern instead of raw connections:

// OLD: Returns ClientConnection directly
ClientConnection conn = client.borrowConnection(uri, worker, pool, true);
try {
    // use connection
} finally {
    client.returnConnection(conn);
}

// NEW: Returns ConnectionToken that wraps the connection
ConnectionToken token = client.borrow(uri, worker, pool, true);
try {
    ClientConnection conn = token.connection().getRawConnection();
    // use connection
} finally {
    client.restore(token);
}

Why tokens? Tokens track which specific borrow operation acquired a connection, enabling:

• Accurate metrics (borrow/restore counts per URI)
• Leak detection
• Proper HTTP/2 multiplexing management

Migration Patterns

Pattern 1: Simple Request

Before:

Http2Client client = Http2Client.getInstance();
ClientConnection connection = null;
try {
    connection = client.borrowConnection(10, uri, Http2Client.WORKER, 
        Http2Client.BUFFER_POOL, OptionMap.create(UndertowOptions.ENABLE_HTTP2, true));
    
    // Send request
    connection.sendRequest(request, callback);
    
} finally {
    if (connection != null) {
        client.returnConnection(connection);
    }
}

After:

Http2Client client = Http2Client.getInstance();
SimpleConnectionHolder.ConnectionToken token = null;
try {
    token = client.borrow(uri, Http2Client.WORKER, Http2Client.BUFFER_POOL, true);
    ClientConnection connection = token.connection().getRawConnection();
    
    // Send request
    connection.sendRequest(request, callback);
    
} finally {
    if (token != null) {
        client.restore(token);
    }
}

Pattern 2: With OptionMap

Before:

OptionMap options = OptionMap.builder()
    .set(UndertowOptions.ENABLE_HTTP2, true)
    .set(Options.KEEP_ALIVE, true)
    .getMap();
    
ClientConnection conn = client.borrowConnection(10, uri, worker, bufferPool, options);

After:

OptionMap options = OptionMap.builder()
    .set(UndertowOptions.ENABLE_HTTP2, true)
    .set(Options.KEEP_ALIVE, true)
    .getMap();
    
ConnectionToken token = client.borrow(uri, worker, bufferPool, options);
ClientConnection conn = token.connection().getRawConnection();

Pattern 3: HTTP/1.1 vs HTTP/2

// HTTP/2 (multiplexed)
ConnectionToken token = client.borrow(uri, worker, bufferPool, true);

// HTTP/1.1 (non-multiplexed)
ConnectionToken token = client.borrow(uri, worker, bufferPool, false);

New Features After Migration

After migrating to SimplePool, you gain access to:

Pool Metrics

SimplePoolMetrics metrics = client.getPoolMetrics();
if (metrics != null) {
    logger.info(metrics.getSummary());
}

Pool Warm-Up

// Pre-establish connections at startup
client.warmUpPool(URI.create("https://api.example.com:443"));

Health Checks

Enable in configuration:

client:
  request:
    healthCheckEnabled: true
    healthCheckIntervalMs: 30000

Configuration

Enable SimplePool features in client.yml:

client:
  request:
    # Pool sizing
    connectionPoolSize: 10
    connectionExpireTime: 1800000
    
    # Metrics (default: false)
    poolMetricsEnabled: true
    
    # Warm-up (default: false)
    poolWarmUpEnabled: true
    poolWarmUpSize: 2
    
    # Health checks (default: true)
    healthCheckEnabled: true
    healthCheckIntervalMs: 30000

Backward Compatibility

During the transition period:

1. Deprecated methods internally redirect to SimplePool when possible
2. Old pool (Http2ClientConnectionPool) is still available as fallback
3. Both pools can coexist during migration

AI Migration Instructions

For automated migration tools and AI assistants, use these patterns:

SEARCH_PATTERN: borrowConnection call

\bclient\.borrowConnection\s*\([^)]*\)

REPLACE_PATTERN: Convert to borrow

1. Change method name from borrowConnection to borrow
2. Change return type from ClientConnection to ConnectionToken
3. Extract connection: token.connection().getRawConnection()
4. Change returnConnection(connection) to restore(token)

SEARCH_PATTERN: returnConnection call

\bclient\.returnConnection\s*\(\s*(\w+)\s*\)

REPLACE_PATTERN: Convert to restore

client.restore($1Token)

where $1Token is the corresponding token variable.

VARIABLE_RENAME

• connection → token
• conn → token
• Add .connection().getRawConnection() when accessing the raw connection

IMPORT_CHANGES

Add:

import com.networknt.client.simplepool.SimpleConnectionHolder;

Troubleshooting

Connection Not Returned to Pool

Symptom: Pool exhaustion, connections not being reused

Cause: restore() not called in finally block

Fix: Always use try-finally:

ConnectionToken token = null;
try {
    token = client.borrow(...);
    // use connection
} finally {
    if (token != null) client.restore(token);
}

Metrics Show Zero

Cause: Metrics not enabled

Fix: Add to configuration:

client:
  request:
    poolMetricsEnabled: true

Related Documentation

• Http2Client Module
• SimplePool Design

Monad Result

The monad-result module provides a functional way to handle success and failure occurrences in your application logic. It is an implementation of a Result monad, similar to Optional in Java, but designed to carry failure information (error status) instead of just being empty.

This approach promotes cleaner code by avoiding the use of exceptions for expected business logic failures, making the control flow more explicit and easier to follow.

Core Components

The module consists of three main parts:

1. Result

An interface that represents the result of an operation. It can be either a Success or a Failure.

2. Success

An implementation of Result that holds the successful value.

3. Failure

An implementation of Result that holds a com.networknt.status.Status object describing the error.

Usage

Creating Results

You can create success or failure results using the static factory methods:

import com.networknt.monad.Result;
import com.networknt.monad.Success;
import com.networknt.monad.Failure;
import com.networknt.status.Status;

// Create a success result
Result<String> success = Success.of("Hello World");

// Create a failure result with a Status object
Status status = new Status("ERR10001");
Result<String> failure = Failure.of(status);

Transforming Results

The Result interface provides several monadic methods to work with the values without explicitly checking for success or failure:

Map

Applies a function to the value if the result is a success. If it’s a failure, it returns the failure as-is.

Result<Integer> length = success.map(String::length);

FlatMap

Similar to map, but the mapping function returns another Result. This is useful for chaining multiple operations that can fail.

Result<User> user = idResult.flatMap(id -> userService.getUser(id));

Fold

Reduces the Result to a single value by providing functions for both success and failure cases. This is often used at the end of a chain to convert the result into a response body or an external format.

String message = result.fold(
    val -> "Success: " + val,
    fail -> "Error: " + fail.getError().getMessage()
);

Lift

Allows combining two Result instances by applying a function to their internal values. If either result is a failure, it returns a failure.

Result<String> result1 = Success.of("Hello");
Result<String> result2 = Success.of("World");
Result<String> combined = result1.lift(result2, (r1, r2) -> r1 + " " + r2);

Conditional Execution

You can perform actions based on the state of the result using ifSuccess and ifFailure:

result.ifSuccess(val -> System.out.println("Got value: " + val))
      .ifFailure(fail -> System.err.println("Error Status: " + fail.getError()));

Why use Monad Result?

1. Explicit Error Handling: Failures are part of the method signature and cannot be accidentally ignored like unchecked exceptions.
2. Improved Readability: Functional chaining (map, flatMap) leads to cleaner logic compared to deeply nested if-else blocks.
3. Composability: It is easy to combine multiple operations into a single result using flatMap and lift.

Dependency

Add the following to your pom.xml:

<dependency>
    <groupId>com.networknt</groupId>
    <artifactId>monad-result</artifactId>
    <version>${version.light-4j}</version>
</dependency>

Data Mask

In a production environment, various logging statements are written to log files or persistent storage to assist in identifying and resolving issues. Since a broad group of people might have access to these logs, sensitive information such as credit card numbers, SIN numbers, or passwords must be masked before logging for confidentiality and compliance.

The mask module provides a utility to handle these masking requirements across different data formats.

StartupHookProvider

The mask module depends on JsonPath, which allows for easy access to nested elements in JSON strings. To ensure consistency and performance, you should configure JsonPath to use the Jackson parser (which is standard in light-4j) instead of the default json-smart parser.

The light-4j framework provides JsonPathStartupHookProvider for this purpose. You can enable it by updating your service.yml:

singletons:
- com.networknt.server.StartupHookProvider:
    - com.networknt.server.JsonPathStartupHookProvider

When using light-codegen, this provider is often included in the generated service.yml but commented out by default.

Configuration (mask.yml)

The masking behavior is fully configurable via mask.yml. It supports four sections: string, regex, json, and map.

Example mask.yml

---
# Replacement rules for specific keys. 
# Key maps to a map of "regex-to-find": "replacement-string"
string:
  uri:
    "password=[^&]*": "password=******"

# Masking based on regex groups. 
# Matching groups in the value will be replaced by stars (*) of the same length.
regex:
  header:
    Authorization: "^Bearer\\s+(.*)$"

# Masking based on JSON Path. 
# Key maps to a map of "json-path": "mask-expression"
json:
  user:
    "$.password": ""
    "$.creditCard.number": "^(?:4[0-9]{12}(?:[0-9]{3})?|5[1-5][0-9]{14})$"

Usage

The com.networknt.mask.Mask utility class provides static methods to apply masking based on the configuration above.

1. Mask with String

Used for simple replacements, typically in URIs or query parameters.

// Uses the 'uri' key from the 'string' section in mask.yml
String maskedUri = Mask.maskString("https://localhost?user=admin&password=123", "uri");
// Result: https://localhost?user=admin&password=******

2. Mask with Regex

Replaces the content of matching groups with the * character. This is useful for headers or cookies where you want to preserve the surrounding structure.

// Uses the 'header' key and 'Authorization' name from the 'regex' section
String input = "Bearer eyJhbGciOiJIUzI1...";
String masked = Mask.maskRegex(input, "header", "Authorization");
// Result: Bearer ****************...

3. Mask with JsonPath

The most powerful way to mask JSON data. It supports single values, nested objects, and arrays.

// Uses the 'user' key from the 'json' section
String json = "{\"username\":\"admin\", \"password\":\"secret123\"}";
String maskedJson = Mask.maskJson(json, "user");
// Result: {"username":"admin", "password":"*********"}

Masking Lists/Arrays

The JSON masking logic automatically handles arrays if the JSON Path targets an array element (e.g., $.items[*].id). It ensures that only the values are masked while preserving the array structure.

Dependency

Add the following to your pom.xml:

<dependency>
    <groupId>com.networknt</groupId>
    <artifactId>mask</artifactId>
    <version>${version.light-4j}</version>
</dependency>

Auto-Registration

The mask module registers itself with the ModuleRegistry automatically when the configuration is loaded. You can verify the loaded masking rules through the Server Info endpoint.

Portal Registry

The portal-registry module enables light-4j services to self-register with a centralized Light Portal controller (light-controller) instead of using local agents like Consul. It addresses the issues of resource utilization, long polling inefficiencies, and complex setup associated with Consul.