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.