Change is happening all around us: new technologies, new methodologies… But how are these changes affecting the ways in which systems are architected, and how do recent developments like patterns and refactoring cause us to think differently about architecture?
When microservices were introduced, one of the benefits was that you could make changes to services easily. Is this the case if there are many consumers dependent on it, although the consumers and services are loosely coupled? In reality, it is not easy to make service changes once it is in production. The root cause is due to designers/developers still following the pattern of the Java EE RMI approach to building microservices and their consumers with POJO.
Example of Evolving a Service
Let’s say we have a restful service called party which can output all the profile information about customers. Here is the output in JSON.
The party service is currently consumed by two applications: a web application and a native mobile application. Both consumers validate the received response prior to processing them. The web application uses id, firstName, lastName, and email fields; the mobile application only uses id and email fields. Neither uses the age field.
One of the most common ways in which we might evolve a service is to add another field to a contract on behalf of one or more consumers. Depending on how the provider and consumers have been implemented, even this simple change can have costly implications for the business and its partners.
In our example, after the party service has been in production for some time, a second mobile application considers using it but asks that a new type field be added to each party. Because of the way the consumers have been built, the change has significant and costly implications both for the provider and the existing consumers. The cost to each varies based on how we implement the change. There are at least two ways in which we can distribute the cost of change between the members of the service community. First, we could modify our original schema and require each consumer to update its copy of the schema in order correctly to validate search results; the cost of changing the system is here distributed between the provider - who, faced with a change request like this, will always have to make some kind of change - and the consumers, who have no interest in the updated functionality. Alternatively, we could choose to add a second operation and schema to the service provider on behalf of the new consumer and maintain the original operation and schema on behalf of the existing consumers. The cost of change is now constrained to the provider but at the expense of making the service more complex and more costly to maintain.
Making schemas both backward and forward-compatible is a well-understood design task, best expressed by the Must Ignore pattern of extensibility. The Must Ignore pattern recommends that schemas incorporate extensibility points, which allow extension elements to be added to types and additional attributes to each element. The pattern also recommends that schema languages define a processing model that specifies how consumers process extensions. The simplest model requires consumers to ignore elements that they do not recognize - hence the name of the pattern. The model may also require consumers to process elements that have a “Must Understand” flag or abort if they cannot understand them.
The schema includes an optional extension object. The extension itself can contain one or more attributes. We can publish a new schema with an additional type attribute that the provider inserts into the extension container when we receive a change request to add a type to each party. This allows the party service to return results that include party type, and consumers using the new schema to validate the entire document. Consumers using the old schema will not break, though they will not process the type. The new results documents look like this:
Note that the first version of the extensible schema is forward-compatible with the second and that the second is backward-compatible with the first. This flexibility, however, comes at the expense of increased complexity. Extensible schemas allow us to make unforeseen changes to a schema language, but at the same time, they provide for requirements that may very well never arise and they obscure the expressive power that comes from a simple design, and frustrate the meaningful representation of business information by introducing meta-informational container attribute into the domain language.
We’ll not discuss schema extensibility further here. Suffice to say, extension points allow us to make backward and forward-compatible changes to schemas and documents without breaking service providers and consumers. Schema extensions do not help us manage the evolution of a system when we need to make what is a breaking change to a
The party service depends on a security subsystem to populate operatorId for consumers to access another system that uses this field to verify if access is allowed. It was designed this way but never used, and the security subsystem is too old to be maintained. The service provider discusses with all consumers to evaluate if this field can be removed. Given that this field is not part of the extension points, this is a breaking change. Although consumers have not used it, all consumers have confirmed that they have to release a new version, which is costly. The party service, in this respect, cannot evolve independently of its consumers. Providers and consumers must all jump at the same time.
Our service community is frustrated in its evolution because each consumer implements a form of “hidden” coupling that naively reflects the entirety of the provider contract in the consumer’s internal logic. The consumers, through their use of JSON schema validation and JSON to POJO static language bindings derived from a schema, implicitly
accept the whole of the provider contract, irrespective of their appetite for processing the component parts.
David Orchard provides some clues as to how we might have avoided this issue when he wrote to the Internet Protocol’s Robustness Principle: “In general, an implementation must be conservative in its sending behavior and liberal in its receiving behavior.” We can augment this principle in the context of service evolution by saying that message receivers should implement “just enough” validation: that is, they should only process data that contributes to the business functions they implement, and should only perform explicitly bounded or targeted validation of the data they receive - as opposed to the implicitly unbounded, “all-or-nothing” validation inherent in schema processing.
We can target or bound consumer-side validation and processing to extract pattern expressions along with the received message’s document tree structure, perhaps using a zero-copy JSON parser like Jsoniter instead of Jackson POJO binding. Using Jsoniter, each consumer of the party service can programmatically assert/process what it expects
to find in the response.
Notice that this approach makes no assertions about attributes in the underlying document for which the consuming application has no appetite. In this way, the validation language explicitly targets a bounded set of required attributes. Changes to the underlying document’s schema will not be picked up by the validation process unless they disturb the explicit expectations described in the consumer side contract, even if those changes extend to deprecating or removing formerly mandatory attributes.
Here is a relatively lightweight solution to our contract and coupling problems, and one that doesn’t require us to add obscure meta-informational elements to a document. So let’s roll back time once again, and reinstate the simple schema described at the beginning of the article. But this time, we’ll also insist that consumers are liberal in their receiving behavior, and only validate and process information that supports the business functions they implement. Now when the provider is asked to add a type to each party, the service can publish a revised schema without disturbing existing consumers. Similarly, upon discovering that the operatorId field is not validated or processed by any of the consumers, the service can revise the service results schema - again without disturbing the rate of evolution of each of the consumers.
Unlike monolithic applications, communications between client and service should be loosely coupled with individual attributes in JSON instead of POJO, which is popular in RMI. By doing so, we can ensure that the service can be evolved easily without multiple versions running in parallel. Given this, when we write light-codegen, we deliberately removed the POJO generator to encourage developers not to bind service response to a POJO, which is time-consuming and tightly couples your consumer to the service.
Also, to give a service provider confidence to evolve, it is wise to ask all consumers to provide test cases that describe consumers’ expectations in their processing as part of the service’s regression test. Once service changes happen, we only need to run the entire regression test cases to ensure nothing is broken on the consumer side. This consumer expectation is called consume contract, which is another topic that is out of the scope of this article.