API Contract Definitions

Different ways of specifying API contracts

Dec 5, 2022

When running one or multiple services, it is essential that they have reliable service contracts defining their exposed APIs. Those contracts mostly consist of declarative interface definitions, which strongly define and type the API exposed by the respective service. As such, it is crucial that the code making up the service exactly implements the interface and therefore fulfills its side of the contract. Regressions need to be detected and changes reflected in a well-communicated update to the contract. Here, we want to look at different ways of specifying contracts for what is one of the most common protocols for exposing service APIs: HTTP.

HTTP works great as a means of communication for microservices because it is open, reliable, programming language-agnostic, and works great over the wire. All these features are crucial to modern services, as they allow engineers to change the underlying technologies (e.g., change the back-end code from Python to Go) without it affecting the contract. Therefore, the API’s consumers don’t even need to know about the implementing technology and the providing team can take independent decisions respectively.

Service contracts usually contain the following four components:

  • Available endpoints and operations on each endpoint
  • Operation parameters input and output for each operation
  • Authentication methods
  • Contact information, license, terms of use, and other information


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Specification and implementation

When working with services and their respective contracts, one has to maintain both the specification and the implementation. Ideally, these should always be in sync, as the best documentation is useless if it does not accurately reflect the reality of the API implementation.

Manual specification

The easiest way of creating a contract is to manually write it, and then write the respective code that should implement the contract. This is quite tedious and error-prone, as you have to basically write everything twice. When you change your implementation, you have to think about also changing the documentation and contract in the exact same way and vice versa. A way better approach is to either pick a technology that is contract-based and incorporates the interface specification in the exposed API or to at least automate either the generation of the contract from the implementation or the other way around.

Automated generation

There are two basic approaches to keeping the contract and the implementation in sync in an automated way. The first one is to write the code first and have the contract generated from that (Implementation First). The second approach is to write the contract and have the respective implementation code generated from that (Contract First).

Using either the contract first or implementation first approach guarantee that there is a single source of truth and that the other part is always in sync. As such, both are viable approaches. However, in general, it is preferred to write the contract first and generate implementation code from it. The reason being that when you begin implementing your service, ideally, the contract has already been defined and communicated with potential consumers of your API to allow them to work independently of your implementation. Having a human- and machine-readable contract checked into your source code repository allows you to track changes to that contract over time and additionally serves as documentation for what the implementation code does (or at least what it should do).


Here, we’ll look at three different technologies that allow you to write a clearly defined and declarative contract for your services: OpenAPI, GraphQL, and gRPC. These all have their advantages and disadvantages, which will be laid out and discussed. Obviously, there are many more technologies which allow declaring contracts, but the ones presented here are three very popular ones which are easy to use and have great communities around them. They will be illustrated along the simple example of an API where one can query Pokémon by their ID.


OpenAPI (formerly known as Swagger) is a very widespread way of specifying REST and other HTTP APIs. It is easy to write because the specification is just a JSON or a YAML file which defines what your API looks like by following a clearly defined specification.

An HTTP endpoint definition in OpenAPI might look as follows:

OpenAPI itself doesn’t come with any tools to generate the specification from your implementation or vice versa. However, because it is such a popular format, there are many tools that allow you to parse your implementation code (and possibly additional annotations) and generate a valid OpenAPI specification from it. A great example of such a tool is springdoc-openapi which takes Java classes with their properties, methods, and annotations and automatically generates an OpenAPI specification from those. There are also tools to do it the other way around. These take an existing OpenAPI spec and generate boilerplate code from it for a compliant implementation. A popular example of such a tool is oapi-codegen which creates Go code from a valid specification.

Obviously, OpenAPI not being directly integrated into the implementation frameworks has a great disadvantage: It does not enforce (e.g., at compile time) that your implementation actually perfectly fulfills the specified contract. However, you can achieve a similar outcome by adding a check for your code’s compliance to your automation pipeline, which prevents releases that diverge from their contract in an unwanted manner.

At this point, it is noteworthy, that REST applications can include so-called HATEOAS links. These are URLs included in the response body to a request, which lead to further endpoints providing actions for an element. If a client automatically follows those links, contracts can rely on that and therefore drop some of the actual URLs and paths from their specification. However, not too many applications in the wild reliably implement HATEOAS links, and they have their caveats and shortcomings.


GraphQL calls itself “a query language for your API.” The technology is about defining a schema which strongly types your endpoint methods and the objects they expect and return.

A simple GraphQL schema might look as follows:

It is not only much more concise than the above OpenAPI specification, but it also has great advantages because it is part of the GraphQL specification. Almost every GraphQL endpoint exposes its schema automatically, which is a direct product of the endpoints it actually exposes. This allows clients to query the contract directly from the endpoint and therefore know that it is always up to date. Tests can be run against that exposed schema, which would detect breaking changes automatically and potentially prevent releasing such. These conventions of how the endpoint exposes its documentation allow us to use comprehensive client frameworks such as apollo-client.

With GraphQL, there are also frameworks that allow writing a schema first and generating the respective boilerplate code from it. A popular tool for doing so is gqlgen in Go.


Another popular technology for declaring contracts is gRPC. It is based on Protocol Buffers, which is a way of specifying how to serialize structured data. The interface of a protocol buffer is defined in a file that might look like this:

One big difference between protocol buffers and the other technologies mentioned is that the data exchanged is in binary format rather than plain text. This makes them very performant but also harder to debug, which makes having a clearly defined schema and API crucial. A compiler of such a Protocol Buffer file is built into the toolchain and lets you generate boilerplate code from the specification and enforce compliance with the defined contract.


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There are many ways of writing contracts for your service APIs. A good contract has the following characteristics:

  • It is human-readable
  • It is machine-readable
  • It is declarative and comprehensive
  • It is tracked via version control
  • It is programming language-agnostic
  • It enforces that the implementation fulfills the contract
  • Breaking changes to the contract are detected and properly communicated to potential consumers

This makes the above technologies excellent choices, and all of them are a great step up from simply writing your contract somewhere in a wiki.

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