My team has been talking about revamping our testing strategy for a core service for some time. We heavily rely on E2E tests against a pleothra of other microservices which makes all of our pipelines unbelievably slow. We're looking to implement contract tests to help elevate our dependency on our E2E test suite.

I haven't had any experience with this testing strategy so far so, I decided take a look at the OpenAPI specification to get my head around how we might implement contract tests using it to help us on our way.

Contract tests are pretty neat, they are the cool cousin to E2E you're not ashamed of. They represent a methodology for ensuring that two separate systems (such microservices) are able to talk with each other. They use the communication between two systems and creates a "contract" that can then verify if both sides are adhering to their agreed upon standards. There is a little more to them than schema tests as the idea is you're going to work with the folks behind the other service and allow changes and development to happen while keeping all the pipelines green!

How does the OpenAPI spec relate to contract tests? One challenge with contract tests is having your API documented in such a way that allows you to create and maintain these contracts with ease. Luckily this is one of the problems that the OpenAPI specification aims to address.

The Open API Spec (v3.0)

Originally known as the "Swagger" spec, named after Swagger the originally creators of the spec. The OpenAPI specification or OAS is defined as:

The OpenAPI Specification (OAS) defines a standard, language-agnostic interface to RESTful APIs which allows both humans and computers to discover and understand the capabilities of the service without access to source code, documentation, or through network traffic inspection.

In essence a OpenAPI YAML/JSON file that is the textual representation of an API interface, its response scenarios and format of messages etc. Think Infrastructure as code but for APIs.

Example structure

Here is a very simple YAML sample file for my-resturants API:

openapi: 3.0.0
  version: 0.0.0
  title: my-resturants
  - url: ''
    summary: Get a list of services
          - $ref: "#/components/parameters/brand"
          description: OK
                $ref: "#/components/schemas/BrandSchema"

There is quite a lot going on here, but you can see on line 11, we're referencing and a separate specification for the brand URL parameter. Same kind of thing on line 18 too, we're referencing a response schema which is defined in a single place (which I've ommited for breviety) then reused, like a variable but for specification information. The rough idea is you specify the possible responses, parameters and all sorts of other things like examples, headers, auth and alike so the specification is a self-contained, single source of truth.

Okay we've got a huge YAML spec file, now what?

Now your API is represented in YAML or JSON, the fun can begin. This file can now be used by a number of tools to spin up things like mock servers, build API docs, and much more!


Need to document your API? Of course you do! Now you've got your spec you can feed the file into any number of doc site generators or even more generic static site generators, like Gatsby! I've had a quick go with RapiDoc and it is awesome.

<!doctype html>
    <meta charset="utf-8">
    <script type="module" src=""></script>
    <rapi-doc spec-url="path/to/api/spec.json"></rapi-doc>

Just like that you have a full API docs site, how cool is that! You can get more bespoke designs working with something like gatsby-source-openapi-aggregate that adds the OpenAPI spec to the sites GraphQL instance.

Mock Servers

Another cool feature of these files is they can be used to spin up cheap and quick mock servers using the example data / responses included in them. Prism is a nifty little tool from the folks at Stoplight that does some cool things like mock servers. You can install the script on your machine via the global installation command:

yarn global add @stoplight/prism-cli

Once that has downloaded you simply pass the API spec path as a parameter:

prism mock specs/example-one.json
open api prism output screenshot

You can see the server being hit here and the validators running against the provided spec file example-one.json.

But now I need to maintain the spec as well as the source code?

Yes. There are tools to build the spec from your source code but that isn't the point of the spec. The added complexity is something many will want to consider.

You can author these specification files from your source code with tools like flasgger for Python. Flasgger is particularly nice as it keeps the specific spec "chunks" next to the endpoint as you define it so makes the code more self-documenting.

from flask import Flask, jsonify
from flasgger import Swagger

app = Flask(__name__)
swagger = Swagger(app)

def colors(palette):
    """Example endpoint returning a list of colors by palette
    This is using docstrings for specifications.
      - name: palette
        in: path
        type: string
        enum: ['all', 'rgb', 'cmyk']
        required: true
        default: all
        description: A list of colors (may be filtered by palette)
          $ref: '#/definitions/Palette'

While there would have to be a little more required to wire up the references here but flasgger certainly is easier than defining the API by hand when retrofitting it to an existing project.

What about contract tests?

Even thought we now we have our API documented and have found some additional uses for the spec, sadly a contract test setup still needs a bit more work. Namely, provider and consumer contracts. As these contract tests more of a methodology than a tool or solution, we need a specific form our contract tests are going to take, and I am sticking to pact.

Now we have a definitive, specification for our API we can start writing consumer and provider Pact tests for the various ways the API endpoints are used. Next on my list is to find a way to use the API spec when creating Pact contracts. Potentially using something like pact-python or pactman. You can find more information on implementations for Pact on theirimplementation guides page.

See also

Here is some helpful further reading or resources you may be interested in around the OpenAPI v3 spec and contract tests.

  • OpenAPI.Tools is a great resource for the multitude of projects that make use of the OAS ecosystem.
  • Postman - A fantastic (albeit slightly pricey) API platform for building and using APIs. My go-to tool for testing and mocking APIs on my desktop. Some great paid features too!
  • OpenAPI Specification - The specification for the specification, how meta.
  • Swagger CodeGen - This allows you to generate API client libraries (SDK generation), server stubs and documentation automatically given an OpenAPI Spec.