Testing JSON APIs with Contract Tests

Testing JSON APIs with Contract Tests

APIs serve as a 'contract' between different services. A broken contract can lead to significant issues in dependent systems. **Contract testing** is an effective way to ensure your API maintains its expected data structure and behavior. In this article, we'll explore strategies like **consumer-driven contracts**, **golden files**, and **differential tests** to keep your JSON contracts stable as you iterate.

Golden Files

Golden files are standard JSON files containing the expected API response, stored under version control. These files act as a reference for the 'correct' response of your API. Every time you make a change, you can compare the new response against the golden file to immediately detect unexpected structural changes. To make these files human-readable and reduce noise in diffs, use a tool like the JSON Prettier to normalize their formatting before committing them to your repository.

Consumer-Driven Contracts

In this approach, the API consumer defines the data structure it expects, and this definition is used for the contract test. The API provider then runs tests against these contracts before releasing a new version, ensuring they don't introduce breaking changes for their consumers. This method is ideal for managing dependencies between services, especially in a microservices architecture.

Differential Tests

You can integrate differential tests into your Continuous Integration (CI) pipeline to automatically compare your API's current responses with your golden files. Tools like the JSON Diff can highlight structural changes, such as fields being added or removed, or type drift. This ensures that any unintended changes to your API are caught in the CI pipeline before they can be deployed to production. These tests also help you observe how your API evolves over time.