ContractTesting

Contract testing is a software testing approach focused on the interactions between services, particularly the agreements or contracts that define how a consumer talks to a provider. A contract specifies the expected requests, responses, and behavior for a given integration, and serves as a behavioral guarantee between teams that own the consumer and the provider.

The core idea is to separate contract verification from broader integration tests. In a typical consumer-driven contract testing pattern, the consumer writes tests that express its expectations and generates a contract. This contract is published to a central repository or broker and used by the provider to verify that it can fulfill the contract. If the provider changes its API in a way that violates the contract, the verification fails, prompting a discussion or rollback before the change reaches production.

Two common approaches exist: consumer-driven contract testing, where consumers define and publish contracts for providers to honor, and provider-driven contracts, where providers declare their API expectations for consumers to implement against. Tools in this space include Pact (with multiple language bindings and broker services), Spring Cloud Contract, and Hoverfly, among others.

Benefits include faster feedback, better isolation of teams, and reduced flaky integration tests by catching incompatibilities early. Limitations involve ensuring contract completeness, handling dynamic or evolving data, and the need for governance to prevent drift. Contract testing is often used in microservice architectures and API integrations, complementing but not replacing end-to-end tests.