Locks API expectations between services using consumer-driven contracts so that when one team changes their implementation, it fails in CI rather than during a coordinated production deployment. Contract testing prevents the common integration failure pattern where both sides of an API appear to work in isolation but break when connected in production.
Use cases
- Splitting a monolith into microservices where each service team needs to evolve their APIs independently
- Preparing a public SDK where you want to catch API drift before external developers encounter it
- Maintaining versioned internal APIs where the provider and consumer teams are in different time zones
- Onboarding a new consumer to an existing API and wanting to verify compatibility quickly without a full integration environment
- Running contract tests in CI so that API changes that break existing consumers are caught before merge
Key features
- Consumer teams write tests that describe the API responses they depend on—these tests define the contract from the consumer's perspective
- The contract verification system (e.g., Pact, Spring Cloud Contract) publishes the contract to a shared broker where the provider can retrieve it
- The provider team runs the contract against their implementation in CI on every change, failing the build if the implementation no longer satisfies any consumer contract
- Block merges on the provider side when a contract test fails, forcing coordination with the affected consumer teams before shipping the breaking change
- Maintain versioned contracts so that provider and consumer can migrate to new contract versions on their own schedule
When to Use This Skill
- When multiple services or teams depend on a shared API and you want to catch breaking changes before production
- When building a public SDK and wanting to verify compatibility with consumer expectations automatically
- When microservices teams work independently and need confidence that integration will work without a full staging environment
Expected Output
Consumer-driven contracts stored in a shared broker, provider-side contract verification running in CI, and blocked merges when contracts are violated.
Frequently Asked Questions
- How does contract testing differ from integration testing?
- Integration testing verifies that two services work together in a shared environment. Contract testing verifies that each service meets its consumers' expectations without requiring a shared environment. Contract tests run in isolation on each team's CI.
- What happens when a consumer needs a breaking change?
- The consumer updates their contract test to reflect the new expectation, the provider fails verification, and both teams coordinate on a migration plan. This is preferable to discovering the breakage in production.
- Can contract testing replace end-to-end integration tests?
- No—contract testing verifies API compatibility, not overall system behavior. End-to-end tests are still needed to verify that the system as a whole behaves correctly. Use contract testing for rapid API evolution, end-to-end testing for business logic verification.
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