Integration testing is a software testing methodology that verifies the correct interaction between two or more components, modules, or services within an application. Unlike unit tests that validate individual functions in isolation, integration tests examine how different parts of a system work together, checking data flow across boundaries, API contracts between services, and the behavior of components when connected to real or simulated external dependencies.
Common strategies for integration testing include top-down testing (starting from higher-level modules and progressively integrating lower-level ones), bottom-up testing (the reverse approach), and big-bang testing (integrating all components simultaneously). In modern microservices architectures, integration tests often focus on verifying HTTP API interactions, message queue communication, and database operations using tools like Testcontainers, WireMock, or Pact for contract testing.
Integration tests occupy an important middle ground in the testing pyramid, sitting between fast but narrow unit tests and comprehensive but slow end-to-end tests. While they take longer to execute than unit tests, they catch a category of defects, such as serialization mismatches, incorrect query logic, and misconfigured service connections, that unit tests simply can't detect. A well-balanced test suite uses integration tests strategically to provide confidence that the system's components collaborate correctly without incurring the overhead of full end-to-end scenarios.