Integration Testing

What is Integration Testing?

Integration testing verifies that different modules, services, or systems work together correctly when integrated. While unit tests validate individual components in isolation, integration tests validate the interfaces and interactions between components. Integration testing catches issues like incorrect API contracts, incompatible data formats, timing problems, and misunderstood assumptions about how components should interact. It sits between unit testing (smallest scope) and end-to-end testing (largest scope).

Integration testing approaches include: big bang integration (testing all components together at once, risky but fast), incremental integration (testing components as they're integrated, catching issues earlier), top-down integration (testing from user interface downward, using stubs for lower components), bottom-up integration (testing from database upward, using drivers for upper components), and sandwich integration (combining top-down and bottom-up). Modern microservices architectures use contract testing and service virtualization for integration testing.

Integration tests often require databases with realistic relationships and data flows. Testing a checkout flow integrating inventory, payment, and shipping services needs customers, products, inventory levels, and valid payment information across all systems. Mock data works for isolated unit tests, but integration tests need production-like data complexity. Synthetic data generation can create coherent datasets across multiple integrated systems, maintaining referential integrity while avoiding the security and compliance risks of using real production data in integration test environments.