Integration testing

Integration testing is a level of software testing that verifies whether separately developed modules, services or components work correctly when combined. A piece of code may behave perfectly on its own yet fail when it has to exchange data with a database, an external API or another module. Integration tests exist to catch exactly those problems at the boundaries between parts.

Where integration testing fits

Software testing is often pictured as a pyramid. At the base sit fast, numerous unit tests that check individual components in isolation. Integration tests occupy the middle layer: fewer in number, slower to run, but able to confirm that components cooperate — that a service really connects to its database, that two modules agree on a data format, that an API call returns what the caller expects. Above them sit end-to-end tests that exercise the whole system through its interface.

Why integration testing matters

Unit tests frequently replace real dependencies with fakes, which means a suite can be fully green while the assembled system is broken. Integration testing closes that gap by exercising the real connections. It is especially valuable in systems built from many services, where most defects hide in the interactions rather than inside any single component. Integration tests pair naturally with practices like TDD and run automatically in CI/CD pipelines, so a broken interaction is caught before it reaches production. The trade-off is speed and setup cost — integration tests need realistic environments and take longer than unit tests — so teams balance the layers: many fast unit tests, a focused set of integration tests around critical boundaries, and a small number of end-to-end checks on the most important flows.