Integration testing

Unit tests prove each piece works against your assumptions about its neighbours. Integration tests prove the assumptions were right. Almost every "all tests pass but production is broken" story lives in that gap — a mock that returns a shape the real service never returns.

These cost more than unit tests and less than end-to-end. Spend them on the seams where your assumptions about someone else's behaviour are most likely wrong.

1. Pick the seams worth the cost

Test the boundaries where you cross into something you do not control:

• Your code against a real database: queries, migrations, transactions, constraints. The highest-value integration tests in most applications, because SQL and ORM behaviour is where mocks lie most.
• Your service against its dependencies: real HTTP, real serialization.
• Message producers against consumers: that the payload one writes is one the other parses.
• Your code against the filesystem, clock, or queue, where behaviour is subtle.