Unit Test vs Integration Test — What's the Difference?
A unit test verifies a single function or component in isolation, typically with dependencies mocked out; an integration test verifies that multiple components work correctly together, often including real dependencies like a database.
Published October 3, 2026
A unit test verifies a single function or component in isolation, typically with dependencies mocked out; an integration test verifies that multiple components work correctly together, often including real dependencies like a database.
Why they differ
- Unit tests are fast and pinpoint exactly which small piece of code broke
- Integration tests catch problems that only appear when real components interact, like a mismatched API contract or a database migration error
How to choose
- Write many fast unit tests for core logic, and fewer, more expensive integration tests for critical end-to-end flows
- A healthy test suite usually has far more unit tests than integration tests, often visualized as a pyramid
FAQ
Should integration tests use a real database?
Generally yes — a real (test) database catches issues, like a broken migration or a query that only fails under real constraints, that a mocked database would silently hide.