How do unit, integration, and end-to-end tests differ?

A unit test checks a small behavior in a controlled scope. An integration test checks interaction across a real boundary, such as application code and a database. An end-to-end test exercises a user jou ey through a larger deployed stack or equivalent environment. Boundaries vary by team, so define them clearly. Use fast focused tests for many rules and a smaller number of broader tests for important interactions and jou eys. A large count of unit tests does not compensate for missing coverage of a critical database constraint or deployment configuration.