# Effective Software Testing

Automated tests are the primary mechanism for verifying that code does what it claims to do and
continues to do so as the codebase evolves. Unit tests cover individual functions in isolation;
integration tests verify that components collaborate correctly; end-to-end tests exercise the
full system from the user's perspective.

The test pyramid suggests writing many fast unit tests, fewer integration tests, and only the
critical paths as end-to-end tests. Inverting the pyramid — relying on slow, brittle end-to-end
tests instead of fast unit tests — makes the suite expensive to run and hard to diagnose when
failures occur.

Good tests are deterministic, isolated, and fast. A test that passes on one run and fails on the
next without a code change is worse than no test at all, because it erodes trust in the entire
suite and trains developers to ignore failures.
