Unit tests isolate logic; integration tests validate databases and queues; end-to-end tests cover critical journeys; contract tests protect API compatibility.

How to apply it

Make time and dependencies controllable. Use isolated fixtures. Prioritize tenant isolation, billing, identity, deletion, migrations, and core workflows. Coverage is a signal, not proof of meaningful assertions.

Worked example

Unit-test entitlement rules, integration-test persistence, and end-to-end-test a gated feature journey.

Common mistakes to avoid

Only testing happy paths or relying on coverage percentage as proof of quality.

Practice task

Implement this concept in the capstone. Add one positive test, one negative or failure-path test, and a short note explaining the trade-off you chose.