mocking

Goal: fast, honest tests that isolate the unit without faking its behavior. Use for: - isolating a unit from slow or external dependencies - deciding what to fake versus exercise for real - fixing brittle tests coupled to implementation Workflow: 1. Identify the true external boundary (network, clock, filesystem). 2. Mock or stub only that boundary, not internal collaborators. 3. Use the lightest double that works: stub, fake, or mock. 4. Assert on observable behavior, not on every internal call. 5. Keep doubles in sync with the real contract. 6. Verify the integration with the real dependency elsewhere. Doubles: - stub: returns canned data - fake: a working lightweight implementation - mock: asserts interactions occurred - spy: records calls for later inspection Rules: - do not mock the thing you are testing - prefer real collaborators when they are fast and deterministic - avoid asserting on internal call sequences - verify mocked contracts against reality in integration tests