Video lesson: Choose Tests by the Failure They Can Catch

Lesson promise

By the end, the learner should be able to explain the core model for choose tests by the failure they can catch, apply it to a concrete input, and identify when its usual shortcut or guarantee stops applying. This is a recording brief; publish it as a playable lesson after the narration and visual sequence have been produced and reviewed.

Narration draft

Tests provide evidence about behavior at different boundaries. Unit tests isolate a small decision; integration tests exercise collaboration between components; end-to-end tests validate a user-visible path. Each level trades feedback speed, realism, and setup cost differently.

A unit test can verify that a retry policy stops after its configured budget. An integration test can verify that the persistence adapter records an idempotency key. An end-to-end test can prove the checkout flow returns the expected result through the deployed browser-facing path.

High coverage does not prove meaningful behavior was asserted. Tests that duplicate implementation details can block harmless refactoring, while a few end-to-end tests may miss many edge cases. Choose representative contracts and include failure and boundary conditions.

Visual sequence

1. Put the input and assumptions on screen. Ask the learner to predict the next state before revealing it.
2. Animate the representation and show the operation one transition at a time.
3. Pause at the boundary case in the companion article and compare the result with the invariant.
4. End with the exercise prompt: A service test passes with a mocked database but production loses updates under concurrent requests. Which test layer was missing, and what invariant should the new test exercise?

Companion material

Use the article, trace, and interactive concept flow as the learner’s written and visual references. The video remains planned until an actual playable media URL and reviewed transcript are available.