Jon Chery
60 lines
2.4 KiB
Markdown
60 lines
2.4 KiB
Markdown
# Test Pyramid — Derived Rules
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> Derives from `domains/testing/first-principles.md` P4 (Fast Feedback), P5 (Coverage of Behavior), P10 (No Test Theater).
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## The Pyramid
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```
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/\
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/e2e\ few, slow, integration
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/------\
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/ integ \ some, medium, contract
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/----------\
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/ unit \ many, fast, isolated
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/--------------\
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```
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- **Unit (many):** test a function/class in isolation. Fast (< 10ms each). The bulk of tests.
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- **Integration (some):** test components together (DB, API client, queue). Medium (< 1s each).
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- **E2E (few):** test the whole system from outside. Slow (> 1s each). The tip of the pyramid.
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## Why a Pyramid (P4 Fast Feedback)
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- A pyramid inverts to a "ice cream cone" (many e2e, few unit) when devs avoid unit tests.
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- Inverted pyramids are slow and flaky. The feedback loop breaks.
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- The pyramid shape preserves fast feedback: most failures are unit failures, found in < 10ms.
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## What Goes Where
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| Test Type | What it Covers | Speed | Count |
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|-----------|----------------|-------|-------|
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| Unit | A function, a class, a pure module | < 10ms | Many |
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| Integration | DB queries, API contract, queue behavior | < 1s | Some |
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| E2E | A user flow, an API request → response end-to-end | > 1s | Few |
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- A unit test does not hit the database. A unit test does not make a network call.
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- An integration test does not test business logic; it tests the integration.
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- An e2e test does not test edge cases; it tests the happy path. Edge cases are unit tests.
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## Anti-Patterns (P10 No Test Theater)
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- **Ice cream cone:** many e2e, few unit. Slow, flaky, no signal.
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- **Cupcake:** same count at every level. No pyramid shape. Slow.
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- **Only unit:** 100% unit coverage, 0% integration. The system is untested as a whole.
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- **Only e2e:** every edge case is an e2e test. The suite takes an hour.
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## Coverage (P5)
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- Unit coverage of behavior: every branch, every edge case, every error path.
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- Integration coverage of contracts: every API endpoint, every DB query, every queue interaction.
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- E2E coverage of flows: the top 3–5 critical user flows. Not every permutation.
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## What Violates the Pyramid
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| Violation | Principle |
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|-----------|-----------|
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| E2E test for an edge case | P4 (slow feedback) |
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| Unit test that hits the DB | P2 (not isolated) |
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| 0 integration tests | P5 (no coverage of contracts) |
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| 500 e2e tests, 50 unit tests | P10 (theater) |
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| A 30-second test suite | P4 (feedback loop broken) | |