Martin Fowler - The Practical Test Pyramid

Comprehensive article on Ham Vocke’s martinfowler.com about structuring an automated test suite around the test pyramid.

Key Takeaways

The test pyramid has three layers: unit, service/integration, UI. Expands to also include contract tests and subcutaneous tests.
Two rules: “write tests with different granularity” and “the more high-level you get the fewer tests you should have.”
Ice cream cone anti-pattern: inverted pyramid — mostly manual and UI tests. Results in slow CI, high maintenance cost, poor failure localisation.
Unit tests: narrowest scope, fastest. Can be sociable (using real collaborators) or solitary (using test doubles).
Integration tests: focus on serialisation/deserialisation boundaries; test ORM mapping, HTTP clients, message parsing against real infrastructure.
Contract tests / CDC tests: consumers publish expectations; providers verify. Pact is the main tool.
End-to-end tests: very few. Test that the system works end-to-end in a production-like topology.
Subcutaneous tests: just below UI; avoid browser overhead.
Acceptance tests: validate features from user perspective; BDD Given/When/Then.

High — read directly from source.