Deep modules

Design deep modules: a lot of behaviour behind a small interface, placed at a clean seam, testable through that interface. The aim is payback for callers, locality for maintainers, and testability for everyone.

Vocabulary

Use these words exactly in module design; do not substitute "component", "service" or "API" for module and interface, or "boundary" for seam. "Boundary" keeps its other senses: a bounded context, and the system boundary where test-first puts its mocks.