Reference
Dependency Categories
When assessing a candidate for deepening, classify its dependencies:
1. In-process
Pure computation, in-memory state, no I/O. Always deepenable — just merge the modules and test directly.
2. Local-substitutable
Dependencies that have local test stand-ins (e.g., PGLite for Postgres, in-memory filesystem). Deepenable if the test substitute exists. The deepened module is tested with the local stand-in running in the test suite.
3. Remote but owned (Ports & Adapters)
Your own services across a network boundary (microservices, internal APIs). Define a port (interface) at the module boundary. The deep module owns the logic; the transport is injected. Tests use an in-memory adapter. Production uses the real HTTP/gRPC/queue adapter.
Recommendation shape: "Define a shared interface (port), implement an HTTP adapter for production and an in-memory adapter for testing, so the logic can be tested as one deep module even though it's deployed across a network boundary."
4. True external (Mock)
Third-party services (Stripe, Twilio, etc.) you don't control. Mock at the boundary. The deepened module takes the external dependency as an injected port, and tests provide a mock implementation.
Testing Strategy
The core principle: replace, don't layer.
- Old unit tests on shallow modules are waste once boundary tests exist — delete them
- Write new tests at the deepened module's interface boundary
- Tests assert on observable outcomes through the public interface, not internal state
- Tests should survive internal refactors — they describe behavior, not implementation