bianhaifeng789-hue/dependency-categories

Dependency Categories

From "A Philosophy of Software Design" (John Ousterhout).

1. In-process ✅ Always deepenable

Pure computation, in-memory state, no I/O.

Strategy: Merge modules directly, test directly.

2. Local-substitutable ✅ Deepenable if stand-in exists

Dependencies with local test stand-ins:

• PGLite for Postgres
• In-memory filesystem
• SQLite for MySQL

Strategy: Deepen module, test with local stand-in.

3. Remote but owned (Ports & Adapters) ✅ Deepenable

Your own services across network boundary (microservices, internal APIs).

Strategy:

• Define port (interface) at module boundary
• Deep module owns logic, transport injected
• Tests: in-memory adapter
• Production: HTTP/gRPC/queue adapter

Recommendation: "Define shared interface (port), implement HTTP adapter for production and in-memory adapter for testing."

4. True external (Mock) ⚠️ Mock at boundary

Third-party services (Stripe, Twilio) you don't control.

Strategy:

• Mock at boundary
• Deepened module takes dependency as injected port
• Tests provide mock implementation

Testing Strategy

Replace, don't layer:

• Old unit tests on shallow modules → DELETE once boundary tests exist
• New tests: deepened module's interface boundary
• Assert on observable outcomes, not internal state
• Tests describe behavior, not implementation

Decision Matrix

| Category | Deepenable? | Test Strategy | |----------|-------------|---------------| | In-process | ✅ Always | Direct test | | Local-substitutable | ✅ If stand-in exists | Stand-in test | | Ports & Adapters | ✅ Yes | In-memory adapter | | True external | ⚠️ Mock boundary | Mock implementation |

Borrowed From

Matt Pocock's improve-codebase-architecture/REFERENCE.md.

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创建时间: 2026-04-15