codebase-design

name	codebase-design
audience	team
source	mattpocock/skills
source_commit	unknown
source_note	Modified locally — see .matt-pocock-attribution.yml for details
description	Shared vocabulary for designing deep modules — a lot of behaviour behind a small interface, placed at a clean seam, testable through that interface. Use when designing or improving a module's interface, finding deepening opportunities, deciding where a seam goes, making code more testable, or when another skill needs the deep-module vocabulary. Pairs with improve-codebase-architecture (which applies it) and domain-model (which supplies the domain nouns). Ported from https://github.com/mattpocock/skills (Matt Pocock).

Design deep modules: a lot of behaviour behind a small interface, placed at a clean seam, testable through that interface. Use this language and these principles wherever code is being designed or restructured. The aim is leverage for callers, locality for maintainers, and testability for everyone.

Glossary

Use these terms exactly — don't substitute "component," "service," "API," or "boundary." Consistent language is the whole point.

• Module — anything with an interface and an implementation. Scale-agnostic: a function, class, package, or tier-spanning slice. Avoid: unit, component, service.
• Interface — everything a caller must know to use the module correctly: the type signature, plus invariants, ordering constraints, error modes, required configuration, and performance characteristics. Avoid: API, signature (too narrow — type-level only).
• Implementation — what's inside a module, its body of code. Distinct from Adapter: a thing can be a small adapter with a large implementation (a Postgres repo) or a large adapter with a small implementation (an in-memory fake). Say "adapter" when the seam is the topic; "implementation" otherwise.
• Depth — leverage at the interface: how much behaviour a caller (or test) can exercise per unit of interface they must learn. Deep = large behaviour behind a small interface. Shallow = interface nearly as complex as the implementation.
• Seam (Michael Feathers) — a place where you can alter behaviour without editing in that place; the location at which a module's interface lives. Where the seam goes is its own design decision. Avoid: boundary (overloaded with DDD's bounded context).
• Adapter — a concrete thing that satisfies an interface at a seam. Names a role (the slot it fills), not its substance.
• Leverage — what callers get from depth: more capability per unit of interface learned. One implementation pays back across N call sites and M tests.
• Locality — what maintainers get from depth: change, bugs, knowledge, and verification concentrate in one place instead of spreading across callers. Fix once, fixed everywhere.

Deep vs shallow

Deep = small interface + lots of implementation. Shallow = large interface + thin implementation (avoid).

  DEEP  [ small interface ]     SHALLOW  [ large interface ]
        [                 ]              [ thin impl        ]
        [   deep impl     ]        ← interface ≈ implementation (avoid)
        [   (hidden)      ]

When designing an interface, ask: Can I reduce the number of methods? Can I simplify the parameters? Can I hide more complexity inside?

Principles

• Depth is a property of the interface, not the implementation. A deep module can be internally composed of small, mockable parts — they just aren't part of the interface. A module can have internal seams (private to its tests) as well as the external seam at its interface.
• The deletion test. Imagine deleting the module. If complexity vanishes, it was a pass-through. If complexity reappears across N callers, it was earning its keep.
• The interface is the test surface. Callers and tests cross the same seam. If you need to test past the interface, the module is probably the wrong shape.
• One adapter means a hypothetical seam. Two adapters means a real one. Don't introduce a seam unless something actually varies across it.

Designing for testability

Good interfaces make testing natural: accept dependencies, don't create them (pass paymentGateway in); return results, don't produce side effects (calculateDiscount(cart): Discount over a mutating applyDiscount); keep the surface small (fewer methods and params = fewer, simpler tests). Worked examples in references/DESIGN-NOTES.md.

Going deeper

• Testability examples, rejected framings, and the relational vocabulary map — references/DESIGN-NOTES.md.
• Deepening a cluster given its dependencies — references/DEEPENING.md (dependency categories, seam discipline, replace-don't-layer testing).
• Exploring alternative interfaces — references/DESIGN-IT-TWICE.md (parallel sub-agents design the interface several radically different ways, then compare on depth and locality).

Integration with Other Skills

Skill	Relationship
improve-codebase-architecture	Applies this vocabulary — surfaces shallow modules, information leakage, and deepening candidates. This skill supplies the words it names them with.
domain-model	Supplies domain nouns (ubiquitous language); this skill supplies module-shape nouns (depth, seam, adapter). They compose — domain names what, this names how it's shaped.
refactor-challenger	Prioritises which deepenings to actually perform; this skill frames the target shape.