codebase-design

Codebase Design

Design deep modules: a lot of behaviour tucked behind a small interface, sited at a clean seam, and tested through that interface. Apply this language and these principles anywhere code is being designed or reshaped. You are chasing three payoffs — leverage for callers, locality for maintainers, and testability for everyone.

Glossary

Use these terms precisely. Don't swap in "component," "service," "API," or "boundary" — the consistency is the whole point.

Module — anything that pairs an interface with an implementation. Intentionally scale-agnostic: a function, a class, a package, or a slice that spans tiers. Avoid: unit, component, service.

Interface — everything a caller must know to use the module correctly: the type signature, yes, but also invariants, ordering constraints, error modes, required configuration, and performance characteristics. Avoid: API, signature (both too narrow — they name only the type-level surface).

Implementation — the code inside a module, its body. Not the same as Adapter: a thing can be a small adapter wrapping a large implementation (a Postgres repo) or a large adapter wrapping a small one (an in-memory fake). Say "adapter" when the seam is the subject; "implementation" otherwise.

Depth — leverage at the interface: how much behaviour a caller (or test) can reach per unit of interface it must learn. A module is deep when a lot of behaviour hides behind a small interface, shallow when the interface is nearly as complex as the implementation.

Seam (Michael Feathers) — a place where behaviour can change without editing that place; the location where a module's interface sits. Choosing where the seam goes is a design decision in its own right, separate from deciding what lives behind it. Avoid: boundary (already overloaded by DDD's bounded context).

Adapter — a concrete thing that satisfies an interface at a seam. It names a role (which slot it fills), not a substance (what's inside).

Leverage — the caller's reward for depth: more capability per unit of interface learned. One implementation pays back across N call sites and M tests.

Locality — the maintainer's reward for depth: change, bugs, knowledge, and verification gather in one place instead of scattering across callers. Fix it once, it's fixed everywhere.

Deep vs shallow

Deep module = small interface + plenty of implementation:

┌─────────────────────┐
│   Small Interface   │  ← Few methods, simple params
├─────────────────────┤
│                     │
│  Deep Implementation│  ← Complex logic hidden
│                     │
└─────────────────────┘

Shallow module = large interface + thin implementation (avoid):

┌─────────────────────────────────┐
│       Large Interface           │  ← Many methods, complex params
├─────────────────────────────────┤
│  Thin Implementation            │  ← Just passes through
└─────────────────────────────────┘

As you shape an interface, ask:

• Can I cut the number of methods?
• Can I simplify the parameters?
• Can I bury more complexity inside?

Principles

• Depth is a property of the interface, not the implementation. A deep module may be built inside from small, mockable, swappable pieces — they simply aren't part of the interface. A module can carry internal seams (private to its implementation, exercised by its own tests) alongside the external seam at its interface.
• The deletion test. Picture deleting the module. If complexity evaporates, it was a pass-through. If complexity resurfaces spread across N callers, it was pulling its weight.
• The interface is the test surface. Callers and tests cross the same seam. If you find yourself wanting to test past the interface, the module is probably the wrong shape.
• One adapter is a hypothetical seam; two adapters make it real. Add a seam only when something genuinely varies across it.

Designing for testability

Good interfaces make tests fall out naturally:

1. Accept dependencies, don't construct them.

   // Testable
   function processOrder(order, paymentGateway) {}
   
   // Hard to test
   function processOrder(order) {
     const gateway = new StripeGateway();
   }
   

2. Return results rather than firing side effects.

   // Testable
   function calculateDiscount(cart): Discount {}
   
   // Hard to test
   function applyDiscount(cart): void {
     cart.total -= discount;
   }
   

3. Small surface area. Fewer methods means fewer tests; fewer params means simpler setup.

Relationships

• A Module has exactly one Interface (the surface it shows callers and tests).
• Depth is a property of a Module, measured against its Interface.
• A Seam is where a Module's Interface lives.
• An Adapter sits at a Seam and satisfies the Interface.
• Depth yields Leverage for callers and Locality for maintainers.

Rejected framings

• Depth as the ratio of implementation lines to interface lines (Ousterhout): rewards padding the implementation. Prefer depth-as-leverage.
• "Interface" as the TypeScript interface keyword or a class's public methods: too narrow — here, interface covers every fact a caller must know.
• "Boundary": overloaded by DDD's bounded context. Say seam or interface.

Going deeper

• Deepening a cluster given its dependencies — see DEEPENING.md: dependency categories, seam discipline, and replace-don't-layer testing.
• Exploring alternative interfaces — see DESIGN-IT-TWICE.md: fan out parallel sub-agents to design the interface several radically different ways, then compare on depth, locality, and seam placement.