codebase-design

name	codebase-design
description	Shared vocabulary for designing deep modules. Use when: design a module interface, decide where a seam goes, make code more testable, reduce coupling, evaluate if a module is too shallow, or another skill needs the deep-module vocabulary.
license	MIT

Codebase Design

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

Use this language and these principles wherever code is designed or restructured. Consistent vocabulary is the whole point — don't substitute "component," "service," "API," or "boundary."

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Glossary

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: type signature + invariants + ordering constraints + error modes + required configuration + performance characteristics. Avoid: API, signature (both too narrow — they refer only to the type-level surface).

Implementation — what is 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). Use "adapter" when the seam is the topic; "implementation" otherwise.

Depth — leverage at the interface: amount of behaviour a caller (or test) can exercise per unit of interface they must learn. A module is deep when a large amount of behaviour sits behind a small interface, shallow when the interface is 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. Avoid: boundary (overloaded with DDD's bounded context).

Adapter — a concrete thing that satisfies an interface at a seam. Describes role (what slot it fills), not substance (what is inside).

Leverage — what callers get from depth: more capability per unit of interface they learn. 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 rather than spreading across callers. Fix once, fixed everywhere.

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Deep vs shallow

Deep module = small interface + lots of implementation:

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

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

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

When designing an interface, ask:

• Can I reduce the number of methods?
• Can I simplify the parameters?
• Can I hide more complexity inside?

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Principles

• Depth is a property of the interface, not the implementation. A deep module can be internally composed of small, mockable, swappable parts — they just are not part of the interface. A module can have internal seams (private to its implementation, used by its own 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 want to test past the interface, the module is probably the wrong shape.
• One adapter means a hypothetical seam. Two adapters means a real one. Do not introduce a seam unless something actually varies across it.

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Designing for testability

1. Accept dependencies, do not create them

// HARD TO TEST: creates its own dependency
function processOrder(order) {
  const gateway = new StripeGateway();     // cannot inject a fake
}

// TESTABLE: accepts dependency
function processOrder(order, paymentGateway: PaymentGateway) {
  // test by passing a mock gateway
}

# HARD TO TEST: creates its own dependency
def send_notification(user):
    client = boto3.client("sns")          # cannot mock
    client.publish(...)

# TESTABLE: accepts dependency
def send_notification(user, sns_client):
    sns_client.publish(...)                # test with moto or MagicMock

2. Return results, do not produce side effects

// HARD TO TEST: mutates input
function applyDiscount(cart): void {
  cart.total -= discount;                  # must inspect cart after call
}

// TESTABLE: returns result
function calculateDiscount(cart): Discount {
  // test by checking returned value
}

3. Small surface area

Fewer methods = fewer tests needed. Fewer params = simpler test setup. If a method has 5+ params, consider an options object or splitting the concern.

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Rejected framings

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

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To explore alternative interface designs by comparing multiple approaches, use skill subagent-driven-development.