/codebase-design

name: codebase-design
description: |
  Canonical deep-module vocabulary (module, interface, depth, seam, adapter,
  leverage, locality) for designing a module's shape — a lot of behaviour
  behind a small interface at a clean seam, testable through that interface.
  The single source of truth for these terms; other skills (architecture,
  codebase-hygiene) point here instead of restating them.
  Use when designing or improving a module's interface, finding deepening
  opportunities, deciding where a seam goes, or making code more testable.
allowed-tools: Read Grep Glob LSP
user-invocable: false

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Codebase Design

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. Deliberately 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, but also invariants, ordering constraints, error modes, required configuration, and performance characteristics. _Avoid_: API, signature (too narrow — they refer only to the type-level surface).

**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). Reach for "adapter" when the seam is the topic; "implementation" otherwise.

**Depth** — leverage at the interface: the amount of behaviour a caller (or test) can exercise per unit of interface they have to 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. Where to put the seam is its own design decision, distinct from what goes behind it. _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's 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.

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?

The Deletion Test

For every module or abstraction, ask: **"If I deleted this module and inlined its code at every call site, where does the complexity go?"**

• **It vanishes** → the module was a pass-through / shallow. It adds indirection without hiding complexity. Delete it or deepen its interface.
• **It reappears across N call sites** → the module is deep. It earns its existence by hiding complexity that would otherwise be duplicated.

This is a falsifiable test, not a matter of taste — apply it before accepting any new module boundary.

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 aren't 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** (above) — falsifiable, canonical.
• **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.** This rule is about **ports** — external dependency seams where you inject an adapter (production vs test, or two real transports). It does NOT apply to every internal seam or test boundary: an in-process deepening needs no adapter (see [DEEPENING.md](DEEPENING.md)), and an ordinary caller or test exercising a public interface is NOT an adapter. Don't introduce a port unless something actually varies across it.

Designing for testability

Good interfaces make testing natural:

1. **Accept dependencies, don't create them.**

   // Testable
   function processOrder(order, paymentGateway) {}

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

2. **Return results, don't produce side effects.**

   // Testable
   function calculateDiscount