Codebase Design

---
name: codebase-design
description: Shared vocabulary for designing deep modules. Use when the user wants to design or improve a module's interface, find deepening opportunities, decide where a seam goes, make code more testable or AI-navigable, or when another skill needs the deep-module vocabulary.
---

# 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.

`references/DESIGN-IT-TWICE.md`'s briefs assume a `CONTEXT.md` glossary (the
`domain-modeling` skill's format) alongside this vocabulary — load
`domain-modeling` too when one exists.

## 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).

**Core** — the application or domain code behind a seam. The core offers its
interface to callers and declares the capability it requires from the outside;
both interface decisions belong on the core side of the seam.

**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?

## 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.** 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.**
  Don't introduce a seam unless something actually varies across it.
- **Hexagonal direction.** Adapters depend toward the core; core code must not
  import infrastructure, framework, storage, clock, or protocol details. An
  adapter translates those external details and does not duplicate business rules.
  Composition selects adapters. Keep a one-adapter case local; a simple module may
  expose its handler directly.

## Designing for testability

Good interfaces make testing natural:

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

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

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

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

   ```typescript
   // Testable
   function calculateDiscount(cart): Discount {}

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

3. **Small surface area.** Fewer methods = fewer tests needed. Fewer params =
   simpler test setup.

## Relationships

- A **Module** has exactly one **Interface** (the surface it presents to
  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**.
- The **Core** offers an **Interface** and declares the capability it requires at
  its **Seam**; **Adapters** depend toward that **Core**, and composition selects
  them.
- **Depth** produces **Leverage** for callers and **Locality** for maintainers.

## Rejected framings

- **Depth as ratio of implementation-lines to interface-lines** (Ousterhout):
  rewards padding the implementation. We 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**.

## Going deeper

- **Deepening a cluster given its dependencies** — see
  [references/DEEPENING.md](references/DEEPENING.md): dependency categories,
  seam discipline, and replace-don't-layer testing.
- **Exploring alternative interfaces** — see
  [references/DESIGN-IT-TWICE.md](references/DESIGN-IT-TWICE.md): spin up
  parallel sub-agents to design the interface several radically different
  ways, then compare on depth, locality, and seam placement. `/design-an-interface`
  automates this pattern — reach for it directly when you just want the parallel
  designs run, not the background here.