Codebase Design

---
name: codebase-design
description: Shared vocabulary for designing deep modules — a small interface hiding a lot of behavior. Use when designing or improving a module's interface, deciding where a seam belongs, deciding whether to extract or merge modules, or making code more testable.
---

# Codebase Design

Design **deep modules**: a lot of behavior 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 — these 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 behavior a caller (or test) can exercise
per unit of interface they have to learn. A module is **deep** when a large amount of behavior
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 behavior 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. **Shallow module** (avoid) = large
interface + little implementation — mostly a pass-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.

## Designing for testability

1. **Accept dependencies, don't create them.** `function processOrder(order, paymentGateway)`,
   not `function processOrder(order) { const gateway = new StripeGateway(); }`.
2. **Return results, don't produce side effects.** `function calculateDiscount(cart): Discount`,
   not `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**. **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 produces Leverage for callers and Locality for
maintainers.

## Rejected framings

- **Depth as ratio of implementation-lines to interface-lines** (Ousterhout): rewards padding
  the implementation. Depth-as-leverage is used instead.
- **"Interface" as a language's `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 [DEEPENING.md](DEEPENING.md): dependency
  categories, seam discipline, and replace-don't-layer testing.
- **Exploring alternative interfaces** — see [DESIGN-IT-TWICE.md](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.

## Why this exists

Operationalizes Engineering #1 (Simplicity) and #10 (Maintainability) in
`products/ald-os/context/product-builder-principles.md` with concrete, testable heuristics
(the deletion test, one-vs-two-adapters) instead of just "keep it simple."

## Source

Adapted from [mattpocock/skills](https://github.com/mattpocock/skills)' `codebase-design`
skill.