codebase-design

Files (5)

• SKILL.md
• openai.yaml · agents/
• deepening.md · references/
• design-it-twice.md · references/
• laravel.md · references/

SKILL.md (first lines)

# 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, action, package, or tier-spanning slice. _Avoid_: unit, component, service.

**Interface**: everything a caller must know to use the module correctly: the method signatures, 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 (an ORM-backed repository) 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?

## Principles

- **Depth is a property of the interface, not the implementation.** A deep module can be internally composed of small, 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.

## Mapping onto a stack

Where these concepts land in a typical application framework:

- **Dependency injection is the seam mechanism**: an interface (or protocol, trait, or function type) plus whatever wires the implementation in. Apply the two-adapter rule before reaching for it: an interface with exactly one implementation, wired up "for mocking", is a hypothetical seam. Framework fakes usually make it unnecessary (next point).
- **Framework fakes are adapters at framework-owned seams.** A test filesystem, queue, mailer, or HTTP stub the framework ships is the second adapter for that seam: provided free. Prefer them over hand-rolled ports for I/O the framework already owns.
- **ORM models are a wide, shared interface**: every attribute, query, and relationship is surface area available to every caller. Depth usually lives *above* them: an action or service whose small interface (`reconcileInvoice(invoice)`) hides the queries, state transitions, and side effects.
- **Controllers, route handlers, CLI commands, jobs, and UI components are entry-point adapters**: thin things that satisfy a transport's interface and delegate to a deep module. If real logic accumulates in one, that's a deepening candidate (the same logic will soon be needed from a second entry point).

**Stack references.** Identify the stack from its manifests. If a matching file exists below, read it for these concepts in that stack's terms.

- Laravel (`laravel/framework` in `composer.json`): [`references/laravel.md`](references/laravel.md)

## Designing for testability

Good interfaces make testing natural:

1. **Accept dependencies, don't create them.** A payment gateway passed in through the constructor or a parameter can be swapped for a fake in tests. One built inside the method from config (`new StripeGateway(secret)`) cannot.

2. **Return results, don't produce side effects.** `calculateDiscount(cart) → Discount` is a pure calculation you assert on directly. `applyDiscount(cart)`, which mutates and persists, can only be tested by inspecting what it changed.

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**.
- **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 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 [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.

From leek/agent-skills, MIT.

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