codebase-design

Files (3)

• SKILL.md
• deepening.md · resources/
• design-it-twice.md · resources/

SKILL.md (first lines)

<!-- Adapted from mattpocock/skills codebase-design at 6654f6b60cd9d5be8b54c6fafe44346dabeb3b76 (MIT). -->

# 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?

## 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

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**.

From bodnarbalazs/dydo, MIT.

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