Module 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.

This is a reference to consult, not a session to run. build-craft and bug-diagnosis both speak this vocabulary; reach for it directly when the words, not the process, are the problem.

Glossary

Use these terms exactly: do not 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, a class, a package, or a 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 name only the type-level surface).
• Implementation — what is inside a module; its body of code. Distinct from Adapter: a thing can be a small adapter with a large implementation (a Postgres 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: how much behaviour a caller or a 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 — 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. It describes a role (what slot it fills), not a substance (what is 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 instead of spreading across callers. Fix once, fixed everywhere.

Deep vs shallow

Deep — a small interface, lots of implementation:

┌─────────────────────┐
│   Small Interface   │  ← few methods, simple params
├─────────────────────┤
│  Deep Implementation│  ← complex logic hidden
└─────────────────────┘

Shallow — a 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 are just not 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 the complexity vanishes, it was a pass-through. If the 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 mean a real one. Do not introduce a seam unless something actually varies across it.

Designing for testability

Good interfaces make testing natural. Each rule below is what build-craft's seam discipline assumes of the code it tests.

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

3. Small surface area. Fewer methods means fewer tests needed; fewer params means simpler 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 the ratio of implementation lines to interface lines (Ousterhout). It rewards padding the implementation. Depth-as-leverage is the measure used here.
• "Interface" as the 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.

Where this is checked

rules/quality.md and agents/reviewer.md make these terms the vocabulary a structural dispute is settled against: a reviewer that flags a module names the term (a pass-through, a seam with nothing varying across it) rather than arguing about taste. The finding carries a path:line like any other.