Codebase Design

Design deep modules: a lot of behaviour behind a small interface, placed at a clean seam, testable through that interface. The aim is leverage for callers, locality for maintainers, testability for everyone.

Glossary

Use these terms exactly. Consistent language is the whole point — do not substitute "component", "service", "API", or "boundary".

• Module — anything with an interface and an implementation. Deliberately scale-agnostic: a function, class, package, or tier-spanning slice.
• Interface — everything a caller must know to use the module correctly. Not just the type signature: invariants, ordering constraints, error modes, required configuration, performance characteristics. Avoid "API" and "signature" — both are too narrow.
• Implementation — what is inside the module.
• Depth — leverage at the interface: how much behaviour a caller or test can exercise per unit of interface they must learn. Deep = large behaviour behind a small interface. Shallow = interface nearly as complex as the implementation.
• Seam (Feathers) — a place where behaviour can be altered without editing in that place; where a module's interface lives. Where to put the seam is its own decision, separate from what goes behind it. Avoid "boundary" — overloaded with DDD's bounded context.
• Adapter — a concrete thing satisfying an interface at a seam. Describes the role it fills, not what is inside it.
• Leverage — what callers get from depth: one implementation paying back across N call sites and M tests.
• Locality — what maintainers get from depth: change, bugs, and verification concentrate in one place. Fix once, fixed everywhere.

Principles

• Depth is a property of the interface, not the implementation. A deep module can be internally composed of small swappable parts; they just are not part of its interface. Modules can have internal seams used by their own tests as well as the external seam at their 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. Needing to test past the interface means the module is the wrong shape.
• One adapter is a hypothetical seam. Two adapters is a real one. Do not introduce a seam unless something actually varies across it.

When shaping an interface, ask: can I reduce the number of methods? Simplify the parameters? Hide more complexity inside?

Designing for testability

• Accept dependencies, do not construct them. processOrder(order, gateway) is testable; a processOrder that news up a StripeGateway inside is not.
• Return results, do not mutate. calculateDiscount(cart): Discount is testable; applyDiscount(cart): void is not.
• Small surface area. Fewer methods means fewer tests; fewer parameters means simpler setup.

Rejected framings

• Depth as a ratio of implementation lines to interface lines (Ousterhout) — rewards padding the implementation. Use depth-as-leverage.
• "Interface" as a language keyword or a class's public methods — too narrow. Interface here is every fact a caller must know.

Pairs with scrutinize when auditing an existing module, and with brainstorming when the shape is still open.