codebase-design

name	codebase-design
description	Shared vocabulary for designing deep modules in a PHP/Laravel codebase. Use when the user wants to design or improve a class or module's interface, find deepening opportunities, decide where a seam goes, make code more testable or AI-navigable, or when another skill (to-spec, implement, architecture-satisfaction) needs the deep-module vocabulary.

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, Laravel 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 Eloquent-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.

Laravel mapping

Where these concepts land in a Laravel codebase:

• The container is the seam mechanism: a PHP interface plus a service-provider binding. Apply the two-adapter rule before reaching for it: an interface with exactly one implementation, bound "for mocking", is a hypothetical seam. Laravel's fakes usually make it unnecessary (next point).
• Framework fakes are adapters at framework-owned seams. Storage::fake(), Queue::fake(), Mail::fake(), Http::fake() are the second adapter for filesystem, queue, mail, and HTTP seams: provided free. Prefer them over hand-rolled ports for I/O the framework already owns.
• Eloquent models are a wide, shared interface: every attribute, scope, and relationship is surface area available to every caller. Depth in a Laravel app usually lives above them: an action or service class whose small interface (ReconcileInvoice::run($invoice)) hides the queries, state transitions, and side effects.
• Controllers, commands, jobs, and Livewire/Filament 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).

Designing for testability

Good interfaces make testing natural:

1. Accept dependencies, don't create them.

   // Testable: gateway injected via constructor (container resolves it)
   public function __construct(private PaymentGateway $gateway) {}
   
   // Hard to test: hard-wired inside
   public function process(Order $order): void
   {
       $gateway = new StripeGateway(config('services.stripe.secret'));
   }
   

2. Return results, don't produce side effects.

   // Testable: pure calculation, assert on the return value
   public function calculateDiscount(Cart $cart): Discount
   
   // Hard to test: mutates and persists as a side effect
   public function applyDiscount(Cart $cart): void
   

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 the PHP 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: dependency categories, seam discipline, and replace-don't-layer testing.
• Exploring alternative interfaces: see 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.