# Language Shared vocabulary for every suggestion this skill makes. Use these terms exactly — don't substitute "component," "service" (we use that narrowly for DI ports), "API," or "boundary" (overloaded with our workspace-tag enforcement). Consistent language is the whole point. This vocabulary is foundational for the skill's reasoning. The project's domain vocabulary lives in [`docs/glossary.md`](../../../docs/glossary.md) — terms like _use case_, _manifest_, _slice_, _binder_, _brand_, _conformance band_, _coverage layer_. Both vocabularies are in scope when proposing deepenings; see the "Mapping to this repo's identifiers" section below for how the abstract terms here land on concrete file shapes. ## Terms **Module** — **in this repo, "module" defaults to "feature"** (`packages//`). The abstract definition (anything with an interface + implementation) still applies at narrower scales — a use case, controller, repository/service port, or binder can also be a module — but **whenever the refactor scope is "the whole thing", say feature**. Reach for "module" only when the abstraction across scales actually matters (e.g., comparing how a use case's depth differs from its containing feature's depth). _Avoid_: unit, component, service (we use "service" for DI ports specifically). **Interface** Everything a caller must know to use the module correctly. Includes the type signature, but also invariants, ordering constraints, error modes, required configuration, performance characteristics, **manifest declarations**, and **DI symbol contract**. _Avoid_: API, signature (too narrow — those 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 Payload-backed repository) or a large adapter with a small implementation (an in-memory mock). 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. A module is **shallow** when the interface is nearly as complex as the implementation. **Seam** _(from Michael Feathers)_ A place where you can alter behaviour without editing in that place. The _location_ at which a module's interface lives. Choosing where to put the seam is its own design decision, distinct from what goes behind it. _Avoid_: boundary (this repo uses "boundary" specifically for ESLint workspace-tag rules — keep it for that meaning). **Adapter** A concrete thing that satisfies an interface at a seam. Describes _role_ (what slot it fills), not substance (what's inside). In this repo every port typically has at least two adapters (real + mock); some have three (real + mock + recording). **Leverage** What callers get from depth. More capability per unit of interface they have to learn. One implementation pays back across N call sites and M tests. **Locality** What maintainers get from depth. Change, bugs, knowledge, and verification concentrate at one place rather than spreading across callers. Fix once, fixed everywhere. ## 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, the module wasn't hiding anything (it was a pass-through). If complexity reappears across N callers, the module 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. In this repo, the typical justification is "one real adapter + one mock for tests" — that's two. - **The manifest is a structural seam.** A feature's `feature.manifest.ts` declares its use cases / events / jobs / channels / required cores / coverage bands. Refactors that move behaviour between features MUST move manifest entries too; the conformance gates enforce this. ## 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. ## Mapping to this repo's identifiers Abstract → concrete translation table. When proposing a deepening, name things using the right column. | Abstract term | Where it lands in this repo | | ------------------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | | **Module** | **Primarily a feature** (`packages//`) — that's the canonical refactor scope. Also: a use case (`*.use-case.ts`), controller (`*.controller.ts`), repository port + adapters (`*.repository.{interface,mock,}.ts`), service port + adapters (`*.service.{interface,mock,}.ts`), binder (`bind-production.ts` / `bind-dev-seed.ts`), manifest (`feature.manifest.ts`), or a core package (`packages/core-/`) — when the refactor operates at those narrower scales. When in doubt, say "feature". | | **Interface** | The exported types from a module file: `IXUseCase = ReturnType`, `IXController`, the `.repository.interface.ts` shape, the manifest's declared keys, the Zod input/output schemas, the DI symbol contract. | | **Implementation** | The factory body, the adapter class body, what the binder wires. | | **Seam** | `.repository.interface.ts`, `.service.interface.ts`, the DI symbol (`*_SYMBOLS.IXRepository`), the manifest entry, a `// ` anchor, the protocol types in `core-shared/di/bind-protocols.ts`. | | **Adapter** | `.repository.ts` (Payload real) ↔ `.repository.mock.ts` (in-memory). For instrumentation: `Noop*` ↔ `Otel*` ↔ `Recording*` (test). For bus: `InMemoryEventBus` ↔ `PayloadJobsEventBus`. | | **Test stand-in** | The `.mock.ts` adapter (constructed directly + injected into the factory). No container rebinding (ADR-012). | ## Rejected framings - **Depth as ratio of implementation-lines to interface-lines** (Ousterhout's original metric): rewards padding the implementation. We use depth-as-leverage instead. - **"Interface" as the TypeScript `interface` keyword or a class's public methods**: too narrow — interface here includes every fact a caller must know, including manifest entries and DI symbols. - **"Boundary"** as a synonym for **seam**: this repo uses "boundary" specifically for ESLint workspace-tag rules (`feature` may depend on `core` + `tooling` only). Keep that meaning intact; say **seam** or **interface** when discussing features. - **"Service" as a generic term**: in this repo, **service** = a DI-injected port for non-collection capabilities (`IAuthenticationService`, `IMailerService`). Not a generic stand-in for "feature" or "the module doing the work." - **"Module" as the canonical noun**: avoid in everyday discourse — say **feature** (or **use case** / **controller** / **package** when narrower). "Module" is the abstract refactor vocabulary's word for the same thing, useful only when the abstraction across scales is the point. ## Cross-references - [`docs/glossary.md`](../../../docs/glossary.md) — project domain vocabulary (use case, manifest, slice, brand, coverage band, etc.) - [`docs/decisions/`](../../../docs/decisions/) — 21 ADRs that constrain the design space - [SKILL.md](SKILL.md) — the skill's process + hard constraints - [DEEPENING.md](DEEPENING.md) — dependency categories + seam discipline - [INTERFACE-DESIGN.md](INTERFACE-DESIGN.md) — parallel sub-agent design exploration