Architecture Language

Shared vocabulary for architectural discussions. Used by deepening-review and interface-design. The point is terminology discipline — substituting "service," "API," "boundary," or "component" produces vague suggestions and re-litigates the same ground.

Scope

This vocabulary governs module topology discussions — depth, seams, adapters. It is not a replacement for paradigm-specific vocabularies:

• architecture-review uses SOLID vocabulary (responsibilities, dependencies, layers) — that's a different lens.
• code-quality-audit uses smell vocabulary (Fowler) — also different.
• refactoring-guide uses Fowler's catalog (Extract Function, Move Method, etc.) — also different.

When discussing should this module be deeper, where should the seam go, what does the interface owe its callers — use this vocabulary. When discussing which SOLID principle is violated or which refactoring technique applies — use the paradigm appropriate to the lens.

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Terms

Module

Anything with an interface and an implementation. Deliberately scale-agnostic — applies equally to a function, class, package, or tier-spanning slice.

Avoid: "unit," "component," "service" (each carries paradigm-specific baggage that derails the conversation).

Interface

Everything a caller must know to use the module correctly. Includes the type signature, but also invariants, ordering constraints, error modes, required configuration, and performance characteristics.

Avoid: "API," "signature" (too narrow — those refer only to the type-level surface).

Implementation

The code inside a module — its body. 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. 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" (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 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.

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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. Apply this to anything that looks shallow.

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. Tests at the interface survive internal refactors; tests past the interface break on every implementation change.

One adapter means a hypothetical seam. Two adapters means a real one

Don't introduce a port unless something actually varies across it. A single-adapter seam is just indirection. The typical justification for a real seam: production adapter + test adapter.

Replace, don't layer (testing)

When a shallow module is folded into a deep one, the old unit tests on the shallow piece become waste — delete them. Write new tests at the deepened interface. Don't keep both layers of tests "to be safe" — they pull in opposite directions on the next refactor.

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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.

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Rejected framings

These come up often and weaken the conversation. Reject them when they appear:

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Quick reference card

When proposing an architectural change, every sentence should be expressible in these terms:

• "This module is shallow — its interface is nearly as complex as its implementation. Apply the deletion test: would deleting it concentrate complexity, or just push it onto the N callers?"
• "Put the seam at the module's external interface. Two adapters: production (HTTP) and test (in-memory). The deep module owns the logic; transport is injected."
• "These tests sit past the interface — they assert on internal state. Replace them with tests at the interface so they survive refactors."

If a sentence is classifying parts of the system as "services," "components," or "boundaries" — rewrite it. (Mentioning a third party as "the Stripe service" or referring to "across the network boundary" descriptively is fine. The discipline is about classifying your own parts in this skill's vocabulary, not policing every word you use.)