The C4 drill-down

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# The C4 drill-down

_System → container → component → code item: the four nesting levels and what belongs at each._

View 4 decomposes the estate in four strict levels:

- **System** — one box when you explain the estate to an outsider. Owns containers.
- **Container** — the independently runnable/deployable unit: a service, a database, a web app, a CLI, a message queue, a static site. The test: \*could this run or ship on its own?\* (All WAD blocks live in the `wcl.wad` namespace, so the kind is plain `container` — it never collides with wdoc's diagram shape of the same name.)
- **Component** — a cohesive part inside a container: a module, a handler family, a store. No deployment identity of its own.
- **Code item** — a diagram of code-level structure: a module graph, a DB schema, a class diagram, or an API surface (`kind` selects the renderer). Code items are **generated by extractor scripts** and document the **public interface only** — never hand-drawn mirrors of internal structure.

The drill-down stops at interfaces: internals below the public surface change too often to document and are better read from the code itself.

## Related

- [The twelve views](../references/concept_twelve_views.md)

- [Relations wire the diagrams](../references/concept_relations_model.md)

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