Domain Modeling

---
name: domain-modeling
description: Maintain a project's ubiquitous language and decision record against GLOSSARY.md
when_to_use: Use when defining or disputing domain terminology, when a design session resolves a term that belongs in GLOSSARY.md, or when a decision made mid-session passes the ADR gate.
license: MIT
metadata:
  author: Matt Pocock
  source: Vendored from https://github.com/mattpocock/skills @ 5beb35b
---

# Domain Modeling

Actively build and sharpen the project's domain model as you design. This is the *active* discipline — challenging terms, inventing edge-case scenarios, and writing the glossary and decisions down the moment they crystallise. (Merely *reading* `GLOSSARY.md` for vocabulary is not this skill — that's a one-line habit any skill can do. This skill is for when you're changing the model, not just consuming it.)

## File structure

Most repos have a single context:

```
/
├── GLOSSARY.md
├── docs/
│   └── adr/
│       ├── 0001-event-sourced-orders.md
│       └── 0002-postgres-for-write-model.md
└── src/
```

If a `GLOSSARY-MAP.md` exists at the root, the repo has multiple contexts. The map points to where each one lives:

```
/
├── GLOSSARY-MAP.md
├── docs/
│   └── adr/                          ← system-wide decisions
├── src/
│   ├── ordering/
│   │   ├── GLOSSARY.md
│   │   └── docs/adr/                 ← context-specific decisions
│   └── billing/
│       ├── GLOSSARY.md
│       └── docs/adr/
```

Create files lazily — only when you have something to write. If no `GLOSSARY.md` exists, create one when the first term is resolved. If no `docs/adr/` exists, create it when the first ADR is needed.

**Legacy name.** Repos set up before this convention carry `CONTEXT.md` / `CONTEXT-MAP.md` (the upstream mattpocock/skills name until 1.3.0). Read either name; when a legacy file exists, keep writing to it rather than creating a second glossary beside it, and offer a `git mv` to the new name once.

## During the session

### Challenge against the glossary

When the user uses a term that conflicts with the existing language in `GLOSSARY.md`, call it out immediately. "Your glossary defines 'cancellation' as X, but you seem to mean Y — which is it?"

### Sharpen fuzzy language

When the user uses vague or overloaded terms, propose a precise canonical term. "You're saying 'account' — do you mean the Customer or the User? Those are different things."

### Discuss concrete scenarios

When domain relationships are being discussed, stress-test them with specific scenarios. Invent scenarios that probe edge cases and force the user to be precise about the boundaries between concepts.

### Cross-reference with code

When the user states how something works, check whether the code agrees. If you find a contradiction, surface it: "Your code cancels entire Orders, but you just said partial cancellation is possible — which is right?"

### Update GLOSSARY.md inline

When a term is resolved, update `GLOSSARY.md` right there. Don't batch these up — capture them as they happen. Use the format in [GLOSSARY-FORMAT.md](./references/GLOSSARY-FORMAT.md).

`GLOSSARY.md` carries no implementation details. Do not treat `GLOSSARY.md` as a spec, a scratch pad, or a repository for implementation decisions. It is a glossary and nothing else.

### Offer ADRs sparingly

Only offer to create an ADR when all three are true:

1. **Hard to reverse** — the cost of changing your mind later is meaningful
2. **Surprising without context** — a future reader will wonder "why did they do it this way?"
3. **The result of a real trade-off** — there were genuine alternatives and you picked one for specific reasons

If any of the three is missing, skip the ADR. In orc, ADRs are authored via the `orc-adr-writing` template into `docs/adr/NNNN-*.md` — see [ADR-FORMAT.md](./references/ADR-FORMAT.md) for the lightweight shape and what qualifies, and `orc-adr-writing` for the full house template.