Domain Modeling

ADR Format

ADRs live in docs/adr/ and use sequential numbering: 0001-slug.md, 0002-slug.md, etc.

Create the docs/adr/ directory lazily: only when the first ADR is needed.

Template

# {Short title of the decision}

{1-3 sentences: what's the context, what did we decide, and why.}

That's it. An ADR can be a single paragraph. The value is in recording that a decision was made and why, not in filling out sections.

Optional sections

Only include these when they add genuine value. Most ADRs won't need them.

• Status frontmatter (proposed | accepted | deprecated | superseded by ADR-NNNN): useful when decisions are revisited

• Considered Options: only when the rejected alternatives are worth remembering

• Consequences: only when non-obvious downstream effects need to be called out

Numbering

Scan docs/adr/ for the highest existing number and increment by one.

When to offer an ADR

All three of these must be true:

1. Hard to reverse: the cost of changing your mind later is meaningful

2. Surprising without context: a future reader will look at the code and wonder "why on earth 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 a decision is easy to reverse, skip it: you'll just reverse it. If it's not surprising, nobody will wonder why. If there was no real alternative, there's nothing to record beyond "we did the obvious thing."

What qualifies

• Architectural shape. "We're using a monorepo." "The write model is event-sourced, the read model is projected into Postgres."

• Integration patterns between contexts. "Ordering and Billing communicate via domain events, not synchronous HTTP."

• Technology choices that carry lock-in. Database, message bus, auth provider, deployment target. Not every library: just the ones that would take a quarter to swap out.

• Boundary and scope decisions. "Customer data is owned by the Customer context; other contexts reference it by ID only." The explicit no-s are as valuable as the yes-s.

• Deliberate deviations from the obvious path. "We're using manual SQL instead of an ORM because X." Anything where a reasonable reader would assume the opposite. These stop the next engineer from "fixing" something that was deliberate.

• Constraints not visible in the code. "We can't use AWS because of compliance requirements." "Response times must be under 200ms because of the partner API contract."

• Rejected alternatives when the rejection is non-obvious. If you considered GraphQL and picked REST for subtle reasons, record it; otherwise someone will suggest GraphQL again in six months.