# DDD & Event Sourcing Patterns

What LLMs get subtly wrong in event-sourced systems. Based on actual code review findings.

## Why This Exists

LLMs know the textbook definitions of event sourcing and DDD. But they make subtle mistakes:

- Adding `DateTime.utc_now()` in apply functions (breaks replay determinism)
- Creating `OrderUpdated` events with a changes map (CRUD-in-disguise)
- Treating projections as source of truth
- Suggesting "fixes" to immutable events
- Making aggregates too large (data grouping vs consistency boundary)
- Multi-aggregate coordination in services without saga patterns

This checklist catches those mistakes.

## Contents

- `DDD-CHECKLIST.md` - Review prompts covering:
  - Replay Determinism (CRITICAL)
  - Event Design
  - Aggregate Boundaries
  - Projections
  - Idempotency
  - Process Managers / Sagas
  - Elixir/OTP Specific
  - Anti-patterns table

## Integration

```yaml
# In your review workflow
patterns-repo: rodin/ddd-patterns
patterns-files: '.'
```

## License

MIT