Aggregates & CQRS
Aggregates are single-writer actors. Tests prove pure business rules and concurrency semantics without a cloud account.
What to unit-test first
| Concern | Assert |
|---|---|
| Happy path decide | Given history + command → expected facts |
| Invariant reject | AGGREGATE_INVARIANT (or domain equivalent) — not retryable |
| OCC conflict | Stale expectedSeq → conflict; reload & retry succeeds |
| Fence | Stale/missing fence fails closed — not treated as OCC |
| Idempotency | Same key does not double-apply effects |
| Fold purity | Fold(events) has no I/O |
Pattern
history facts → fold state → decide(command) → facts
append(expectedSeq, fenceToken?) → store accepts/rejects
projection.fold(facts) → read modelUse in-memory EventStore + ActorHost for pure tests; use SQLite partition tests when proving durability (see Durability).
Multi-aggregate
Cross-aggregate workflows are eventually consistent. Test sagas with correlation ids and failure compensation — do not pretend distributed transactions exist.
Anti-patterns
- Calling wall-clock inside pure decide
- Treating
STALE_FENCEas “retry with same token” - Asserting on projection without replaying from facts
- Sharing one aggregate stream across unbounded entity ids