Projection
Projection
Section titled “Projection”A query-optimized read model built by a projector from event streams. Projections are the “read side” of CQRS.
Characteristics
Section titled “Characteristics”- Derived: Built from events, not the source of truth
- Disposable: Can be rebuilt by replaying events
- Optimized: Structured for specific query patterns
- Eventually consistent: May lag behind writes
Examples
Section titled “Examples”| Events | Projection | Query Pattern |
|---|---|---|
OrderCreated, ItemAdded | Order summary table | Get order by ID |
PlayerRegistered, FundsDeposited | Player balance view | Get current balance |
HandStarted, BetPlaced | Active hands dashboard | List active hands |
Projection vs Event Store
Section titled “Projection vs Event Store”| Aspect | Projection | Event Store |
|---|---|---|
| Source of truth | No | Yes |
| Structure | Query-optimized | Append-only log |
| Rebuildable | Yes (replay events) | No (is the source) |
| Consistency | Eventually | Immediately |
In Angzarr
Section titled “In Angzarr”Projectors output to:
- Databases (PostgreSQL, Redis)
- APIs (REST endpoints)
- Files (reports, exports)
- External systems (webhooks, queues)