Read Model Projections

Also Known As

TL;DR

Explanation

In CQRS/Event Sourcing, the write model stores domain events. Projections listen to those events and build optimised read models — denormalised, pre-computed, shaped exactly for query needs. A dashboard projection might aggregate order totals per customer; a search projection might build Elasticsearch documents; a reporting projection might maintain monthly revenue. Projections can be rebuilt by replaying all events. Each projection is independent — one event can feed many projections without coupling.

Diagram

flowchart LR
    CMD[Command] --> AGG[Aggregate]
    AGG --> STORE[(Event Store<br/>OrderPlaced<br/>PaymentReceived)]
    STORE -->|project| P1[CustomerStats<br/>projection]
    STORE -->|project| P2[DashboardSummary<br/>projection]
    STORE -->|project| P3[SearchIndex<br/>projection]
    P1 --> R1[(Redis<br/>O of 1 lookup)]
    P2 --> R2[(Postgres<br/>pre-aggregated)]
    P3 --> R3[(Elasticsearch)]
style STORE fill:#6e40c9,color:#fff
style R1 fill:#238636,color:#fff
style R2 fill:#238636,color:#fff
style R3 fill:#1f6feb,color:#fff

Common Misconception

Why It Matters

Common Mistakes

• Projections that query the write model — projections should be fed by events, not queries.
• Not making projections idempotent — event replays must produce the same result; use event IDs to detect re-processing.
• One monolithic projection — separate projections for each use case, each independently rebuildable.
• Not handling projection failures — failed projections cause stale read models; monitor projection lag.

Code Examples

// Slow read — joins at query time:
SELECT c.name, SUM(o.total) as lifetime_value,
       COUNT(o.id) as order_count,
       MAX(o.created_at) as last_order
FROM customers c
JOIN orders o ON o.customer_id = c.id
GROUP BY c.id;
-- Full table scan + aggregation on every dashboard load

// Projection updated on each OrderPlaced event:
class CustomerStatsProjection {
    public function onOrderPlaced(OrderPlaced $event): void {
        $stats = $this->store->find($event->customerId) ?? [
            'order_count' => 0, 'lifetime_value' => 0, 'last_order' => null
        ];
        $stats['order_count']++;
        $stats['lifetime_value'] += $event->total;
        $stats['last_order'] = $event->occurredAt;
        $this->store->save($event->customerId, $stats);
    }
}
// Dashboard query: single key lookup — O(1) regardless of order count

References

• ↗ https://martinfowler.com/bliki/CQRS.html

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