Event-Driven Architecture

Event Sourcing

Store events as facts, rebuild state by replaying them.

interface DomainEvent { type: string; aggregateId: string; data: unknown; timestamp: number; }
class OrderAggregate {
    apply(event: DomainEvent) {
        switch(event.type) {
            case 'OrderCreated': this.state.status = 'created'; break;
            case 'OrderShipped': this.state.status = 'shipped'; break;
        }
    }
    static fromEvents(events: DomainEvent[]) { /* replay */ }
}

CQRS

• Command side: writes events to event store
• Query side: reads from materialized projections
• Projection: listens to events, updates read DB

Saga Pattern (distributed transactions)

try {
    await paymentService.charge(order);
    await inventoryService.reserve(order);
} catch { // compensating transactions
    await inventoryService.release(order);
    await paymentService.refund(order);
}

Best Practices

• Events are immutable, past-tense (OrderCreated not CreateOrder)
• Idempotent consumers (same event twice = same result)
• Correlation IDs for distributed tracing
• Dead letter queues for failed messages