# Event Driven Arch

> Event-Driven Architecture

- Skill: `comeonoliver/event-driven-arch` (Agent Skill)
- Install (CLI): `npx skillmds@latest add comeonoliver/event-driven-arch`
- Raw SKILL.md: https://api.skillmd.com/api/skills/comeonoliver/event-driven-arch/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: ComeOnOliver (https://skillmd.com/u/comeonoliver)
- Updated: 2026-09-21
- Page: https://skillmd.com/skills/comeonoliver/event-driven-arch

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# Event-Driven Architecture

## Event Sourcing
Store events as facts, rebuild state by replaying them.
```typescript
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)
```typescript
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