2nd/10_Wiki/Topics/AI_and_ML/Event Sourcing Pattern.md

---
id: wiki-2026-0508-event-sourcing-pattern
title: Event Sourcing Pattern
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [event sourcing, ES, append-only, event store, projection, snapshot]
duplicate_of: none
source_trust_level: A
confidence_score: 0.95
verification_status: applied
tags: [architecture, event-sourcing, cqrs, domain-driven-design, audit, kafka]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: TypeScript / Java
  framework: EventStoreDB / Kafka / Axon
---

# Event Sourcing

## 매 한 줄
> **"매 state 의 store 의 X — 매 event 의 sequence 의 store"**. 매 current state = 매 replay 의 result. 매 audit + 매 temporal query + 매 CQRS 의 partner. 매 modern: 매 Kafka, EventStoreDB, Axon.

## 매 핵심

### 매 vs CRUD
- **CRUD**: 매 row 의 update.
- **ES**: 매 immutable event 의 append.
- **Projection**: 매 event → 매 read model.

### 매 component
- **Event store**: 매 append-only.
- **Aggregate**: 매 command → events.
- **Projection / read model**: 매 query.
- **Snapshot**: 매 replay 최적화.
- **Outbox**: 매 publish reliability.

### 매 응용
1. **Banking**: 매 audit.
2. **E-commerce**: 매 order lifecycle.
3. **Compliance**: 매 immutable history.
4. **Collaboration**: 매 Figma.
5. **Game state**: 매 replay.

### 매 trade-off
- ✅ Audit, replay, temporal query, decouple.
- ❌ Complexity, eventual consistency, schema evolution.

## 💻 패턴

### Event store (TypeScript)
```typescript
type DomainEvent = { aggregateId: string; type: string; payload: object; version: number; timestamp: Date };

class EventStore {
  private events: DomainEvent[] = [];
  
  append(event: DomainEvent) {
    const last = this.events.filter(e => e.aggregateId === event.aggregateId).pop();
    const expectedVersion = (last?.version ?? 0) + 1;
    if (event.version !== expectedVersion) throw new Error('Concurrency conflict');
    this.events.push(event);
  }
  
  load(aggregateId: string): DomainEvent[] {
    return this.events.filter(e => e.aggregateId === aggregateId);
  }
}
```

### Aggregate (replay)
```typescript
class Order {
  state = { id: '', status: 'NEW', items: [] as any[], version: 0 };
  
  static fromHistory(events: DomainEvent[]): Order {
    const o = new Order();
    events.forEach(e => o.apply(e));
    return o;
  }
  
  apply(event: DomainEvent) {
    switch (event.type) {
      case 'OrderCreated': this.state.id = event.payload.id; break;
      case 'ItemAdded': this.state.items.push(event.payload); break;
      case 'OrderPlaced': this.state.status = 'PLACED'; break;
    }
    this.state.version = event.version;
  }
  
  addItem(item: any): DomainEvent {
    if (this.state.status !== 'NEW') throw new Error('Cannot modify');
    const event = { aggregateId: this.state.id, type: 'ItemAdded', payload: item, version: this.state.version + 1, timestamp: new Date() };
    this.apply(event);
    return event;
  }
}
```

### Projection
```typescript
class OrderListProjection {
  private orders = new Map<string, any>();
  
  handle(event: DomainEvent) {
    switch (event.type) {
      case 'OrderCreated':
        this.orders.set(event.aggregateId, { id: event.aggregateId, status: 'NEW', total: 0 });
        break;
      case 'ItemAdded':
        const o = this.orders.get(event.aggregateId);
        if (o) o.total += event.payload.price;
        break;
      case 'OrderPlaced':
        this.orders.get(event.aggregateId).status = 'PLACED';
        break;
    }
  }
  
  query() { return Array.from(this.orders.values()); }
}
```

### Snapshot
```typescript
class SnapshotStore {
  async save(agg: any, version: number) {
    await db.snapshots.put({ aggregateId: agg.id, snapshot: agg.state, version });
  }
  
  async loadOrReplay(aggId: string, eventStore: EventStore): Promise<Order> {
    const snap = await db.snapshots.get(aggId);
    const o = new Order();
    if (snap) o.state = snap.snapshot;
    const events = eventStore.load(aggId).filter(e => e.version > (snap?.version ?? 0));
    events.forEach(e => o.apply(e));
    return o;
  }
}
```

### Kafka publish (outbox)
```typescript
async function placeOrder(orderId: string) {
  await db.transaction(async tx => {
    const events = order.place();
    await tx.events.append(events);
    await tx.outbox.insert(events.map(e => ({ topic: 'orders', payload: e })));
  });
}
// 매 outbox poller → Kafka
```

### Schema evolution
```typescript
function upgradeEvent(e: any) {
  if (e.type === 'OrderPlacedV1') {
    return { ...e, type: 'OrderPlaced', payload: { ...e.payload, currency: 'USD' } };
  }
  return e;
}
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| Audit-critical | ES + projection |
| CRUD simple | Stay CRUD |
| Temporal query | ES |
| Collaboration | ES + CRDT |
| Distributed system | ES + Kafka outbox |
| Compliance | ES + immutable store |

**기본값**: 매 high-stakes domain = ES + CQRS + outbox + snapshot. 매 simple = CRUD.

## 🔗 Graph
- 부모: [[Architecture]] · [[Domain-Driven-Design]]
- 변형: [[CQRS]]
- 응용: [[Kafka]]
- Adjacent: [[Event-Driven-Architecture]]

## 🤖 LLM 활용
**언제**: 매 audit. 매 temporal. 매 collaboration.
**언제 X**: 매 simple CRUD.

## ❌ 안티패턴
- **No snapshot**: 매 replay slow.
- **No version check**: 매 concurrency conflict.
- **Mutate event**: 매 ES violation.
- **No upgrade path**: 매 schema break.
- **Synchronous projection**: 매 latency.

## 🧪 검증 / 중복
- Verified (Fowler ES, Greg Young, Vernon DDD).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-04-20 | Auto-reinforced |
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — ES + 매 store / aggregate / projection / snapshot / outbox code |