2nd/10_Wiki/Topics/Architecture/Event Mediator.md

---
id: wiki-2026-0508-event-mediator
title: Event Mediator
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Mediator Pattern, Event Bus]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [architecture, design-pattern, event-driven, decoupling]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: typescript
  framework: nodejs
---

# Event Mediator

## 매 한 줄
> **"매 N×N component coupling 의 N×1 mediator hub 의 collapse"**. Event Mediator 매 multiple components 의 direct coupling 의 elimination — 매 components 매 mediator 를 publish/subscribe, 매 mediator 매 routing/orchestration 의 own. GoF Mediator pattern 의 event-driven evolution.

## 매 핵심

### 매 Mediator vs Observer
- **Observer**: 매 1 subject → N observers, 매 unidirectional broadcast.
- **Mediator**: 매 N senders ↔ N receivers, 매 hub 의 bidirectional routing.
- **Event Bus**: 매 Mediator 의 generic implementation — 매 string-keyed event types.

### 매 동기 vs 비동기
- **Sync mediator**: 매 in-process method dispatch — 매 EventEmitter, MediatR.
- **Async mediator**: 매 message queue 의 backed — 매 RabbitMQ, Kafka, Redis Streams.
- **Hybrid**: 매 in-process sync + cross-service async (e.g., NestJS CQRS).

### 매 응용
1. UI components decoupling — chat rooms, form fields.
2. Microservices orchestration — saga coordinator.
3. CQRS command/query bus — MediatR.
4. Game event systems — unit death, achievement triggers.

## 💻 패턴

### Typed Event Bus (TypeScript)
```typescript
type EventMap = {
  'user.signup': { userId: string; email: string };
  'order.placed': { orderId: string; total: number };
  'cache.invalidate': { key: string };
};

class EventMediator<M> {
  private handlers = new Map<keyof M, Set<(p: any) => void | Promise<void>>>();

  on<K extends keyof M>(event: K, handler: (payload: M[K]) => void | Promise<void>) {
    if (!this.handlers.has(event)) this.handlers.set(event, new Set());
    this.handlers.get(event)!.add(handler);
    return () => this.handlers.get(event)!.delete(handler);
  }

  async emit<K extends keyof M>(event: K, payload: M[K]) {
    const hs = this.handlers.get(event);
    if (!hs) return;
    await Promise.all([...hs].map(h => h(payload)));
  }
}

const bus = new EventMediator<EventMap>();
bus.on('user.signup', async ({ userId, email }) => {
  await sendWelcomeEmail(email);
});
```

### Mediator with Request/Response (MediatR-style)
```typescript
interface IRequest<TResponse> { __response?: TResponse }
interface IHandler<TReq extends IRequest<TRes>, TRes> {
  handle(req: TReq): Promise<TRes>;
}

class Mediator {
  private handlers = new Map<Function, IHandler<any, any>>();
  register<T extends IRequest<R>, R>(reqCtor: new (...a: any[]) => T, h: IHandler<T, R>) {
    this.handlers.set(reqCtor, h);
  }
  async send<R>(req: IRequest<R>): Promise<R> {
    const h = this.handlers.get(req.constructor);
    if (!h) throw new Error(`No handler for ${req.constructor.name}`);
    return h.handle(req);
  }
}

class GetUserQuery implements IRequest<User> { constructor(public id: string) {} }
class GetUserHandler implements IHandler<GetUserQuery, User> {
  async handle(q: GetUserQuery) { return db.users.findById(q.id); }
}
```

### Saga Mediator (orchestrated)
```typescript
class OrderSaga {
  constructor(private bus: EventMediator<OrderEvents>) {
    bus.on('order.created', this.onCreated.bind(this));
    bus.on('payment.failed', this.onPaymentFailed.bind(this));
  }
  async onCreated({ orderId }: { orderId: string }) {
    await this.bus.emit('payment.requested', { orderId });
  }
  async onPaymentFailed({ orderId, reason }: any) {
    await this.bus.emit('order.cancelled', { orderId, reason });
    await this.bus.emit('inventory.released', { orderId });
  }
}
```

### Middleware Pipeline
```typescript
type Middleware<E> = (event: E, next: () => Promise<void>) => Promise<void>;

class PipelineMediator<E> {
  private middlewares: Middleware<E>[] = [];
  use(m: Middleware<E>) { this.middlewares.push(m); }
  async dispatch(event: E) {
    let i = -1;
    const run = async (idx: number): Promise<void> => {
      if (idx <= i) throw new Error('next() called twice');
      i = idx;
      const fn = this.middlewares[idx];
      if (fn) await fn(event, () => run(idx + 1));
    };
    await run(0);
  }
}
// usage: logging, validation, retry, dead-letter
```

### Cross-Service Mediator (Kafka)
```typescript
import { Kafka } from 'kafkajs';

const kafka = new Kafka({ brokers: ['localhost:9092'] });
const producer = kafka.producer();
const consumer = kafka.consumer({ groupId: 'order-svc' });

await consumer.subscribe({ topic: 'orders', fromBeginning: false });
await consumer.run({
  eachMessage: async ({ message }) => {
    const evt = JSON.parse(message.value!.toString());
    await handler[evt.type]?.(evt.payload);
  },
});

await producer.send({
  topic: 'orders',
  messages: [{ key: orderId, value: JSON.stringify({ type: 'order.placed', payload }) }],
});
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| 2-3 components 의 direct call | 매 mediator skip — 매 over-engineering |
| 5+ components, fan-out broadcast | 매 in-process EventEmitter |
| CQRS / clean architecture | 매 MediatR-style request bus |
| Cross-service, durability | 매 Kafka / RabbitMQ |
| Long-running workflow | 매 saga orchestrator (Temporal, Inngest) |

**기본값**: 매 typed in-process EventEmitter (Node) 또는 MediatR (.NET) — 매 cross-service 의 Kafka.

## 🔗 Graph
- 부모: [[Design-Patterns]] · [[Event-Driven-Architecture]]
- 변형: [[Observer-Pattern]] · [[Pub-Sub]]
- 응용: [[CQRS]] · [[Microservices]]
- Adjacent: [[Event-Sourcing]]

## 🤖 LLM 활용
**언제**: 매 N×N coupling 의 emerge — 매 5+ components 매 mutual callbacks. CQRS / saga implementation.
**언제 X**: 매 simple 2-component dependency — 매 direct injection 매 sufficient. Hot-path latency-critical (mediator dispatch overhead).

## ❌ 안티패턴
- **God Mediator**: 매 mediator 매 business logic 의 absorb — 매 anemic handlers, 매 mediator 의 1000+ lines. Mediator 의 routing only.
- **Event soup**: 매 untyped string events — 매 'user_signup' vs 'userSignup' typo 의 silent failure. Typed map 의 use.
- **Sync chain pretending async**: 매 emit() 매 await chain 의 200ms latency — 매 async queue 의 use.
- **Lost events**: 매 in-memory bus 매 crash 의 lose — 매 durability 매 required 의 persistent queue.

## 🧪 검증 / 중복
- Verified (GoF Design Patterns; MediatR docs; NestJS CQRS).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — Event Mediator pattern 의 typed/saga/Kafka 의 5 patterns |