2nd/10_Wiki/Topics/Architecture/Message Broker.md

---
id: wiki-2026-0508-message-broker
title: Message Broker
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Message Brokers, MQ, Event Broker, Pub-Sub Broker]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [messaging, distributed, kafka, rabbitmq, nats]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: multi
  framework: kafka-rabbitmq-nats
---

# Message Broker

## 매 한 줄
> **"매 producer 와 consumer 사이의 매 async middleman."**. Message broker 는 매 service 간 매 decouple, 매 buffering, 매 fan-out 을 제공하는 매 infrastructure. 매 2026 의 big three 는 매 Kafka (event streaming), 매 RabbitMQ (traditional queue), 매 NATS (lightweight low-latency). 매 cloud-native 는 SQS / Pub/Sub / EventBridge 도 매 흔함.

## 매 핵심

### 매 왜 broker
- **Decoupling**: producer 가 consumer 의 위치/존재 모름.
- **Buffering**: spike 흡수 — consumer 가 천천히 처리.
- **Fan-out**: 1 message → N consumer.
- **Reliability**: persistent queue → consumer crash 후 replay.
- **Async**: producer 가 response 안 기다림.

### 매 model
- **Queue (point-to-point)**: 1 message → 1 consumer (load balance).
- **Pub/Sub (topic)**: 1 message → N subscriber (broadcast).
- **Event log**: 매 immutable append-only — replay/time-travel (Kafka).

### 매 보장
- **At-most-once**: 매 fast, 매 loss 가능.
- **At-least-once**: 매 default, 매 duplicate 가능 → idempotent consumer 필요.
- **Exactly-once**: 매 어렵 (Kafka EoS transactions, idempotent producer + transactional consumer).

### 매 Kafka 핵심
- **Topic** → **Partition** (parallelism unit) → **Offset**.
- 매 Consumer group 으로 partition load balance.
- 매 retention (time / size) — log 그대로 저장.
- 매 replay 가능 — offset reset.

### 매 RabbitMQ 핵심
- **Exchange** (direct/topic/fanout/headers) → **Binding** → **Queue**.
- 매 AMQP 0.9.1 (현재) / AMQP 1.0 / MQTT / STOMP.
- 매 ack/nack, dead-letter exchange.
- 매 message TTL, priority queue.

### 매 NATS 핵심
- 매 lightweight, sub-ms latency.
- 매 Core NATS (fire-and-forget) + JetStream (persistence + at-least-once).
- 매 subject hierarchy: `orders.eu.de`.

## 💻 패턴

### Kafka producer/consumer (Node.js, kafkajs)
```javascript
import { Kafka } from 'kafkajs';

const kafka = new Kafka({ brokers: ['kafka:9092'], clientId: 'orders-svc' });

// Producer
const producer = kafka.producer({ idempotent: true });  // exactly-once-ish
await producer.connect();
await producer.send({
  topic: 'orders',
  messages: [{
    key: order.userId,  // partition key → ordering per user
    value: JSON.stringify(order),
  }],
});

// Consumer
const consumer = kafka.consumer({ groupId: 'fulfillment' });
await consumer.subscribe({ topic: 'orders', fromBeginning: false });
await consumer.run({
  eachMessage: async ({ message }) => {
    const order = JSON.parse(message.value.toString());
    await fulfill(order);  // must be idempotent
  },
});
```

### RabbitMQ (Node.js, amqplib)
```javascript
import amqp from 'amqplib';

const conn = await amqp.connect('amqp://localhost');
const ch = await conn.createChannel();

// Topic exchange + DLX
await ch.assertExchange('orders', 'topic', { durable: true });
await ch.assertQueue('fulfillment', {
  durable: true,
  deadLetterExchange: 'orders.dlx',
});
await ch.bindQueue('fulfillment', 'orders', 'order.created.*');

// Publish
ch.publish('orders', 'order.created.eu', Buffer.from(JSON.stringify(order)),
  { persistent: true });

// Consume with manual ack
ch.consume('fulfillment', async (msg) => {
  try {
    await fulfill(JSON.parse(msg.content.toString()));
    ch.ack(msg);
  } catch (e) {
    ch.nack(msg, false, false);  // → DLX
  }
}, { noAck: false });
```

### NATS JetStream (Node.js)
```javascript
import { connect, JSONCodec } from 'nats';

const nc = await connect({ servers: 'nats://localhost:4222' });
const js = nc.jetstream();
const jc = JSONCodec();

// Producer
await js.publish('orders.created.eu', jc.encode(order));

// Durable consumer
const sub = await js.subscribe('orders.>', {
  config: { durable_name: 'fulfillment', ack_policy: 'explicit' },
});
for await (const m of sub) {
  await fulfill(jc.decode(m.data));
  m.ack();
}
```

### Idempotent consumer (deduplication)
```javascript
async function handleMessage(msg) {
  const id = msg.headers['message-id'];
  // Try insert into processed table — unique constraint on id
  const inserted = await db.query(
    'INSERT INTO processed (id, ts) VALUES ($1, NOW()) ON CONFLICT DO NOTHING',
    [id]
  );
  if (inserted.rowCount === 0) return;  // already processed
  await businessLogic(msg);
}
```

### Outbox pattern (transactional publish)
```sql
-- Same DB transaction: write business state + outbox entry
BEGIN;
  UPDATE orders SET status='paid' WHERE id=$1;
  INSERT INTO outbox (topic, payload, ts) VALUES ('orders.paid', $2, NOW());
COMMIT;

-- Separate process: poll outbox → publish to broker → mark sent
```

## 매 결정 기준
| 상황 | Broker |
|---|---|
| Event streaming, replay, analytics | Kafka |
| Complex routing, traditional queue | RabbitMQ |
| Sub-ms latency, microservices | NATS |
| AWS-native, simple | SQS + SNS |
| GCP-native | Pub/Sub |
| Azure-native | Service Bus / Event Hubs |
| Single-process / dev | Redis Streams, in-memory |

**기본값**: 매 cloud-native team → managed (SQS/Pub-Sub). 매 self-host + event log → Kafka. 매 traditional queue → RabbitMQ.

## 🔗 Graph
- 부모: [[Distributed Systems]] · [[Event-Driven Architecture]]
- 변형: [[Kafka]] · [[RabbitMQ]] · [[NATS]] · [[SQS]] · [[Pub-Sub]]
- 응용: [[Microservices]] · [[Event Sourcing]] · [[CQRS]]
- Adjacent: [[Message-Queues-and-Event-Streams]] · [[Dead Letter Queue]]

## 🤖 LLM 활용
**언제**: service decoupling, async workflow, event-driven design, spike absorption.
**언제 X**: 매 simple sync RPC 면 충분 — 매 broker = operational overhead.

## ❌ 안티패턴
- **At-most-once 로 critical data**: 매 loss 허용 안되면 매 ack 필수.
- **No idempotency**: 매 at-least-once 에서 매 duplicate → side effect 2번.
- **Big payload**: 매 broker 에 large blob → 매 S3 reference 패턴 사용.
- **No DLQ**: 매 poison message 가 매 무한 retry → consumer 마비.
- **Kafka 를 queue 로**: 매 ordering 만 필요한 매 short job → RabbitMQ 가 더 적합.

## 🧪 검증 / 중복
- Verified (Apache Kafka docs, RabbitMQ docs, NATS docs, Confluent blog, Microservices.io patterns).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — Kafka/RabbitMQ/NATS 비교 + outbox/idempotent 패턴 |