2nd/10_Wiki/Topics/Architecture/Message-Queues-and-Event-Streams.md

---
id: wiki-2026-0508-message-queues-and-event-streams
title: Message Queues and Event Streams
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Queue vs Stream, MQ vs Streaming]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [messaging, queues, streams, kafka, sqs]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: multi
  framework: kafka-sqs-rabbitmq
---

# Message Queues and Event Streams

## 매 한 줄
> **"매 Queue 는 work 를 distribute, 매 Stream 은 history 를 record."**. Queue 와 stream 은 매 둘 다 producer-consumer 를 decouple 하지만 매 mental model 이 다르다 — 매 queue 는 "처리할 일" (소비 후 사라짐), 매 stream 은 "발생한 사건" (immutable log, 매 replay 가능). 매 modern system 은 둘을 매 함께 사용.

## 매 핵심

### 매 Queue 의 본질
- **Consume = delete** (또는 ack 후 hidden).
- 매 1 message → 매 1 worker (work distribution).
- 매 short retention (ack 후 사라짐).
- 매 Examples: SQS, RabbitMQ classic, Redis list (BRPOP).

### 매 Stream 의 본질
- **Append-only immutable log**.
- 매 모든 consumer 가 매 모든 event 읽음 (각자 offset).
- 매 long retention (days~weeks~forever).
- 매 replay 가능.
- 매 Examples: Kafka, Kinesis, Pulsar, Redis Streams.

### 매 비교
| 측면 | Queue | Stream |
|---|---|---|
| Consume | destructive | non-destructive |
| Retention | 짧음 (ack 후) | 긺 (time/size 기반) |
| Replay | X | O |
| Ordering | 약함 (per-queue) | 강함 (per-partition) |
| Throughput | 중 | 매 high (millions/sec) |
| Use case | task distribution | event sourcing, analytics, ML |

### 매 함께 쓰기
- 매 Stream → consumer 가 매 work item 추출 → 매 Queue 에 push (workflow orchestration).
- 매 Queue 에서 매 ack 후 매 audit event 를 매 stream 에 publish.

## 💻 패턴

### Queue: SQS worker (Node.js)
```javascript
import { SQSClient, ReceiveMessageCommand, DeleteMessageCommand } from '@aws-sdk/client-sqs';

const sqs = new SQSClient({});
const QueueUrl = process.env.QUEUE_URL;

while (true) {
  const { Messages } = await sqs.send(new ReceiveMessageCommand({
    QueueUrl,
    MaxNumberOfMessages: 10,
    WaitTimeSeconds: 20,  // long polling
    VisibilityTimeout: 60,
  }));
  if (!Messages) continue;

  for (const msg of Messages) {
    try {
      await processJob(JSON.parse(msg.Body));
      await sqs.send(new DeleteMessageCommand({ QueueUrl, ReceiptHandle: msg.ReceiptHandle }));
    } catch (e) {
      // Don't delete → message becomes visible again after VisibilityTimeout → retry → DLQ after maxReceiveCount
    }
  }
}
```

### Stream: Kafka consumer with offset (Node.js)
```javascript
import { Kafka } from 'kafkajs';

const consumer = new Kafka({ brokers: ['kafka:9092'] }).consumer({
  groupId: 'analytics-v2',
});
await consumer.subscribe({ topic: 'user-events', fromBeginning: true });
// fromBeginning: true → re-read entire history (replay)

await consumer.run({
  eachMessage: async ({ topic, partition, message }) => {
    const event = JSON.parse(message.value.toString());
    await projectIntoDB(event);
    // Offset auto-committed (or manual via heartbeat)
  },
});
```

### Stream: replay from specific offset
```javascript
// Reset consumer group to specific offset (admin operation)
import { Kafka } from 'kafkajs';
const admin = new Kafka({ brokers: ['kafka:9092'] }).admin();
await admin.connect();
await admin.resetOffsets({
  groupId: 'analytics-v2',
  topic: 'user-events',
  earliest: true,  // or specific offset / timestamp
});
// Next consumer.run() will re-read from beginning
```

### Redis Streams (XADD / XREADGROUP)
```javascript
import Redis from 'ioredis';
const redis = new Redis();

// Producer
await redis.xadd('events', '*', 'type', 'order.created', 'data', JSON.stringify(order));

// Consumer group
await redis.xgroup('CREATE', 'events', 'workers', '$', 'MKSTREAM').catch(() => {});

// Consume
while (true) {
  const res = await redis.xreadgroup(
    'GROUP', 'workers', 'worker-1',
    'COUNT', 10, 'BLOCK', 5000,
    'STREAMS', 'events', '>'
  );
  if (!res) continue;
  for (const [, entries] of res) {
    for (const [id, fields] of entries) {
      await process(fields);
      await redis.xack('events', 'workers', id);
    }
  }
}
```

### Outbox → Stream (CDC pattern)
```yaml
# Debezium reads DB WAL → publishes to Kafka topic
# → other services subscribe to data changes without polling
connector: debezium-postgres
config:
  database.hostname: db
  database.dbname: orders
  table.include.list: public.orders, public.outbox
  topic.prefix: cdc
# → cdc.public.orders, cdc.public.outbox topics created
```

### Choice: 매 task vs event
```
Q1: "결과를 매 누가 처리하면 끝나나?" → Queue (1 worker)
Q2: "여러 service 가 매 동일 event 에 react?" → Stream (fan-out)
Q3: "역사를 replay 해야?" → Stream
Q4: "Order 가 strict (per-key)?" → Stream (partition by key)
Q5: "단순 background job?" → Queue
```

## 매 결정 기준
| 요구 | 선택 |
|---|---|
| Background job (이메일, 썸네일) | Queue (SQS, BullMQ) |
| Event sourcing / CDC | Stream (Kafka) |
| Real-time analytics | Stream (Kafka, Kinesis) |
| 매 strict ordering per user | Stream + partition key |
| Pub/Sub broadcast | Stream 또는 Pub/Sub |
| Workflow orchestration | Queue + worker (Temporal, Step Functions) |
| 매 audit log | Stream (immutable retention) |

**기본값**: 매 task 면 queue, 매 event 면 stream. 매 둘 다 필요하면 매 함께 사용.

## 🔗 Graph
- 부모: [[Message Broker]] · [[Distributed Systems]]
- 변형: [[Kafka]] · [[SQS]]
- 응용: [[Event Sourcing]] · [[CQRS]] · [[CDC]]
- Adjacent: [[Dead Letter Queue]]

## 🤖 LLM 활용
**언제**: messaging architecture 결정, queue vs stream 선택, replay 요구사항 평가.
**언제 X**: 매 simple sync RPC 로 충분한 매 internal call.

## ❌ 안티패턴
- **Stream 을 queue 로**: 매 short retention 만 쓰면 매 stream 의 강점 (replay) 못 살림.
- **Queue 로 broadcast**: 매 fan-out 에 queue → 매 consumer 마다 별도 queue 만들어야 → 매 stream 이 정답.
- **Partition key 무시**: 매 stream 에서 매 ordering 필요한데 매 random key → 매 race condition.
- **Retention infinity**: 매 cost 폭주 — 매 compaction / time-based 설정.

## 🧪 검증 / 중복
- Verified (Kafka docs, AWS SQS/Kinesis docs, Confluent blog, Martin Kleppmann "DDIA" Ch 11).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — queue vs stream 비교 + SQS/Kafka/Redis Streams 패턴 |