2nd/10_Wiki/Topics/Coding/Backend_WebSocket_Scaling.md

---
id: backend-websocket-scaling
title: WebSocket Scaling — Pub/Sub / Sticky / Heartbeat
category: Coding
status: draft
source_trust_level: B
verification_status: conceptual
created_at: 2026-05-09
updated_at: 2026-05-09
tags: [backend, websocket, scaling, pubsub, vibe-coding]
tech_stack: { language: "TS / Node / Redis", applicable_to: ["Backend"] }
applied_in: []
aliases: [WS scaling, Redis pub/sub, sticky session, socket.io adapter]
---

# WebSocket Scaling

> 1대 서버 = 수만 connections OK. **N 대 서버 분산 시** 메시지 중계가 핵심. Redis pub/sub, NATS, Kafka 사용. **Sticky session + heartbeat + reconnect with backoff**.

## 📖 핵심 개념
- 1 connection = 한 process 메모리. 1대 ~50K 까지 보통.
- Multi-node 시 client A → server 1, client B → server 2 — 어떻게 broadcast?
- Heartbeat: idle conn 정리, NAT timeout 방지.

## 💻 코드 패턴

### 단일 서버 — ws
```ts
import { WebSocketServer } from 'ws';

const wss = new WebSocketServer({ port: 8080 });
const rooms = new Map<string, Set<WebSocket>>();

wss.on('connection', (ws, req) => {
  const room = new URL(req.url!, 'http://x').searchParams.get('room')!;
  if (!rooms.has(room)) rooms.set(room, new Set());
  rooms.get(room)!.add(ws);

  ws.on('message', (data) => {
    for (const peer of rooms.get(room)!) {
      if (peer !== ws && peer.readyState === ws.OPEN) peer.send(data);
    }
  });

  ws.on('close', () => rooms.get(room)?.delete(ws));
});
```

### Multi-node — Redis pub/sub
```ts
import Redis from 'ioredis';

const pub = new Redis(url);
const sub = new Redis(url);

sub.subscribe('room:42', () => {});
sub.on('message', (channel, msg) => {
  const room = channel.split(':')[1];
  for (const peer of localRooms.get(room) ?? []) peer.send(msg);
});

// 메시지 도착하면
pub.publish(`room:${room}`, JSON.stringify(message));
```

### Heartbeat (양방향)
```ts
function heartbeat(ws: WebSocket) {
  let alive = true;
  ws.on('pong', () => { alive = true; });

  const t = setInterval(() => {
    if (!alive) { ws.terminate(); return; }
    alive = false;
    ws.ping();
  }, 30_000);

  ws.on('close', () => clearInterval(t));
}
```

### Reconnect (client) with backoff
```ts
class ReconnectingWS {
  private retries = 0;
  connect() {
    this.ws = new WebSocket(this.url);
    this.ws.onopen = () => { this.retries = 0; };
    this.ws.onclose = () => {
      const delay = Math.min(1000 * 2 ** this.retries, 30_000) + Math.random() * 1000;
      this.retries++;
      setTimeout(() => this.connect(), delay);
    };
  }
}
```

### Backpressure
```ts
// ws send 가 buffer 한계 초과 시
if (ws.bufferedAmount > 1_000_000) {
  // drop or disconnect — 느린 client 가 메모리 고갈
  ws.close(1009, 'too slow');
}
```

### LB sticky (cookie or IP hash)
```nginx
upstream ws_backend {
    ip_hash; # 간단
    server ws1:8080;
    server ws2:8080;
}
server {
    location /ws {
        proxy_pass http://ws_backend;
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "upgrade";
        proxy_read_timeout 3600s;
    }
}
```

### Auth (during handshake)
```ts
wss.on('connection', (ws, req) => {
  const token = new URL(req.url!, 'http://x').searchParams.get('token');
  const user = verifyJwt(token);
  if (!user) return ws.close(4401, 'unauth');
  ws.user = user;
});
```

## 🤔 의사결정 기준
| 규모 | 솔루션 |
|---|---|
| <10K conn | 단일 Node + ws |
| <100K | 다중 Node + Redis pub/sub |
| 100K+ | NATS / Kafka / 전용 service (Centrifugo, Soketi) |
| Pub/sub + 메시지 영속 | Kafka / Redis Streams |
| 게임 (low latency) | UDP / WebRTC |
| Chat / 알림 | WebSocket / SSE |

## ❌ 안티패턴
- **Heartbeat 없음**: NAT timeout (60s+) 후 dead conn 남음.
- **Reconnect 즉시 무한**: 서버 다운 시 thundering herd.
- **Auth 만 메시지 첫 번째로**: 무인증 conn 점유. handshake 에서.
- **Send back-pressure 무시**: 메모리 폭발.
- **Single-node assumption**: 두 서버 띄우면 broadcast 안 됨.
- **메시지에 PII 그대로**: TLS 필수 (wss).
- **Reconnect 시 missed message 무시**: server 측 message id 큐 + replay.

## 🤖 LLM 활용 힌트
- Heartbeat (30s) + reconnect (exponential + jitter) + sticky (ip_hash).
- N 노드 = Redis pub/sub.

## 🔗 관련 문서