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

id, title, category, status, source_trust_level, verification_status, created_at, updated_at, tags, tech_stack, applied_in, aliases

id	title	category	status	source_trust_level	verification_status	created_at	updated_at	tags	tech_stack	applied_in	aliases
backend-websocket-scaling	WebSocket Scaling — Pub/Sub / Sticky / Heartbeat	Coding	draft	B	conceptual	2026-05-09	2026-05-09	backend websocket scaling pubsub vibe-coding	language applicable_to TS / Node / Redis Backend		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

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

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)

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

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

// 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)

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)

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

• Backend_SSE_Server_Sent_Events
• Realtime_Architecture_Patterns
• WebSocket_Authentication_Patterns