[G1-Sync] Manual knowledge update

10_Wiki/Topics/Coding/Backend_WebSocket_Scaling.md

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+---
+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
+- [[Backend_SSE_Server_Sent_Events]]
+- [[Realtime_Architecture_Patterns]]
+- [[WebSocket_Authentication_Patterns]]