Fan-Out Patterns: Server-Side Rooms vs Pub-Sub Broker for Multi-Server Scale

A WebSocket server with in-memory rooms works on a single instance, but when scaled horizontally, clients connected to different server instances cannot receive messages meant for their room because each server has only local room state.

Use a pub-sub broker (Redis Pub/Sub, NATS) as the shared message bus between server instances. Each server subscribes to room topics and forwards messages to its locally connected clients.

import { createClient } from 'redis';

const pub = createClient();
const sub = createClient();
await pub.connect();
await sub.connect();

// Subscribe this server instance to a room
async function joinRoom(roomId: string, ws: WebSocket) {
  localRooms.get(roomId)?.add(ws);

  // Only subscribe Redis channel once per room per server
  if (!redisSubscriptions.has(roomId)) {
    await sub.subscribe(roomId, (message) => {
      for (const client of localRooms.get(roomId) ?? []) {
        if (client.readyState === WebSocket.OPEN) client.send(message);
      }
    });
    redisSubscriptions.add(roomId);
  }
}

// Publish to all servers
async function broadcastToRoom(roomId: string, data: unknown) {
  await pub.publish(roomId, JSON.stringify(data));
}

In-memory rooms are local to the process. Redis Pub/Sub broadcasts a message to all subscribers across all server instances simultaneously. Each instance filters to only its local WebSocket clients for final delivery.