Scaling WebSocket ServersLesson 6.2

How to use Redis pub/sub to broadcast across WebSocket servers

Redis pub/sub model, PUBLISH and SUBSCRIBE commands, channel naming, ioredis client, publish on message receive, subscribe and forward pattern, serialization overhead

Redis as the Cross-Server Message Bus

Each server instance publishes incoming messages to Redis. All instances subscribe and forward messages to their local sockets:

const Redis = require('ioredis');
const pub = new Redis();
const sub = new Redis();

const CHANNEL = 'chat:global';

// Subscribe on startup
sub.subscribe(CHANNEL);
sub.on('message', (channel, rawMsg) => {
  const msg = JSON.parse(rawMsg);
  // Forward to all LOCAL clients
  for (const client of wss.clients) {
    if (client.readyState === WebSocket.OPEN) {
      client.send(rawMsg); // already serialized
    }
  }
});

// When a client sends a message, publish to Redis
ws.on('message', (data) => {
  const msg = JSON.parse(data);
  pub.publish(CHANNEL, JSON.stringify({
    ...msg,
    originServer: process.env.SERVER_ID
  }));
});

Add an originServer field so each server can skip re-delivering to the sender if they are on the same instance. Use per-room channels (chat:room:roomId) to avoid every server processing every message - subscribe only to rooms that have local clients.