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.