Scaling WebSocket ServersLesson 6.3

Socket.IO Redis adapter for horizontal scaling

@socket.io/redis-adapter installation, createAdapter configuration, io.adapter(), pub/sub wiring, room broadcast across nodes, adapter internals, cluster mode

Drop-In Redis Adapter for Socket.IO

Socket.IO's Redis adapter replaces the in-memory broadcast mechanism with Redis pub/sub automatically. No changes to your event handlers are needed:

npm install @socket.io/redis-adapter ioredis

const { createClient } = require('redis');
const { createAdapter } = require('@socket.io/redis-adapter');
const { Server } = require('socket.io');
const http = require('http');

const server = http.createServer();
const io = new Server(server);

const pubClient = createClient({ url: 'redis://localhost:6379' });
const subClient = pubClient.duplicate();

await Promise.all([pubClient.connect(), subClient.connect()]);

io.adapter(createAdapter(pubClient, subClient));

io.on('connection', (socket) => {
  socket.join('general');
  // io.to('general').emit() now works across all nodes
  io.to('general').emit('user:joined', { id: socket.id });
});

server.listen(3000);

Two separate Redis clients are required: one for publishing and one for subscribing, because a Redis connection in subscribe mode cannot issue other commands. The adapter handles all routing internally. io.to(room).emit() just works across nodes.