---

name: socket-io description: | Socket.IO real-time bidirectional communication. Rooms, namespaces, acknowledgments, middleware, scaling with Redis adapter, and TypeScript types.

USE WHEN: user mentions "Socket.IO", "socket.io", "real-time chat", "live updates", "bidirectional WebSocket", "rooms", "namespaces"

DO NOT USE FOR: SSE (server-sent events) - use sse; WebRTC - use webrtc; raw WebSocket without Socket.IO - use framework WS skills allowed-tools: Read, Grep, Glob, Write, Edit

Socket.IO

Server Setup

import { Server } from 'socket.io';
import { createServer } from 'http';

const httpServer = createServer(app);

interface ServerToClientEvents {
  message: (data: { user: string; text: string; timestamp: number }) => void;
  userJoined: (user: string) => void;
}

interface ClientToServerEvents {
  sendMessage: (text: string, callback: (status: 'ok' | 'error') => void) => void;
  joinRoom: (room: string) => void;
}

const io = new Server<ClientToServerEvents, ServerToClientEvents>(httpServer, {
  cors: { origin: process.env.CLIENT_URL },
});

// Authentication middleware
io.use((socket, next) => {
  const token = socket.handshake.auth.token;
  try {
    socket.data.user = verifyToken(token);
    next();
  } catch {
    next(new Error('Authentication failed'));
  }
});

io.on('connection', (socket) => {
  console.log(`${socket.data.user.name} connected`);

  socket.on('joinRoom', (room) => {
    socket.join(room);
    socket.to(room).emit('userJoined', socket.data.user.name);
  });

  socket.on('sendMessage', (text, callback) => {
    const message = { user: socket.data.user.name, text, timestamp: Date.now() };
    io.to('general').emit('message', message);
    callback('ok');
  });

  socket.on('disconnect', () => {
    console.log(`${socket.data.user.name} disconnected`);
  });
});

Client Setup

import { io, Socket } from 'socket.io-client';

const socket: Socket<ServerToClientEvents, ClientToServerEvents> = io(API_URL, {
  auth: { token: getAuthToken() },
  reconnection: true,
  reconnectionDelay: 1000,
});

socket.on('connect', () => console.log('Connected'));
socket.on('message', (data) => addMessage(data));
socket.on('connect_error', (err) => console.error(err.message));

// With acknowledgment
socket.emit('sendMessage', 'Hello!', (status) => {
  if (status === 'error') showError('Failed to send');
});

Scaling with Redis Adapter

import { createAdapter } from '@socket.io/redis-adapter';
import { createClient } from 'redis';

const pubClient = createClient({ url: process.env.REDIS_URL });
const subClient = pubClient.duplicate();
await Promise.all([pubClient.connect(), subClient.connect()]);

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

React Hook

function useSocket(room: string) {
  const [messages, setMessages] = useState<Message[]>([]);

  useEffect(() => {
    socket.emit('joinRoom', room);
    socket.on('message', (msg) => setMessages((prev) => [...prev, msg]));
    return () => { socket.off('message'); };
  }, [room]);

  const send = (text: string) => socket.emit('sendMessage', text, () => {});
  return { messages, send };
}

Anti-Patterns

Anti-Pattern	Fix
No authentication	Use io.use() middleware with JWT
Broadcasting to all instead of rooms	Use socket.to(room).emit()
No reconnection handling	Enable reconnection, show UI indicator
Single server without adapter	Use Redis adapter for horizontal scaling
No acknowledgments for critical events	Use callback parameter for delivery confirmation

Production Checklist

• Authentication middleware on connection
• Redis adapter for multi-server deployment
• Reconnection with exponential backoff
• Room-based message scoping
• Connection state UI indicator
• Rate limiting on message events