Websocket Patterns

Authentication (Pre-Connection)

// Server middleware (runs before connection event)
io.use((socket, next) => {
  const token = socket.handshake.auth.token;
  try {
    socket.data.user = jwt.verify(token, SECRET);
    next();
  } catch (err) {
    next(new Error('Auth failed'));
  }
});

// Client
const socket = io({ auth: { token: getToken() } });

Message Patterns

Rooms (Pub/Sub)

// Join room
socket.join(roomId);

// Emit to room (including sender)
io.to(roomId).emit('event', data);

// Emit to room (excluding sender)
socket.to(roomId).emit('event', data);

Acknowledgements (Request-Response)

// Server: callback = ack function
socket.on('send-msg', (msg, callback) => {
  if (!msg.text) return callback({ error: 'Required' });
  callback({ success: true, id: saved.id });
});

// Client: wait for ack
socket.emit('send-msg', msg, (response) => {
  if (response.error) handleError();
});

Broadcast

io.emit('event', data);              // All clients
socket.broadcast.emit('event', data); // All except sender

Heartbeat/Ping-Pong (RFC 6455 §5.5.2-5.5.3)

Socket.io handles automatically via pingInterval/pingTimeout. Native WebSocket:

// Client
const ws = new WebSocket('ws://localhost');
ws.addEventListener('open', () => {
  setInterval(() => ws.send('ping'), 30000);
});

Error Handling

Connection Errors

// Client
socket.on('connect_error', (err) => {
  if (err.message === 'Auth failed') redirectLogin();
});

// Server
socket.on('error', (err) => {
  logger.error({ socketId: socket.id, err });
});

Message Validation

socket.on('chat-msg', (msg) => {
  const result = schema.safeParse(msg);
  if (!result.success) return socket.emit('error', 'Invalid');
  // Process
});

Scalability (Multi-Server)

Redis Adapter (Socket.io)

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

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

io.adapter(createAdapter(pubClient, subClient));
// Now events sync across all server instances

Sticky Sessions (Required)

Use IP-based routing or session ID cookies to route client to same server instance.

Resources

• RFC 6455: https://datatracker.ietf.org/doc/html/rfc6455
• Socket.io Docs: https://socket.io/docs/v4/
• MDN WebSocket API: https://developer.mozilla.org/en-US/docs/Web/API/WebSocket
• Redis Adapter: https://socket.io/docs/v4/redis-adapter/