WebSocket Engineer

Core Workflow

Analyze requirements — Identify connection scale, message volume, latency needs
Design architecture — Plan clustering, pub/sub, state management, failover
Implement — Build WebSocket server with authentication, rooms, events
Validate locally — Test connection handling, auth, and room behavior before scaling (e.g., npx wscat -c ws://localhost:3000); confirm auth rejection on missing/invalid tokens, room join/leave events, and message delivery
Scale — Verify Redis connection and pub/sub round-trip before enabling the adapter; configure sticky sessions and confirm with test connections across multiple instances; set up load balancing
Monitor — Track connections, latency, throughput, error rates; add alerts for connection-count spikes and error-rate thresholds

Reference Guide

Load detailed guidance based on context:

Topic	Reference	Load When
Protocol	`references/protocol.md`	WebSocket handshake, frames, ping/pong, close codes
Scaling	`references/scaling.md`	Horizontal scaling, Redis pub/sub, sticky sessions
Patterns	`references/patterns.md`	Rooms, namespaces, broadcasting, acknowledgments
Security	`references/security.md`	Authentication, authorization, rate limiting, CORS
Alternatives	`references/alternatives.md`	SSE, long polling, when to choose WebSockets

Code Examples

Server Setup (Socket.IO with Auth and Room Management)

import { createServer } from "http";
import { Server } from "socket.io";
import { createAdapter } from "@socket.io/redis-adapter";
import { createClient } from "redis";
import jwt from "jsonwebtoken";

const httpServer = createServer();
const io = new Server(httpServer, {
  cors: { origin: process.env.ALLOWED_ORIGIN, credentials: true },
  pingTimeout: 20000,
  pingInterval: 25000,
});

// Authentication middleware — runs before connection is established
io.use((socket, next) => {
  const token = socket.handshake.auth.token;
  if (!token) return next(new Error("Authentication required"));
  try {
    socket.data.user = jwt.verify(token, process.env.JWT_SECRET);
    next();
  } catch {
    next(new Error("Invalid token"));
  }
});

// Redis adapter for horizontal scaling
const pubClient = createClient({ url: process.env.REDIS_URL });
const subClient = pubClient.duplicate();
await Promise.all([pubClient.connect(), subClient.connect()]);
io.adapter(createAdapter(pubClient, subClient));

io.on("connection", (socket) => {
  const { userId } = socket.data.user;
  console.log(`connected: ${userId} (${socket.id})`);

  // Presence: mark user online
  pubClient.hSet("presence", userId, socket.id);

  socket.on("join-room", (roomId) => {
    socket.join(roomId);
    socket.to(roomId).emit("user-joined", { userId });
  });

  socket.on("message", ({ roomId, text }) => {
    io.to(roomId).emit("message", { userId, text, ts: Date.now() });
  });

  socket.on("disconnect", () => {
    pubClient.hDel("presence", userId);
    console.log(`disconnected: ${userId}`);
  });
});

httpServer.listen(3000);

Client-Side Reconnection with Exponential Backoff

import { io } from "socket.io-client";

const socket = io("wss://api.example.com", {
  auth: { token: getAuthToken() },
  reconnection: true,
  reconnectionAttempts: 10,
  reconnectionDelay: 1000,       // initial delay (ms)
  reconnectionDelayMax: 30000,   // cap at 30 s
  randomizationFactor: 0.5,      // jitter to avoid thundering herd
});

// Queue messages while disconnected
let messageQueue = [];

socket.on("connect", () => {
  console.log("connected:", socket.id);
  // Flush queued messages
  messageQueue.forEach((msg) => socket.emit("message", msg));
  messageQueue = [];
});

socket.on("disconnect", (reason) => {
  console.warn("disconnected:", reason);
  if (reason === "io server disconnect") socket.connect(); // manual reconnect
});

socket.on("connect_error", (err) => {
  console.error("connection error:", err.message);
});

function sendMessage(roomId, text) {
  const msg = { roomId, text };
  if (socket.connected) {
    socket.emit("message", msg);
  } else {
    messageQueue.push(msg); // buffer until reconnected
  }
}

Constraints

MUST DO

Use sticky sessions for load balancing (WebSocket connections are stateful — requests must route to the same server instance)
Implement heartbeat/ping-pong to detect dead connections (TCP keepalive alone is insufficient)
Use rooms/namespaces for message scoping rather than filtering in application logic
Queue messages during disconnection windows to avoid silent data loss
Plan connection limits per instance before scaling horizontally

MUST NOT DO

Store large state in memory without a clustering strategy (use Redis or an external store)
Mix WebSocket and HTTP on the same port without explicit upgrade handling
Forget to handle connection cleanup (presence records, room membership, in-flight timers)
Skip load testing before production — connection-count spikes behave differently from HTTP traffic spikes

Output Templates

When implementing WebSocket features, provide:

Server setup (Socket.IO/ws configuration)
Event handlers (connection, message, disconnect)
Client library (connection, events, reconnection)
Brief explanation of scaling strategy

Knowledge Reference

Socket.IO, ws, uWebSockets.js, Redis adapter, sticky sessions, nginx WebSocket proxy, JWT over WebSocket, rooms/namespaces, acknowledgments, binary data, compression, heartbeat, backpressure, horizontal pod autoscaling

Documentation

Websocket Engineer