Websockets

WebSockets

WebSockets provide a persistent connection between a client and server that both parties can use to start sending data at any time.

When to Use

• Chat Apps: Real-time messaging.
• Gaming: Multiplayer state synchronization (low latency).
• Collaborative Editing: Google Docs style co-authoring.
• Financial Dashboards: High-frequency trading updates.

Quick Start

// Server (Node.js with 'ws')
import { WebSocketServer } from "ws";

const wss = new WebSocketServer({ port: 8080 });

wss.on("connection", function connection(ws) {
  ws.on("message", function message(data) {
    console.log("received: %s", data);
    // Echo back
    ws.send(`Echo: ${data}`);
  });

  ws.send("Welcome!");
});

// Client (Browser)
const socket = new WebSocket("ws://localhost:8080");

socket.addEventListener("message", (event) => {
  console.log("Server says:", event.data);
});

Core Concepts

Handshake

Starts as an HTTP Request (Upgrade: websocket). If server accepts (101 Switching Protocols), the specific TCP connection becomes a WebSocket connection.

Heartbeat (Ping/Pong)

Essential to keep connections alive through proxies/firewalls that drop idle TCP connections.

Common Patterns

Socket.IO

A library that adds features on top of raw WebSockets: Auto-reconnection, Rooms, Fallback to HTTP Long-Polling.

Pub/Sub

Users subscribe to channels (e.g., chat_room_1). The server routes messages only to sockets in that channel (using Redis for scaling across multiple servers).

Best Practices

Do:

• Handle Scaling early. Sticky Sessions (if using Socket.IO) or a Redis Adapter are needed for multi-server setups.
• Secure with WSS (WebSocket Secure) always.
• Authenticate via a Token in the Query Parameter or initial handshake message (Headers are limited in JS WebSocket API).

Don't:

• Don't use WebSockets for simple notifications (Use SSE/Push API).
• Don't assume the connection is infinite. Handle disconnects gracefully.

Troubleshooting

Error	Cause	Solution
Connection Closed 1006	Abnormal closure (often network or server crash).	Implement auto-reconnect logic with backoff.
Load Balancer drops	LB timeout.	Configure Application Load Balancer (ALB) sticky sessions and timeout increase.

References

• MDN WebSockets API
• Socket.IO