What is WebSocket?

WebSocket is a protocol that provides full-duplex communication channels over a single TCP connection. Unlike HTTP, both client and server can send messages at any time.

HTTP vs WebSocket

Aspect	HTTP	WebSocket
Connection	New for each request	Persistent
Direction	Request-response	Bidirectional
Overhead	Headers on every request	Low after handshake
Use case	Traditional web	Real-time apps

WebSocket Handshake

Client                                  Server
   │                                      │
   │─── HTTP Upgrade Request ────────────►│
   │    Upgrade: websocket                │
   │    Connection: Upgrade               │
   │                                      │
   │◄── HTTP 101 Switching Protocols ─────│
   │                                      │
   │◄─────── WebSocket Messages ─────────►│
   │         (bidirectional)              │

Connection Lifecycle

┌─────────┐    ┌───────────────┐    ┌─────────┐
│ Opening │───►│   Connected   │───►│ Closing │
└─────────┘    └───────────────┘    └─────────┘
                      │
                      ▼
              ┌───────────────┐
              │  Messages     │
              │  (send/recv)  │
              └───────────────┘

Code Example (JavaScript)

// Client
const ws = new WebSocket('wss://example.com/socket');

ws.onopen = () => {
  ws.send(JSON.stringify({ type: 'subscribe', channel: 'prices' }));
};

ws.onmessage = (event) => {
  const data = JSON.parse(event.data);
  console.log('Received:', data);
};

ws.onclose = () => {
  console.log('Disconnected');
};

Use Cases

Use Case	Example
Chat applications	Slack, WhatsApp Web
Live updates	Stock tickers, sports scores
Gaming	Multiplayer games
Collaborative editing	Google Docs
Notifications	Real-time alerts

Scaling WebSockets

Challenge

Each server maintains connections; users on different servers can't communicate.

Solutions

┌─────────┐     ┌─────────┐
│Server A │◄───►│ Redis   │◄───►│Server B │
│(users)  │ pub │ Pub/Sub │ sub │(users)  │
└─────────┘     └─────────┘     └─────────┘

Use pub/sub (Redis, Kafka) to broadcast across servers.

Interview Tips

Explain full-duplex vs request-response
Describe the HTTP upgrade handshake
Discuss scaling challenges (sticky sessions, pub/sub)
Compare with SSE and long polling
Give use cases: chat, live updates, gaming

WebSockets

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