WebSockets

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

WebSocket Handshake

Connection Lifecycle

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

Scaling WebSockets

Challenge

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

Solutions

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