# WebSocket Protocol

## What is WebSocket?

WebSocket provides full-duplex communication channels over a single TCP connection. Unlike HTTP, WebSocket allows real-time bidirectional communication.

## Connection Flow

```
Client                          Server
  │                              │
  │──── HTTP Upgrade Request ────▶│
  │        Upgrade: websocket     │
  │                              │
  │◀─── HTTP 101 Switching -------│
  │        Protocol: ws          │
  │                              │
  │◀═════ WebSocket Open ═════════▶│
  │                              │
  │◀═════ Messages ══════════════▶│
  │                              │
  │◀═════ Connection Close ═══════▶│
```

## WebSocket Headers

### Upgrade Request
```
GET /ws HTTP/1.1
Host: api.example.com
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
Sec-WebSocket-Version: 13
```

### Server Response
```
HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=
```

## Implementation Examples

### Server (Node.js + ws)
```javascript
const WebSocket = require('ws');
const wss = new WebSocket.Server({ port: 8080 });

wss.on('connection', (ws) => {
    console.log('Client connected');
    
    ws.on('message', (message) => {
        console.log('Received:', message.toString());
        ws.send('Echo: ' + message);
    });
    
    ws.on('close', () => {
        console.log('Client disconnected');
    });
    
    // Send periodic message
    const interval = setInterval(() => {
        ws.send('Ping: ' + Date.now());
    }, 30000);
    
    ws.on('close', () => clearInterval(interval));
});
```

### Client (Browser)
```javascript
const ws = new WebSocket('ws://localhost:8080');

ws.onopen = () => {
    console.log('Connected to WebSocket');
    ws.send('Hello Server!');
};

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

ws.onerror = (error) => {
    console.error('WebSocket error:', error);
};

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

### Client (Python)
```python
import asyncio
import websockets

async def main():
    async with websockets.connect('ws://localhost:8080') as ws:
        await ws.send('Hello Server!')
        response = await ws.recv()
        print(f"Received: {response}")

asyncio.run(main())
```

## Socket.IO (Higher Level)

### Server
```javascript
const { Server } = require('socket.io');
const io = new Server(3000, {
    cors: { origin: '*' }
});

io.on('connection', (socket) => {
    console.log('User connected:', socket.id);
    
    socket.on('message', (data) => {
        console.log('Message:', data);
        io.emit('message', data);
    });
    
    socket.on('disconnect', () => {
        console.log('User disconnected');
    });
    
    socket.on('join-room', (room) => {
        socket.join(room);
        socket.to(room).emit('user-joined', socket.id);
    });
});
```

### Client
```javascript
import { io } from 'socket.io-client';

const socket = io('http://localhost:3000');

socket.on('connect', () => {
    console.log('Connected:', socket.id);
    socket.emit('message', 'Hello everyone!');
});

socket.on('message', (data) => {
    console.log('Received:', data);
});
```

## WebSocket vs HTTP

| Aspect | WebSocket | HTTP |
|--------|----------|------|
| Connection | Persistent | Request-Response |
| Direction | Bidirectional | Client → Server |
| Overhead | Low (after handshake) | High (headers each request) |
| Use Case | Real-time apps | REST APIs, simple requests |
| Browser Support | Modern browsers | All browsers |

## Best Practices

1. **Heartbeat/Ping-Pong** - Keep connections alive
2. **Reconnection Logic** - Handle connection drops
3. **Message Queueing** - Queue messages during disconnect
4. **Authentication** - Validate on connection
5. **Rate Limiting** - Prevent abuse
6. **Compression** - Consider permessage-deflate