Networking Basics for Engineers

const net = require('net');      // TCP
const dgram = require('dgram');  // UDP

// TCP Server — reliable, connection-oriented
const tcpServer = net.createServer((socket) => {
  socket.on('data', (data) => {
    socket.write(`Echo: ${data}`);  // ACK পাঠায়
  });
});
tcpServer.listen(3000);

// UDP Server — fast, connectionless, fire and forget
const udpServer = dgram.createSocket('udp4');
udpServer.on('message', (msg, rinfo) => {
  // No ACK! Just process and move on
  console.log(`From ${rinfo.address}: ${msg}`);
});
udpServer.bind(4000);

# Layer 7 Load Balancer — URL দেখে intelligent routing
upstream api_backend {
    least_conn;  # Algorithm: least connections
    server 192.168.1.10:3000 weight=3;  # বেশি powerful server
    server 192.168.1.11:3000 weight=2;
    server 192.168.1.12:3000 weight=1;
}

server {
    listen 80;
    # API requests → backend
    location /api/ {
        proxy_pass http://api_backend;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_connect_timeout 5s;
    }
    # Static files → CDN / local cache
    location /static/ {
        root /var/www/static;
        expires 30d;
    }
}

const WebSocket = require('ws');
const http = require('http');

// WebSocket — HTTP upgrade করে persistent bidirectional connection
// REST API তে server প্রথমে push করতে পারে না — WebSocket পারে!
const server = http.createServer();
const wss = new WebSocket.Server({ server });
const clients = new Set();

wss.on('connection', (ws) => {
  clients.add(ws);
  ws.on('message', (msg) => {
    // Server push করছে — সব connected clients এ broadcast
    clients.forEach((c) => {
      if (c.readyState === WebSocket.OPEN) c.send(`${msg}`);
    });
  });
  ws.on('close', () => clients.delete(ws));
});
server.listen(3000);
// Use: Chat apps, Live feeds, Notifications, Collaborative tools