Week 1 Day 2: The Client-Server Model - How the Web Works

Today we cover the most basic pattern in distributed systems: Client-Server.

1. The Components

Client: The device requesting information (Your browser, mobile app, IoT device).
Server: The computer listening for requests and providing resources (AWS EC2, your laptop).
Network: The internet/wires connecting them.

2. The Request-Response Cycle

User Action: You type google.com and hit Enter.
DNS Resolution: Browser asks “Where is google.com?” (IP Address).
Request: Browser sends an HTTP Request to that IP. GET / HTTP/1.1
Processing: Server reads request, fetches data from Database.
Response: Server sends back HTML/JSON. 200 OK
Rendering: Browser draws the page.

3. Statelessness

The web is primarily stateless. The server treats every request as a new interaction. It doesn’t “remember” the previous request by default. Why? Scalability. If Server A handles Request 1, Server B can handle Request 2 without needing Server A’s memory.

To “remember” things (like “Who is logged in?”), we send Cookies or Tokens with every request.

4. Communication Protocols

HTTP/HTTPS: Standard web traffic.
WebSockets: Real-time, two-way communication (Chat apps).
TCP/UDP: Low-level transport. TCP is reliable (web), UDP is fast (gaming/video).

5. Coding Snippet (Node.js)

A simple server:

const http = require('http');

const server = http.createServer((req, res) => {
  if (req.url === '/') {
    res.writeHead(200, {'Content-Type': 'text/plain'});
    res.end('Hello, Client!');
  } else {
    res.writeHead(404);
    res.end('Not Found');
  }
});

server.listen(3000, () => {
  console.log('Server running at http://localhost:3000/');
});

Tomorrow: We dive deeper into HTTP & REST APIs! 🔗

Next Step

Next: Http Rest →