Client-Server Model Explained

The Client-Server Model

In the client-server model, clients initiate requests and servers respond with data or actions. The client knows the server's address; the server does not need to know the client's address in advance. This asymmetry is the foundation of web architecture.

The Request-Response Cycle

Flow:

1. DNS Lookup — Resolve hostname to IP address
2. TCP Handshake — SYN → SYN-ACK → ACK (3-way handshake)
3. TLS Handshake — Certificate exchange, cipher negotiation (HTTPS)
4. HTTP Request — Client sends GET/POST with headers and body
5. Server Processing — Auth, business logic, DB query, response assembly
6. HTTP Response — Status code, headers, body returned to client

Stateless vs Stateful Servers

Stateless Server	Stateful Server
No session data stored locally	Session data held in memory
Any server can handle any request	Sticky sessions required
Horizontal scaling is trivial	Harder to scale horizontally
State lives in DB / Redis / JWT	Faster for in-memory ops
REST APIs, microservices	WebSocket servers, game servers

TIP: Design stateless by default. Move session state to a shared store (Redis) so any server replica can serve any request. This is the single biggest enabler of horizontal scaling.

Thin vs Thick Clients

Model	Logic Location	Examples	Tradeoffs
Thin client	Server-side rendering, minimal JS	Traditional web, server-side React	Simple client; server under load; less offline capability
Thick client	Heavy client-side processing	SPAs, mobile apps, Electron	Rich UX, offline-capable; harder to update, security exposure
Hybrid	Split by concern	React + REST API, Flutter + gRPC	Best of both; most common modern architecture

Connection Types

• Short-lived (HTTP/1.1) — new TCP connection per request (or pooled with keep-alive)
• Persistent (HTTP/2) — single TCP connection multiplexes many requests simultaneously
• Long-polling — client holds request open; server responds when data is ready
• WebSocket — full-duplex; either side can push at any time after upgrade handshake
• Server-Sent Events (SSE) — one-way push from server to client over HTTP

// server.ts
import http from 'http';

const server = http.createServer((req, res) => {
  // Stateless — no memory of previous requests
  const url = new URL(req.url!, 'http://localhost');
  const name = url.searchParams.get('name') ?? 'World';

  res.writeHead(200, { 'Content-Type': 'application/json' });
  res.end(JSON.stringify({ message: `Hello, ${name}!` }));
});

server.listen(3000, () => console.log('Server on :3000'));

// client.ts
const res = await fetch('http://localhost:3000?name=Tekivex');
const data = await res.json();
console.log(data.message); // "Hello, Tekivex!"