The client-server model is a classic architecture of distributed systems. It is based on dividing responsibilities between a client (who requests services) and a server (who responds to those requests). This pattern is present in almost all the modern web and in multiple types of applications.

# History and Evolution

In the 60s and 70s, mainframes dominated: central computers with dumb terminals. As PCs emerged, it became viable to divide processing: clients could interact with centralized servers. With the arrival of the Internet, this model exploded in popularity and became the basis of web architecture.

# General Operation

• The client initiates a request over a network.

• The server receives it, processes it, and responds.

• Everything happens under protocols like HTTP, TCP/IP, etc.

# Everyday Analogy

Imagine a restaurant. You (client) order a dish. The waiter (server) takes the order, takes it to the kitchen, and then returns with the dish. The client never gets involved with how the food is cooked; they only make requests and receive responses.

# Technical Example (Node.js)

const express = require('express');
const app = express();

app.get('/api/time', (req, res) => {
  res.send(new Date().toLocaleTimeString());
});

app.listen(3000, () => console.log('Server listening on port 3000'));

<button onclick="requestTime()">What time is it?</button>
<script>
function requestTime() {
  fetch('http://localhost:3000/api/time')
    .then(response => response.text())
    .then(alert);
}
</script>