1.1 · Client–Server Model

Goal: explain how clients and servers cooperate over a network.

Core idea

Two roles in network applications:

Role	Responsibilities
Client	Initiates requests; usually drives the user interface
Server	Listens for requests; provides resources or services

┌──────────┐  request   ┌──────────┐
│  Client  │ ─────────▶ │  Server  │
│ (browser)│            │ (web app) │
│          │ ◀───────── │          │
└──────────┘  response  └──────────┘

Concrete examples

Service	Client	Server
Web browsing	Chrome / Firefox	Apache / Nginx
Email	Outlook	SMTP / IMAP server
Database	PHP app	MySQL server
Gaming	Player's PC	Game server
File share	File manager	NAS / file server

Properties of the model

• Loose coupling — server doesn't know which client will connect next.
• Many-to-one — many clients can share one server.
• Stateless protocols (like HTTP) — server doesn't have to remember clients between requests.
• Scaling — add more servers behind a load balancer.

Connectionless vs connection-oriented

• HTTP uses TCP (connection-oriented, reliable).
• DNS uses UDP (connectionless, fast).

Worked example · Browsing a web page

1. Client (browser) resolves the domain via DNS → IP address.
2. Client opens a TCP connection to the server's port (80 or 443).
3. Client sends GET /index.html HTTP/1.1.
4. Server responds with 200 OK + the HTML content.
5. Browser parses HTML; for each <img>, <link>, <script> it makes more requests.
6. Browser renders the page.

Common student mistakes

• Treating "client" and "user" as the same word — the client is the software.
• Believing servers are physically different from clients — any computer can play either role.
• Forgetting that a single server can run many services on different ports.

Exam-style question

Q (4 marks): Explain the client-server model and give two real-life examples.

Sample answer:

In the client-server model, the client initiates a request for a service and the server waits for incoming requests and responds with the requested resource. The two communicate over a network using a protocol (e.g. HTTP). The model decouples user devices from centralised resources and allows many clients to share one server.

Examples:

• Web browsing: a browser (client) requests a page from a web server (Apache/Nginx) over HTTPS.
• Email: an email client (Outlook / Apple Mail) connects to an SMTP server to send messages and an IMAP server to retrieve them.

Key takeaways

• Client requests; server responds.
• Used across web, email, gaming, database, file sharing.

➡️ Next: 1.2 HTTP Request / Response