The big idea: A server is any machine running software that waits for requests and responds to them. A client is whatever makes the request.
It is a role, not a kind of hardware. A laptop can be a server; one machine can run several servers at once.
| Server | Provides | Typical requests |
|---|---|---|
| Web | Pages and assets over HTTP/HTTPS | "Send me /index.html" |
| File | Shared storage | "Open this document" |
| Database | Query results | "All orders from March" |
| Sending and storing email | "Any new messages?" | |
| DNS | Names turned into IP addresses | "What is bbc.co.uk?" |
| A shared printer queue | "Add this to the queue" |
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What it buys
- One copy of the data — no versions to reconcile
- Access control and backup in one place
- Powerful hardware shared across many users
- Update once, and everyone has the new version
What it costs
- A single point of failure
- A bottleneck under load
- A single, high-value target for attack
- Clients are useless without the network
How that is answered
- Redundant servers, so one failure is survivable
- A load balancer spreading requests between them
- Which also removes the bottleneck
- At the cost of complexity and money
Client–server against peer-to-peer: Client–server centralises: control, backup and consistency are easy, and the server is a single point of failure.
Peer-to-peer has every machine act as both. It has no single failure point and gets faster as more peers join — but security, consistency and backup all become much harder.
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What a server must cope with
- Concurrent requests — thousands may arrive in the same second
- So it must not process them strictly one after another
- Statelessness helps: if each request carries everything it needs, any server can answer it
- Which is what makes a load balancer possible at all
- Sessions are the exception, and are why state is often pushed into a shared store
Scaling up or out: Up — a bigger machine. Simple, but there is a ceiling, and it is still one point of failure.
Out — more machines behind a load balancer. No ceiling and no single failure, but the application must be written to allow it.
Naming both, and the ceiling, is what a full answer does.
How this is tested — you must describe a server by what it DOES, and weigh centralisation honestly. It comes up two ways:
Paper 1 Section A
- Describe the function of servers, 3-4 marks
- Name the server providing a described service
- State one advantage of client–server
Paper 1 Section B — case study
- Compare client–server with peer-to-peer
- Explain how a single point of failure is addressed
The classic trap: Defining a server as "a powerful computer". It is a role: software that waits for requests and responds. One machine can run several, and a laptop can be one.
IB-style questionDescribe[5 marks]
A school replaces files kept on individual laptops with a central file server. Describe the function of the server and evaluate the change.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.