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.
A role, not hardware: 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" |
Two more servers the guide names
| Server | Provides | Typical requests |
|---|---|---|
| DHCP | Hands each device that joins the network an IP address, subnet mask, gateway and DNS server, for a set time (a lease) | "I have just joined: which address can I use?" |
| Proxy | Sits between users and the internet, forwarding their requests: it can filter sites, cache popular pages and hide internal addresses | "Fetch this page for me" |
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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
Judging any server: the four factors in the guide
| Factor | The question to ask | Example |
|---|---|---|
| Function | What service does it provide, to whom? | A mail server stores and forwards email for every staff account |
| Scalability | Can it serve more users by adding machines (scale out) or a bigger one (scale up)? | Extra web servers behind a load balancer for a sales day |
| Reliability | What happens if it fails? | Two DHCP or DNS servers, so one failure does not stop the network |
| Security | Who can reach it, and is its traffic protected? | A proxy that blocks harmful sites; HTTPS on a web server |
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 answer broken into clear points, with guidance on how each point earns marks.