4.9.4b Client-Server Computing — AQA 7517

Client-Server Model

The client-server model is a distributed application structure that separates computing into two roles:

Client: requests a service (e.g. web browser, email client, game client)
Server: provides the service — holds resources and responds to requests (e.g. web server, file server, database server, game server)

Communication happens over a network (LAN or WAN). The server is always on, listening for client requests. Clients connect when needed.

Types of Server

Server type	Function	Example
Web server	Serves HTML/CSS/JS in response to HTTP requests	Apache, Nginx
File server	Centralised file storage/sharing over a network	NAS, Windows Server
Database server	Manages and queries databases	MySQL, PostgreSQL, SQL Server
Mail server	Sends/receives/stores email	Postfix, Exchange
DNS server	Resolves domain names to IP addresses	ISP DNS, Google 8.8.8.8
Proxy server	Intermediary — caches content, filters, anonymises	Squid
Authentication server	Verifies user identity	Active Directory, LDAP
Print server	Manages print queue for network printers	Windows print server

Client-Server vs Peer-to-Peer (P2P)

Feature	Client-Server	Peer-to-Peer
Structure	Centralised — dedicated server	Decentralised — all equal peers
Security	Easier to secure — centralised control	Harder to secure — no central authority
Performance	Can scale with powerful server	Can degrade as more peers added
Administration	Centralised management	Each node manages itself
Cost	High (dedicated server hardware)	Low (uses existing hardware)
Use case	Business networks, websites	BitTorrent, some gaming

Cloud Computing

Cloud computing delivers computing services over the internet (the "cloud") — servers, storage, databases, networking, software, analytics. Users pay on-demand rather than owning hardware.

Cloud service models

Model	You manage	Provider manages	Example
IaaS (Infrastructure as a Service)	OS, apps, data	Hardware, virtualisation, networking	AWS EC2, Azure VMs
PaaS (Platform as a Service)	Apps and data	OS, runtime, middleware, hardware	Heroku, Google App Engine
SaaS (Software as a Service)	Just use the app	Everything	Google Docs, Office 365, Gmail

Cloud deployment models

Public cloud: shared infrastructure owned by a third party (AWS, Azure, GCP)
Private cloud: dedicated infrastructure for one organisation — more control and security
Hybrid cloud: combination of public and private cloud

Advantages and disadvantages of cloud computing

Advantages	Disadvantages
Scalable — pay for what you use	Requires reliable internet connection
No upfront hardware cost	Ongoing subscription costs
Access from anywhere	Data may be stored in unknown locations
Provider handles maintenance and backups	Privacy/security concerns — data held by third party
Automatic software updates	Less control over infrastructure

Exam tip: AQA 7517 expects you to compare client-server with peer-to-peer and explain cloud service models (IaaS, PaaS, SaaS). Key: client-server = centralised, easier to administer/secure; P2P = decentralised, lower cost. For cloud: IaaS = virtual hardware; PaaS = platform to build on; SaaS = ready-to-use software. Common exam question: advantages/disadvantages of cloud computing for a given organisation scenario.

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Worksheet — 4.9.4b Client-Server Computing

8 questions · instantly marked · AQA 7517 standard

Q1Explain the client-server model, identifying the role of both the client and the server.[2]

✅ Mark scheme

Mark scheme

Client-server: distributed architecture where tasks are split between two roles [1]. Client: makes requests for services (e.g. web browser sends HTTP request) [1]; Server: provides services, stores resources, always-on and listens for incoming connections [1]; communication over a network (LAN or WAN) [1].

Q2Compare client-server networking with peer-to-peer (P2P) networking. Include two advantages of each.[4]

✅ Mark scheme

Mark scheme

Client-server advantages: centralised management — easier to administer and backup [1]; better security — data held on secure central server [1]; scales well with a powerful server [1]. P2P advantages: no need for dedicated server hardware — lower cost [1]; no single point of failure — if one peer goes down, others continue [1]; easy to set up for small networks [1]. Award 2 marks per model, max 6.

Q3A school uses a file server and a web server. Explain the function of each.[2]

✅ Mark scheme

Mark scheme

File server: centrally stores files that network users can access, share, and edit [1]; allows controlled access and centralised backups [1]. Web server: stores and serves web pages (HTML/CSS/JS) in response to HTTP requests from clients (browsers) [1]; processes the request and returns the requested content [1].

Q4Explain what cloud computing is and describe two cloud service models.[4]

✅ Mark scheme

Mark scheme

Cloud computing: delivery of computing services (servers, storage, software) over the internet [1]; on-demand, pay-as-you-go model [1]. IaaS (Infrastructure as a Service): provides virtualised hardware (servers, storage, networking) — customer manages OS and applications [1]; e.g. AWS EC2 [1]. PaaS (Platform as a Service): provides a platform for developing/deploying apps — customer manages app and data [1]; e.g. Heroku [1]. SaaS (Software as a Service): ready-to-use application delivered over internet — provider manages everything [1]; e.g. Google Docs [1]. Award 2 marks per model described, max 6.

Q5A business is considering moving its IT infrastructure to the cloud. Give three advantages and two disadvantages.[3]

✅ Mark scheme

Mark scheme

Advantages (any 3 × 1 mark): scalable — pay for what you use; no large upfront hardware cost; access from anywhere with internet; automatic updates and maintenance by provider; disaster recovery managed by provider; reduced IT staffing costs. Disadvantages (any 2 × 1 mark): requires reliable internet connection; data stored by third party — privacy/compliance concerns; ongoing subscription costs can exceed owned hardware long-term; less control over infrastructure configuration.

Q6Explain the function of a proxy server and give one reason why a school might use one.[2]

✅ Mark scheme

Mark scheme

Proxy server: acts as an intermediary between clients and web servers [1]; can cache frequently-visited pages (faster for subsequent requests), filter/block websites, and anonymise client IPs [1]. School use: content filtering — blocks inappropriate websites (e.g. social media during lessons) [1].

Q7Explain the difference between a public cloud and a private cloud. State when a private cloud might be preferred.[2]

✅ Mark scheme

Mark scheme

Public cloud: infrastructure shared among multiple organisations, owned by a third-party provider (AWS, Azure) [1]; resources provisioned on demand over internet [1]. Private cloud: dedicated infrastructure for a single organisation [1]; greater control and security — data stays in-house [1]. Private preferred: organisations handling sensitive data (banking, healthcare, government) where regulatory compliance or security requirements demand internal control [1].

Q8Explain how a DNS server is used in the client-server model when a user visits a website.[2]

✅ Mark scheme

Mark scheme

DNS (Domain Name System) server translates domain names to IP addresses [1]. When user types a URL: browser checks local cache; if not found, sends query to DNS resolver [1]; resolver queries root → TLD → authoritative DNS server [1]; receives IP address; browser sends HTTP request to web server at that IP [1].

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Client-Server Model

Types of Server

Client-Server vs Peer-to-Peer (P2P)

Cloud Computing

Cloud service models

Cloud deployment models

Advantages and disadvantages of cloud computing

Worksheet — 4.9.4b Client-Server Computing

Mini Test — Client-Server Computing