How do the client-server and peer-to-peer models work, and how does the web fit in?
Understand the client-server and peer-to-peer models, web technologies including HTML, CSS and JavaScript, the role of web servers, and the use of APIs and thin versus thick clients.
A focused answer to AQA A-Level Computer Science 4.8.6, covering the client-server and peer-to-peer models, web technologies including HTML, CSS and JavaScript, the role of web servers, and thin versus thick clients.
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What this dot point is asking
AQA wants you to compare the client-server and peer-to-peer models, describe the web technologies (HTML, CSS and JavaScript) and the role of the web server, and explain thin versus thick clients.
Client-server and peer-to-peer
The two models suit different needs. Client-server is the natural choice where control and reliability matter, such as a school network where logins, files and permissions must be managed centrally and data backed up in one place. Peer-to-peer suits situations with no central authority and where resilience through having no single point of failure is valuable, such as file-sharing networks or small home setups, but it scatters the data and makes consistent security much harder to enforce.
Web technologies and the web server
Keeping these three concerns separate is a deliberate design principle: structure (HTML), presentation (CSS) and behaviour (JavaScript) are split so each can be changed without disturbing the others, for example restyling a whole site by editing only the CSS. The web itself is a large client-server system layered on top of the internet, so this dot point connects directly to TCP/IP: the browser's HTTP request travels down the protocol stack, across the network, and the server's response comes back the same way.
Thin and thick clients
The thin versus thick decision is a balance between central control and local independence. A thin client model (for example terminals in a call centre) keeps software, data and updates in one place, simplifying maintenance and security, at the cost of total dependence on the server and network. A thick client (a typical desktop PC running local applications) keeps working if the network drops and spreads the processing load, but every machine must be patched and managed individually. Many modern systems are hybrids, doing some processing in the browser (thick-like) while relying on servers for data and heavy computation.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20194 marksCompare the client-server and peer-to-peer models of networking, giving one advantage of each and one situation where each is the better choice.Show worked answer →
In the client-server model, clients request services from one or more central servers that hold resources and manage security. Its advantage is centralised management, backup and security, since resources are in one place. It is the better choice for an organisation such as a school or business that needs controlled access, central data and reliable backups.
In the peer-to-peer model, every device is an equal peer acting as both client and server, sharing resources directly. Its advantage is that there is no single point of failure and no costly central server. It is the better choice for small home networks or for distributed file sharing, where the cost and administration of a dedicated server are not justified.
Markers reward the central-server versus equal-peers distinction, a valid advantage of each, and an appropriate scenario for each model.
AQA 20214 marksDescribe the roles of HTML, CSS and JavaScript in a web page, and explain the difference between a thin client and a thick client.Show worked answer →
HTML defines the structure and content of a web page (headings, paragraphs, links, images). CSS defines the styling and layout, controlling appearance such as colours, fonts and positioning, kept separate from the content. JavaScript runs in the browser to add client-side interactivity, such as validating a form or updating the page without reloading.
A thin client does little processing itself and relies on a powerful server for most computation and storage; it is cheap and easy to manage centrally but useless without the server and network. A thick (fat) client does most of its own processing and storage locally; it can work offline and reduces server load but is more expensive and harder to maintain across many machines.
Markers reward the structure, style and behaviour roles of the three technologies and the server-dependent versus self-sufficient distinction for thin and thick clients.
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Sources & how we know this
- AQA A-level Computer Science (7517) specification — AQA (2015)