ACCA P3考试：INFORMATION TECHNOLOGY

　　Decentralised (distributed) systems

　　In these systems, each user has local processing power and will hold data locally.

　　The main advantages of such systems are:

　　• Resilience: if one machine breaks down, others are unaffected.

　　• Easy expansion: simply add another computer.

　　• Flexibility: local users can decide which programs and software should be installed to meet local needs.

　　• They are more useful where each location can operate more or less separately from others.

　　The main disadvantages are:

　　• More difficult to control: data storage and processing are in many locations and correct access, processing and back-up of data are more difficult to enforce.

　　• Multiple versions of data: users might have their own version of data that should be uniform.

　　• Potentially higher costs: each local computer has to have sufficient processing power and each location might require an IT expert.

　　Hybrid systems

　　In these systems some data and processing are local and some are centralised. For example, web-browsing and word-processing might be local but critical business applications might be centralised.

　　CLIENT-SERVER AND PEER-TO-PEER SYSTEMS

　　These concepts are similar to centralised and decentralised, but are not quite identical.

　　In a client-server arrangement, a powerful computer (the server) is dedicated to providing a service to other computers in the network (the clients). Typical services provided are:

　　• File storage (file servers)

　　• Handling printing (print server)

　　• Handling the sending and receiving of emails (mail servers).

　　There is an element of centralisation here, but although files might be held centrally on the server they will often be processed locally. For example, a report will be held on the server, but when it is being edited it is downloaded to the user’s local machine (client). The edited version will be saved back to the server where other users can then access it. Obviously there will be great disruption if the server fails. Access rights to files are set centrally and typically enforced by users’ log-on information.

　　Traditionally, in client server networks each client would have had a copy of, say, Word for Windows. Documents would have been downloaded from the server for local editing then saved back to the server. The disadvantage of this is that each machine in the network needs a copy of Word and if the company was upgrading its software all copies of the program would have to be changed. Providing the software initially for all machines and its subsequent management is very expensive. With cloud computing, this approach has changed. There is only one copy of the software on the server within a web-based interface. Users log into the web system and their processing is then carried out on the server or a ‘cloud’ of servers. It appears to each user that they have a local version of the software, but what they are really seeing is the program operating in the server. Client machines can be ‘thin-clients’ which are not very powerful as they do not have to store much data and software nor do they have to carry out much processing. Hardware, software and maintenance costs are greatly reduced, though the system is vulnerable to service disruption.

　　Hotmail and Gmail provide examples of this approach. Whenever you want to write an email you log into the web email account and the processing is carried by the system’s computer cloud – not your computer. All it has to do is to handle the interface.

　　In peer-to-peer networks, two or more computers are connected directly without the need for a server. Access rights to files are given by individual users to specified other users. This is a simpler system to set-up, requiring no specialist operating system or specialist staff and many home systems are like this. It is a much more distributed system than client server systems and therefore has back-up and security issues.