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October 28, 2014  |  By  | 


16 2.2 The Client-Server Model in a Distributed Computing System A distributed computingsystem is a set of application and system programs, and data dispersed across a number of independent personalcomputersconnected by a communication network. In order to provide requested services to users the system and relevant application programs must be executed. Because services are provided as a result of executing programs on a number of computers with data stored on one or more locations, the wholecomputingactivity is called distributed computing. 2.2.1 Basic Concepts The problem is how to formalise the development of distributed computing. The above shows that the main issue of distributed computing is programs in execution, which are called processes. The second issue is thatthese processes cooperate or compete in order to provide the requested services. This means thatthese processes are synchronised. A natural model of distributedcomputing is the client-servermodel, which is able to deal with the problems generated by distribution, could be used to describe computationprocesses and theirbehaviourwhen providing services to users, and allowsdesign of system and application software for distributedcomputingsystems. According to this model there are two processes, the client, which requests a service fromanother process, and the server, which is the service provider. The server performs the requested service and sends back a response. This response could be a processing result, a confirmation of completion of the requested operation or even a notice about a failure of an operation. From the user’s point of view a distributed computing system can provide the following services: printing,electronic mail, file service, authentication, naming, database service and computingservice. These services are provided by appropriate servers. Because of the restricted number of servers (implied by a restricted number of resources on whichthese servers were implemented),clients compete for these servers. An association between this abstract model and its physical implementation is shown in Figure 2.1. In particular the basic items of the model: the client and server, and request and response are shown. In this case, the client and server processes execute on two different computers. They communicate at the virtual (logical) level by exchanging requests and responses. In order to achieve this virtual communication, physical messages are sent betweenthese two processes. This implies that operating systems of computers and a communication system of a distributed computing system are actively involved in the service provision.