The various models that are used for building distributed computing systems can be classified into 5 categories:

1.Minicomputer Model

• The minicomputer model is a simple extension of the centralized time-sharing system.
• A distributed computing system based on this model consists of a few minicomputers interconnected by a communication network were each minicomputer usually has multiple users simultaneously logged on to it.
• Several interactive terminals are connected to each minicomputer.Each user logged on to one specific minicomputer has remote access to other minicomputers.
• The network allows a user to access remote resources that are available on some machine other than the one on to which the user is currently logged.The minicomputer model may be used when resource sharing with remote users is desired.
• The early ARPA net is an example of a distributed computing system based on the minicomputer model.

2.Workstation Model

• A distributed computing system based on the workstation model consists of several workstations interconnected by a communication network.
• An organization may have several workstations located throughout an infrastructure were each workstation is equipped with its own disk & serves as a single-user computer.
• In such an environment,at any one time a significant proportion of the workstations are idle which results in the waste of large amounts of CPU time.

• Therefore,the idea of the workstation model is to interconnect all these workstations by a high-speed LAN so that idle workstations may be used to process jobs of users who are logged onto other workstations & do not have sufficient processing power at their own workstations to get their jobs processed efficiently.

• Example:Sprite system & Xerox PARC.

3.Workstation–Server Model

• The workstation model is a network of personal workstations having its own disk & a local file system.
• A workstation with its own local disk is usually called a diskful workstation & a workstation without a local disk is called a diskless workstation.Diskless workstations have become more popular in network environments than diskful workstations,making the workstation-server model more popular than the workstation model for building distributed computing systems.
• A distributed computing system based on the workstation-server model consists of a few minicomputers & several workstations interconnected by a communication network.
• In this model,a user logs onto a workstation called his or her home workstation.Normal computation activities required by the user's processes are performed at the user's home workstation,but requests for services provided by special servers are sent to a server providing that type of service that performs the user's requested activity & returns the result of request processing to the user's workstation.
• Therefore,in this model,the user's processes need not migrated to the server machines for getting the work done by those machines.
• Example:The V-System.

4.Processor–Pool Model:

• The processor-pool model is based on the observation that most of the time a user does not need any computing power but once in a while the user may need a very large amount of computing power for a short time.
• Therefore,unlike the workstation-server model in which a processor is allocated to each user,in processor-pool model the processors are pooled together to be shared by the users as needed.
• The pool of processors consists of a large number of microcomputers & minicomputers attached to the network.
• Each processor in the pool has its own memory to load & run a system program or an application program of the distributed computing system.
• In this model no home machine is present & the user does not log onto any machine.
• This model has better utilization of processing power & greater flexibility.
• Example:Amoeba & the Cambridge Distributed Computing System.

5.Hybrid Model:

• The workstation-server model has a large number of computer users only performing simple interactive tasks &-executing small programs.
• In a working environment that has groups of users who often perform jobs needing massive computation,the processor-pool model is more attractive & suitable.
• To combine Advantages of workstation-server & processor-pool models,a hybrid model can be used to build a distributed system.
• The processors in the pool can be allocated dynamically for computations that are too large or require several computers for execution.
• The hybrid model gives guaranteed response to interactive jobs allowing them to be more processed in local workstations of the users