Distributed Computing system models can be broadly classified into five categories. They are

• Minicomputer model
• Workstation model
• Workstation - server model
• Processor - pool model
• Hybrid model

1. Minicomputer Model

The minicomputer model is a simple model. Its extension of the centralized time-sharing system. A distributed computing system based on this model has a few minicomputers (they may be large supercomputers as well) interconnected to each other by a communication network.

Each minicomputer in generally has multiple users. All these users can simultaneously be logged on to it. For this, several interactive terminals are connected to each minicomputer. Each user is logged on to one specific minicomputer, he/she can remotely access other minicomputers.

The network allows a sharing of various resources, the 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 (such as sharing of information databases of different types, with each type of database located on a different machine) with remote users is desired.

The best example of a distributed computing system based on the minicomputer model is an ARPAnet. The figure below shows the minicomputer model. In these interactive terminals connected to the minicomputer and minicomputer to the communication network.

2. Workstation Model

A distributed computing system based on the workstation model made up of multiple workstations interconnected by a communication network. Organizations, big industries can have several workstations located throughout a building or campus, each workstation has its own disk and serving as a single-user computer.

It has been often found that in such an environment, at any one time a number of the workstations are idle (not being used), resulting 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.

3. Workstation - Server Model

The workstation model is a network of personal workstations. Each has its own disk and a local file system. A workstation with its own local disk is basically called a diskful workstation and a workstation without a local disk is called a diskless workstation. When we talk about the publicity of high-speed networks, disk-less workstations have become more popular in network environments than diskful workstations.

So, the workstation-server model becomes more popular than the workstation model for building distributed computing systems. A distributed computing system based on the workstation-server model in which few minicomputers and some workstations (most of which are diskless, but a few of which may be diskful) are interconnected by a communication network.

Remember that when diskless workstations are used on a network, the file system of these workstations must be chosen either by a diskful workstation or a minicomputer having a disk for file storage. One or more of the minicomputers are used for making the file system. Other minicomputers may be used for providing other types of services, such as database service and print service.

So, each minicomputer is used as a server machine to provide one or more types of services. Therefore in the workstation-server model, in addition to the workstations, there are specialized machines (maybe specialized workstations) for running server processes (called servers) for managing and providing access to shared resources. For reasons, such as higher reliability and better scalability, multiple servers are often used for managing the resources of a particular type in a distributed computing system.

For example, there may be multiple file servers, each running on a separate minicomputer and cooperating through the network, and managing the files of all the users in the system. Because of this reason, a difference is often made between the services that are provided to clients and the servers that provide them. That is, a service is an abstract entity that is provided by one or more servers.

For example, one or more file servers may be used in a distributed computing system to provide file service to the users. 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 own home workstation, but user sends requests for services provided by special servers (such as a file server or a database server) and server gives a response to each service of a user from its own machine. Therefore, in this model, the user's processes need not migrated to the server machines for getting the work done by those machines. The figure below will get a clear understanding of workstation-server model.

4. Processor - Pool Model

The processor-pool model is based on the observation of the user’s actual requirements. In most of the time, a user does not need any computing power but the user may need a very large amount of computing power for a short time (e.g., when he has to recompile a program which consisting of a large number of files after changing a basic shared declaration).

Sometimes user wants only a single service rather than whole workstation like he only wants to use a high-performance graphics terminal with high speed and clarity. In such cases, instead of the workstation-server model in which a processor is allocated to each user, the processor pool model is used. In the processor-pool model, the processors are pooled together to be shared by the users as needed.

Each processor from the pool has different strength and feature. The pool of processors consists of a huge number of microcomputers and minicomputers attached to the network. Each processor in the pool has its own memory to load and run a system program or an application program of the distributed computing system.

In the above figure number of processors are pooled together having own memory. There are different servers also connected in a link. At the left side of the processor pool model, many user terminals are located.

5. Hybrid Model

All four models described above in detail. The most widely used model for building distributed computing systems is a workstation-server model. The reason behind is that a large number of computer users only perform simple interactive tasks such as editing jobs, sending electronic mails, and executing small programs. The workstation-server model is ideal and most popular for such simple tasks. However, in a working environment that has groups of users who often perform jobs which required massive computation, the processor-pool model is more attractive and suitable.

As per the user’s requirement, a possible combination is to provide each user with a personal workstation and to have a processor pool in addition. Although this solution is more expensive than either a pure workstation model or a pure processor pool model, it combines the advantages of both of the others. Below figure provides the combination of a number of workstations and pool of processors having a specialty are working together.