Client Server Model.

• There are lots of clients and servers that can be part of a network, and the distinctions between them have become a bit more complex over time. 
• Generally there are four types of computers that are commonly used as servers.

Mainframe Computer

• A mainframe is a very large general-purpose computer (usually costing millions of dollars) that is capable of performing very many simultaneous functions, supporting very many simultaneous users, and storing huge amounts of data.

Microcomputer Computer

• A microcomputer is the type of computer you use. Microcomputers used as servers can range from a small microcomputer, similar to a desktop one you might use, to one costing $20,000 or more.

Cluster Computer 

• A cluster is a group of computers (often microcomputers) linked together so that they act as one computer. 
• Requests arrive at the cluster and are distributed among the computers so that no one computer is overloaded. 
• Each computer is separate, so that if one fails, the cluster simply bypasses it. 
• Clusters are more complex than single servers because work must be quickly coordinated and shared among the individual computers. 
• Clusters are very scalable because one can always add one more computer to the cluster.

Virtual Server Computer

• A virtual server is one computer (often a microcomputer) that acts as several servers. 
• Using special software, several operating systems are installed on the same physical computer so that one physical computer appears as several different servers to the network. 
• These virtual servers can perform the same or separate functions (e.g., a printer server, Web server, file server). 
• This improves efficiency (when each server is not fully used, there is no need to buy separate physical computers) and may improve effectiveness (if one virtual server crashes, it does not crash the other servers on the same computer).

There are five types of computers that are commonly used as clients

Microcomputer

• A microcomputer is the most common type of client today. This includes desktop and portable computers, as well as Tablet PCs that enable the user to write with a pen-like stylus instead of typing on a keyboard.

Terminal Computer

• A terminal is a device with a monitor and keyboard but no central processing unit (CPU). 
• Dumb terminals, so named because they do not participate in the processing of the data they display, have the bare minimum required to operate as input and output devices (a TV screen and a keyboard). 
• In most cases when a character is typed on a dumb terminal, it transmits the character through the circuit to the server for processing. Every keystroke is processed by the server, even simple activities such as the up arrow.

Network Computer

• A network computer is designed primarily to communicate using Internet-based standards (e.g., HTTP, Java) but has no hard disk. It has only limited functionality.
• A transaction terminal is designed to support specific business transactions, such as the automated teller machines (ATM) used by banks. Other examples of transaction terminals are point-of-sale terminals in a supermarket.

Handheld Computer

• A handheld computer, Personal Digital Assistant (PDA), or mobile phone can also be used as a network client.

Client-Server Model/Architectures

• Most organizations today are moving to client-server architectures. Client-server architectures attempt to balance the processing between the client and the server by having both do some of the logic. 
• The client is responsible for the presentation logic, whereas the server is responsible for the data access logic and data storage. 
• The application logic may either reside on the client, reside on the server, or be split between both.
• Here the client software accepts user requests and performs the application logic that produces database requests that are transmitted to the server. 
• The server software accepts the database requests, performs the data access logic, and transmits the results to the client.
• The client software accepts the results and presents them to the user. When you used a Web browser to get pages from a Web server, you used a client-server architecture. 
• Likewise, if you’ve ever written a program that uses SQL to talk to a database on a server, you’ve used a client-server architecture.

Peer-to-Peer Architectures

• Peer-to-peer (P2P) architectures are very old, but their modern design became popular because with a P2P architecture, all computers act as both a client and a server. Therefore, all computers perform all four functions: application logic, presentation logic, data access logic, and data storage. 
• With a P2P application sharing network, other users in the network can use others’ computers to access application logic as well.