Online ordering and inventory system

5.3 Client-server Interaction

5.3.1 Characteristics of Clients and Servers

In general, client software has the following characteristics:

· It is an application program that becomes a client temporarily when remote access is needed, but performs other computation locally.

· It is invoked by a user and executes for one session.

· It runs locally on the user's computer.

· It actively initiates contact with a server (CONNECT primitive).

· It can access multiple services as needed.

In general, server software has the following characteristics.

· It is a special-purpose program dedicated to providing one service.

· It is invoked automatically when a system boots, and continues to execute through many sessions.

· It runs on a shared computer.

· It waits passively for contact from arbitrary remote clients (LISTEN primitive).

· It accepts contact from arbitrary clients, but offers a single service.

Note that the word server is (strictly) referring to a piece of software. However, a computer running one or more servers is often (incorrectly) called a server. Like most application programs, a client and a server use a transport protocol to communicate.

Figure 12 illustrates a client and a server using the TCP/IP protocol stack.

Figure 11: A client and a server

5.3.2 The client Server Architecture

The Internet revolves around the client-server architecture. Your computer runs software called the client and it interacts with software known as the server located at a remote computer. The client is usually a browser such as Internet Explorer, Netscape Navigator or Mozilla. Browsers interact with the server using a set of instructions called protocols. These protocols help in the accurate transfer of data through requests from a browser and responses from the server. There are many protocols available on the Internet. The WWW, which is a part of the Internet, brings all these protocols under one roof. You can, thus, use HTTP, FTP, Telnet, email etc. from one platform - your web browser.

5.3.3 Some common Internet protocols

· HTTP: used on the WWW for transferring web pages and files contained in web pages such as images.

· FTP: employed for transferring files from one machine to the other.

· SMTP: used for email.

· Telnet Protocol: Used to open telnet sessions.

The web employs a connection-less protocol, which means that after every client-server interaction the connection between the two is lost.
Let us now examine the client-server inter-communication with three models

Figure 12: the client-server architecture