Client Versus Server

A computer's software largely performs data processing. It reads data in some form, processes that data, and then returns the results. On a single machine, all of the data input, output, and processing is performed on the same machine. A client/server model is one way of extending the software model to work on several machines. The client/server model divides up the data processing and transfers data back and forth over the network.

The server sends the data to the client (possibly processing it first) which the. client then processes. The client can send the data back to the server for further processing or display the results. The World Wide Web is based on this client/ server model. The Web server is essentially a distributor of information where the information is stored on the server and sent to the client upon request. The protocol for distributing this information is called the Hyper Text Transfer Protocol (HTTP).

The client must always send a request and the server must always send some sort of response. Even if you don't want the server to return any content, you must at least tell the browser to expect no content returned; you cannot just send nothing at all.

Now, we would like to present a few familiar examples of the Web in action. These should help you understand how the Web works and will prevent you from making mistakes in your CGI programs based on misconceptions about the Web protocol. Additionally, these examples will give you a good idea of what roles both the client and the server play in common transactions. This knowledge will help you think of creative applications to extend the Web either by extending the client, the server, or both.

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