2. Server Processing of Passed Data: Gateway Programs

Sending data to the server is one thing, but in most cases the server cannot itself process the data -- most HTTP server program are designed only to serve out documents, and are not designed to process data sent from a client. Therefore, if you want to do server-side processing of the data sent from a fill-in HTML FORM (or by any other mechanism), you need three things:

• a second program to do process the data sent by the client,
• a mechanism by which the server can forward the data to this second program. Such secondary programs are called gateway programs, because they act as a gateway between the Web and other resources on the HTTP server machine, such as databases
• a way whereby this second program can return data to the client, so that the gateway program can return results of its analysis to the user.

Figure: Flow of Data to Gateway Programs

This Figure illustrates the flow of data when a user accesses a CGI program. The solid line shows to data flow using HTTP and CGI. HTTP transfers data from the client to the HTTP server, and back again. The CGI mechanisms control the flow of data from the server to the gateway program (shown as the prism) and back again. These are called gateway programs because they generally act as gateways between the World Wide Web and server-side resources such as databases, feedback forms, clickable imagemaps, and so on.