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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:

An example of this flow of information is shown in the attached figure -- the arrows show the flow of data, while the small ovals straddling the lines show the protocols and mechanisms by which the data are communicated from client(browser)-to-server-to-gateway and back again.
[Figure showing flow of data to gateway programs]

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.

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