[Network] CGI(Common Gate Interface)

Figure 8-9 shows the basic mechanics behind server and gateway application interactions.

 

This simple protocol (request in, hand off, and respond) is the essence behind the oldest and one of the most common server extension interfaces, CGI.

 

8.3.1 Common Gateway Interface (CGI)

The Common Gateway Interface was the first and probably still is the most widely used server extension. It is used throughout the Web for things like dynamic HTML, credit card processing, and querying databases.

Since CGI applications are separate from the server, they can be implemented in almost any language, including Perl, Tcl, C, and various shell languages. And because CGI is simple, almost all HTTP servers support it. The basic mechanics of the CGI model are shown in Figure 8-9.

 

CGI processing is invisible to users. From the perspective of the client, it's just making a normal request. It is completely unaware of the hand-off procedure going on between the server and the CGI application. The client's only hint that a CGI application might be involved would be the presence of the letters "cgi" and maybe "?" in the URL.

So CGI is wonderful, right? Well, yes and no. It provides a simple, functional form of glue between servers and pretty much any type of resource, handling any translation that needs to occur. The interface also is elegant in protecting the server from buggy extensions (if the extension were glommed onto the server itself, it could cause an error that might end up crashing the server).

However, this separation incurs a cost in performance. The overhead to spawn a new process for every CGI request is quite high, limiting the performance of servers that use CGI and taxing the server machine's resources. To try to get around this problem, a new form of CGI—aptly dubbed Fast CGI— has been developed. This interface mimics CGI, but it runs as a persistent daemon, eliminating the performance penalty of setting up and tearing down a new process for each request.

 

Source:

www.staroceans.org.s3-website-us-east-1.amazonaws.com/e-book/O%27Reilly%20-%20HTTP%20-%20The%20Definitive%20Guide.pdf