All Categories :
Intranets
Chapter 9
How Common Gateway Interface (CGI)
Works
CONTENTS
In general, Web software is not particularly database friendly-it
often happens that valuable corporate information is not always
accessible to many people who need it.
In the earliest days of the Web, it was exceedingly difficult
for anyone to gain access to those databases. As technology developed,
however, and it became easier to access databases, an increasing
number of databases became available, both on the Internet and
then later on intranets as well. Web server software was developed
to make it easier to access databases for the Web.
There are a number of ways in which someone from an intranet can
tap into corporate databases. An exceedingly popular one is called
the Common Gateway Interface (CGI). CGI allows any executable
to run and have its output sent back to the requesting client.
Therefore, it allows intranet programmers to write programs and
scripts that will allow people on an intranet to use their Web
browsers to easily query databases by filling out forms on Web
pages-and they will then send those results back in HTML form
that the browsers can understand.
Essentially, CGI is an interface that delivers information from
the server to your program and from your program back to the requesting
client. It is not a programming language. The program does all
the processing. CGI only gets data to the program. On an intranet,
the data accessed is often from a database of some sort. You've
no doubt used CGI many times without knowing it. On the Internet,
for example, if you've filled out a form on a Web page in order
to register to use a site, and then later received an e-mail notification
with a password for you to use, you've probably used CGI. CGI
most likely took the information you filled in the form, performed
several actions on it (including putting the information in a
database), automatically created a password, and then automatically
sent you mail.
On your corporate intranet, if you have one, you may well have
used CGI programs or scripts as well. If you've queried a corporate
database from a Web page and gotten information from it, then
there's a good chance that a CGI program or script is what did
the work for you.
Intranet programmers can use a variety of technologies to make
use of CGI. One of the simplest is to use what's called an interpreted
language. An interpreted language, such as the popular Perl
used on UNIX systems, is often favored because scripts written
with them are easy to debug, modify, and maintain.
CGI can also be accessed with more sophisticated computer languages,
such as C, C++, or Fortran. When a programmer writes a program
in a language such as C to be accessed by CGI, the program must
first be compiled. That means running it through a program
called a compiler that can change the application into a machine-readable
language that the computer can understand.
Essentially, CGI is an interface that delivers information from
the server to your program and from your program back to the requesting
client. It is not a programming language. The program does all
the processing. CGI only gets data to the program. CGI is a standard
that allows programmers to write programs that can access information
servers and databases, and then send the information to users
on an intranet. Using CGI, Web-based intranet technologies can
communicate with non-TCP/IP resources and databases. Using CGI,
an intranet programmer can write an application that searches
a database and displays the result in HTML format. CGI is used
to allow people to fill out corporate forms on an intranet, and
have that information entered into a database.
- For CGI to be used, an intranet programmer first writes a
program or a script. Since interpreted languages are easier to
debug, modify, and maintain than compiled programs, they are used
more frequently than compiled programs. Perl is probably the most
popular language for writing scripts. Languages such as C, C++,
or Fortran can be used to access CGI as well after they have been
compiled.
- Next, the script or compiled program is put on an intranet
Web server in a special directory (usually called /cgi-bin) which
holds all the CGI programs and scripts. For security reasons,
programs put in other directories won't run. If multiple directory
placements were allowed, it would be difficult for intranet maintainers
to track them all and therefore realize if an unauthorized user
posted a rogue CGI program.
- After posting the CGI program to /cgi-bin, it is linked to
the URL in HTML on an intranet Web page.
- When someone clicks on the URL, the server launches (via HTML's
GET or POST) the CGI program residing on the Web server. If, for
example, the CGI program's function is for searching a database,
the CGI program could send an HTML form to the client. From the
client, the data on the completed query form is sent (with HTTP
request headers) back to the CGI program using STDIN or environment
variables. The data is formatted as encoded name/value pairs.
- The CGI program contacts the database and requests the information.
The database sends the information to the CGI program. The information
can be in a variety of formats, such as text, graphics, sound
and video files, and URLs. The CGI program returns the results
to the server (via STDOUT) which, in turn, would send it on to
the browser.
- The CGI program also formats the data, for example, taking
the information and putting it into HTML format so that the user
can read it using a Web browser. The user can use that HTML page
as they can any other.
Contact
reference@developer.com with questions or comments.
Copyright 1998
EarthWeb Inc., All rights reserved.
PLEASE READ THE ACCEPTABLE USAGE STATEMENT.
Copyright 1998 Macmillan Computer Publishing. All rights reserved.
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