Common Gateway Interface

The Common Gateway Interface is a standard method for web server software to delegate the generation of web content to executable files. These files are known as CGI scripts usually written in a scripting language.

For example, let’s say you put a Python script on your computer. On your computer, you also start up a web server such as Apache. This means your computer can now satisfy web requests! Whenever your computer receives a request, it’ll look at the path the request contains. If the path is a path to your Python script, it’s because of CGI that your computer can execute that script and return its output as a response to the computer that made the request.

CGI Drawback

The problem with CGI is that calling an executable generally means the invocation of a newly created process on the server. Thus, more time might be spent starting the process than generating the output. Alternatives came about to address this speed and memory drawback.

Alternative 1: Embed the langue interpreter inside the web server

One alternative involves embedding language interpreters inside the web server. Apache web servers allow third-party software to run inside of them via Apache modules. One such module is mod_ruby, which, as its name suggests, embeds the Ruby interpreter inside the Apache web server to interpret, surprise surprise, Ruby.

Alternative 2: Use a persistent process to evaluate requests

CGI spawns a new process per request. FastCGI addresses this speed problem by using a persistent process to handle multiple requests. These processes are owned by the FastCGI server. Separating the application processes from the web server process can be very helpful. The application and the web server can be restarted separately, which is helpful for busy websites.

Running CGI scripts on Mac OS 10.8

1) Create an Apache configuration file

$ sudo vim /etc/apache2/users/`whoami`.conf
<Directory "/Library/WebServer/CGI-Executables">
    Options Indexes MultiViews
    AllowOverride All
    Order allow,deny
    Allow from all
</Directory>

2) Change the permissions of the configuration file so that visitors can access them.

$ sudo chown root:wheel /etc/apache2/users/*
$ sudo chmod 644 /etc/apache2/users/*

3) Put an executable Python script in the cgi-bin.

$ cd /Library/WebServer/CGI-Executables
$ vim script.py

#! /usr/bin/python

print 'Content-Type: text/html'
print
print """\
<html>
<body>
<h1>Hello World!</h1>
</body>
</html>
"""

4) The following is a Ruby CGI script:

#! /usr/bin/ruby                                                                                             
require 'cgi'
cgi = CGI.new
puts cgi.header
puts "<html><body>This is a test</body></html>"