Introduction to the Common Gateway Interface

Introduction to the Common
Gateway Interface

CSC 492 Topics in Perl

Joe Reynoldson

2/7/04

Static Web Resources



Definition
: resources which are written by web
developers and served via a web server unchanged


Static web resources can include HyperText Markup
Language (HTML) files, images, or multimedia files
such as MPeg4 video or Flash animations


Static resources are stored on a server file system,
and must be accessible by the web server

Static Web Resources (continued)


Advantages


Serving files is a
simple process which
requires little work for
the server


Resources which
rarely change require
less work to maintain


Authors do not have to
be programmers


Disadvantages


Information which
changes regularly falls
out of date quickly


Does not allow for
interactivity with the
client


Can’t be customized
for individual clients

Dynamic Web Resources


Definition:

Resources which are generated
dynamically as a result of a web server
processing a request from a web browser


Servers can dynamically generate pages
based on a variety of conditions


Contents of an input form


Location of the client


Identity of the client


Date or time


Dynamic data source (database)

Dynamic Web Resources (continued)


Advantages


A single program can
serve a variety of
clients


Resources which
constantly change are
always up to date


Increases the
interactivity of a web
site


Disadvantages


This process requires
more work for the
server which can
degrade response time


Increases complexity
of an application


Often requires
programmers in
addition to web
developers

The Common Gateway Interface
(CGI)


Definition:
An application programming interface
(API) which defines the minimum requirements for an
external program to generate dynamic web resources


CGI programs can be written in many languages, but
Perl is well suited and most often used


CGI programs can do many things


Search online databases


Online purchasing


Sending email


Posting comments


Generate graphics


CGI (continued)


Advantages:


Can be written in
almost any language


Widely available and
well documented


Simple API


Disadvantages


Slower than embedded
server side alternatives


Requires web
developer with
programming
knowledge


Embedded Server Side Programming
Techniques


Definition: Modules which embed a programming interface
within the web server


Examples


Mod_perl


An Apache (web server) module which embeds a perl
interpreter


Perl CGI applications can be executed internally


Java Server Pages (JSP)


Also embeds simplified Java code within HTML files


Compiled and served by a JSP server (such as Tomcat from
http://jakarta.apache.org
)


HyperText PreProcessor (PHP)


Can embed commands to be processed within HTML


A free module available at http://www.php.net/ and used mostly
with the Apache web server

Embedded Server Side Programming
(continued)


Advantages


Faster


Writing the application
logic doesn’t require
web design skills


Writing the interface
doesn’t require
programming skills


Logic and interface
can be maintained
separately


Disadvantages


Requires support for a
particular module


More complicated
overall design

CGI Request Overview

Client

(Web Browser)

1) HTTP Request

Web Server

CGI Program

4) HTTP Response

2) Call CGI

3) HTML Results

A Simple Example


A user enters a HyperText Transfer Protocol (HTTP)
request for a file named
hello.pl

from a server
named
www.usd.edu


The file is stored in a subfolder named
cgi
-
bin

of
the home directory for a user named
jreynold


We'll cover details of HTTP later


http://www.usd.edu/~jreynold/cgi
-
bin/hello.pl



The server receives the request, and translates it to
the appropriate program stored on the server


/home/j/jreynold/www/cgi
-
bin/hello.pl



A Simple Example (continued)

A Simple Example (continued)


The web browser receives and marks up the
web page appropriately


A Simple Example (continued)


The first thing to check when a CGI program doesn't work is
permissions


sunlight% ls
-
l hello.pl

-
rwx
------

1 jreynold other 544 Feb 9 13:16 hello.pl

sunlight% chmod 755 hello.pl

sunlight%



Be sure to verify the folder permissions too (including
jreynold
,
www
, and
cgi
-
bin

in this case)


The program must be executable by the web server


On our server, this means the permissions must be

rwxr
-
xr
-
x
for the file


All of the parent folders must be executable as follows:

rwx
--
x
--
x


Now then, as I was saying...

A Simple Example (continued)


The web browser receives and marks up the
web page appropriately


A Simple Example (continued)


Uh oh, that wasn't the fix. Yes it was, stay
calm ... stay calm.


Right, so the first line might be a problem
since Komodo defaults to CRLF for end of
line.


What's the fix? Add the wT flags.

A Simple Example (continued)

A Simple Example (continued)


The web browser receives and marks up the
web page appropriately

A Simple Example (continued)


The client may be unaware that a CGI
program was ever utilized

A Short To Do List


Learn HTTP and HTML


Learn a little UNIX


Learn some Perl (

)


Learn CGI