Introduction to the Common Gateway Interface

1
Introduction to the Common Gateway Interface

• CSC 492 Topics in Perl
• Joe Reynoldson
• 2/7/04

2
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

3
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• Information which changes regularly falls out of
  date quickly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
• Cant be customized for individual clients

4
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)

5
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

6
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

7
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

8
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

9
Embedded Server Side Programming (continued)

• Advantages
• Faster
• Writing the application logic doesnt require web
  design skills
• Writing the interface doesnt require programming
  skills
• Logic and interface can be maintained separately

• Disadvantages
• Requires support for a particular module
• More complicated overall design

10
CGI Request Overview
Client (Web Browser)
Web Server
CGI Program
2) Call CGI
1) HTTP Request
3) HTML Results
4) HTTP Response
11
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

12
A Simple Example (continued)
13
A Simple Example (continued)

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

14
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
  1316 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...

15
A Simple Example (continued)

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

16
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.

17
A Simple Example (continued)
18
A Simple Example (continued)

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

19
A Simple Example (continued)

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

20
A Short To Do List

• Learn HTTP and HTML
• Learn a little UNIX
• Learn some Perl (?)
• Learn CGI