Introducing "CGI Programming on the Web"


    by Andy Oram
     _________________________________________________________________

     Give yourself a challenge. All you need is a Web server, a
     scripting language of your choice and a few lunch hours...

   The World Wide Web Web spent its early years as a publishing medium.
   You wrote what you want to say, you put it up and people read it.

   But along with the logarithmic growth of the Web, a lot of people
   realized that it could play a much more dynamic role. It could grow up
   to turn into a useful institutional asset, providing instant
   information within and between far-flung institutions. And the Common
   Gateway Interface (CGI) was the first technology to exploit this new
   use. In this article I'll introduce the new O'Reilly & Associates book
   on the subject, CGI Programming on the World Wide Web , by Shishir
   Gundavaram.

   One might say that dynamic information on the Web is just another step
   in fulfilling the promise of computers. Ever since batch jobs gave way
   to interactive programs, computers have acted as silent robots
   searching out the information people want. CGI lets a network offer
   the same dynamic, automated and customized access.

   Consider Dr. Georgiana Henk, a (fictitious) medical researcher with a
   busy schedule, a myriad of research projects, and an
   I-want-to-know-now attitude. Back in the 1980's, thanks to interactive
   spreadsheet-like software, she got used to pulling up a window on her
   computer and instantly viewing the results of today's study. Enter the
   Internet. Now that she maintains two offices in different cities and
   is using a network instead of a single computer, why can't she still
   query a database and see ever-changing information? If someone has
   written the proper CGI script, she can.

   Many people like to use this expanded Web capability for fun. They
   tell you "Four hundred and seventy-six people have logged into this
   site," or offer a guestbook to sign, or let you spin a virtual wheel
   and visit a random link on the Web. But as we saw above, CGI has its
   serious side, too.

   Of course, the person who maintains Dr. Henk's research or offers
   financial data to the organization's accountants could write a
   stand-alone program, sending forth socket calls (interfacing to the
   network) in its guts. For large amounts of data or routine, batch-like
   applications, that's still the way to go. But for small amounts of
   data where appearance is important, a stand-alone program is going to
   look old and worn the day it is released.

   Why not provide the convenience and attractive interface of a familiar
   Web browser? You get rich support for graphics, while links can take
   the users wherever they want to go through the data. People appreciate
   getting all their information through the same tool, and you can
   concentrate on the details of managing your information.

  What CGI Stands For

   Since you may find a name like Common Gateway Interface disorienting,
   let's take it apart and see what each word implies:

   Common - Assures you that CGI can be used by many languages and
   interact with many different types of systems. It doesn't tie you down
   to one way of doing what you want.

   Gateway - Suggests that CGI's strength lies not in what it does by
   itself, but in the potential access it offers to other systems such as
   databases and graphic generators.

   Interface - Simply means that CGI provides a well-defined way to call
   up its features, in other words, that you can write programs that use
   it. You can learn a lot from the CGI name once you look behind the
   austere exterior.

  What You Can Do With CGI

   Got your browser at hand? Try out some of the examples that we
   describe in our book.

  Quiz

   First, to exercise those memory neurons and show an application of a
   special quiz-generating program that Shishir wrote, take a quiz about
   the Web.

   The program takes quiz information from a file, generates forms that
   are shown to the user, accepts information from the forms, and grades
   the quiz. Try out this World Wide Web Quiz.

  Game

   Remember the game of Concentration, which little kids play? You turn
   up tiles and try to find pairs that match. A Web version of the game
   requires each tile to convey the state of the game so that the server
   can generate a new board after each click. Play with us.

  Database Queries Using SQL

   Looking up data and pretty-printing it for a browser to display is
   where CGI shines. Not many achievements will impress your colleagues
   as much as when you write a program that lets them access a large
   database at the click of a button and see dynamically generated output
   in their favorite browser.

   If you have an Oracle or Sybase database, free extensions to Perl and
   Tcl let you issue SQL to create and retrieve data. Our book shows you
   how to use some of these extensions in the context of a CGI program.
   For people who lack the commercial databases, Shishir has written a
   library that accepts SQL queries and creates or returns data from a
   simple flat file. To try it out, visit the Employee Database.

  Searching

   If you have a large Web site, your users probably wish they could
   search through files to find the ones that contain a particular
   string. Our book shows you how to provide this valuable feature. The
   program accepts a search string from the user and creates HTML output
   that shows this string in context within each file that contains it.
   The program could easily be extended to accept regular expressions,
   but Shishir sticks to fixed strings for security reasons. Take a look
   at this Search Gateway.

  Animation with CGI and Server Push

   I don't know if people will choose CGI over Java to do animation. The
   Java model is certainly appealing, because the server sends a program
   to the client and leaves it up to client power to execute it. But
   animation in CGI is quite easy. Here's an example that uses the
   Server Push Method.

  Dynamic Graphing and Plotting

   Another feature is determining how many people actually look at the
   examples in this article. Well, we can find out on an hourly basis. We
   just have to keep track of hits and plot the results on a timeline
   when each user clicks the URL. Try this Web Plotting Graph.

   Many more examples of dynamic image production are shown, in languages
   such as PostScript, gnuplot (a free language that's good for simple
   plotting), pgperl (a more powerful plotting library that is available
   in Perl), and the gd Graphics Library, which can be called from
   several languages including Perl and Tcl.

  Forms

   Simple forms are not hard to do in CGI. A typical book about the Web
   sticks to the simplest case, patiently taking you through each step of
   taking data from the form, converting characters, and so on. And you
   know what? There are freely-available tools to do just that. Decoding
   is the starting point for CGI, not the endpoint.

   We take you through all the basic steps of handling forms (in several
   different programming languages!) and on to more complicated
   applications such as the popular guestbook. Here's a survey you can
   try, Favorite Ice Cream Survey.

   Experienced CGI programmers know that form processing gets tough when
   you have multiple forms. This is because a separate run of the CGI
   program starts each time a form is submitted. How can you accumulate
   results from many different forms when the data is read by several
   different instances of your program? "Magic cookies"are one solution,
   shown by this TV and Movie Survey.

  Gateways and Remote Communications

   You're not going to create every Web application from scratch; you
   probably have legacy systems you want to hook up to. The databases
   mentioned earlier in this article are one example. Another is an
   Interface to Usenet News.

   This program uses a simple library based on sockets to communicate
   with an NNTP server.

  Mail

   Everybody has to offer a form that you can fill out in order to send
   mail to the Webmaster, even if the editor is primitive and the whole
   process is much more trouble than a real mailer. This application is
   just another gateway. You can let Shishir know what you think of
   his examples.

  That's not all, folks!

   That's it for the more spectacular displays. But our book discusses a
   number of introductory nuts-and-bolts issues too, such as configuring
   the system to run CGI scripts, generating HTTP headers, server-side
   includes, and debugging through a program that Shishir wrote, CGI
   Lint.

  The Language Issue

   You must learn a programming language to do anything non-trivial with
   CGI scripting. But you have your choice of languages: Perl, Tcl,
   Visual Basic, C, Applescript, and just about anything else. That's
   what the C in CGI stands for. Remember, it's a common interface.

   If you're not already wedded to one language for all time, this is a
   good point in your career to learn Perl. Most of the examples in the
   book are written in Perl, and when you see the marvelous things it can
   do to a Web page in two or three lines, you may well wonder why anyone
   would want to use any other language for CGI. But we recognize that
   people have pre-existing expertise in other languages, so we've
   provided some tips for getting started and doing common tasks in these
   languages. We also introduce some of the more obscure special-purpose
   languages that let you develop fancy pages quickly. Regardless of the
   language you ultimately choose, this book speaks valuable secrets to
   you.

   Give yourself a challenge. Find out what you can offer to your
   organization through dynamic Web pages. All you need is a Web server,
   a scripting language of your choice, and a few lunch hours. Rarely
   have so many applications become available using such simple tools.
   Eventually you may have the satisfaction of hearing from people all
   over the world who find your CGI scripts invaluable.

   Andy Oram is co-author of Managing Projects with Make and an editor at
   O'Reilly & Associates