CGI turns the Web from a simple collection of static hypermedia documents into a whole new interactive medium, in which users can ask questions and run applications. Let's take a look at some of the possible applications that can be designed using CGI.
One of the most prominent uses of CGI is in processing forms. Forms are a subset of HTML that allow the user to supply information. The forms interface makes Web browsing an interactive process for the user and the provider. Figure 1.2 shows a simple form.
As can be seen from the figure, a number of graphical widgets are available for form creation, such as radio buttons, text fields, checkboxes, and selection lists. When the form is completed by the user, the Submit Order! button is used to send the information to the server, which executes the program associated with the particular form to "decode" the data.
Generally, forms are used for two main purposes. At their simplest, forms can be used to collect information from the user. But they can also be used in a more complex manner to provide back-and-forth interaction. For example, the user can be presented with a form listing the various documents available on the server, as well as an option to search for particular information within these documents. A CGI program can process this information and return document(s) that match the user's selection criteria.
Web gateways are programs or scripts used to access information that is not directly readable by the client. For example, say you have an Oracle database that contains baseball statistics for all the players on your company team and you would like to provide this information on the Web. How would you do it? You certainly cannot point your client to the database file (i.e., open the URL associated with the file) and expect to see any meaningful data.
CGI provides a solution to the problem in the form of a gateway. You can use a language such as oraperl (see Chapter 9, Gateways, Databases, and Search/Index Utilities, for more information) or a DBI extension to Perl to form SQL queries to read the information contained within the database. Once you have the information, you can format and send it to the client. In this case, the CGI program serves as a gateway to the Oracle database, as shown in Figure 1.3.
Similarly, you can write gateway programs to any other Internet information service, including Archie, WAIS, and NNTP (Usenet News). Chapter 10, Gateways to Internet Information Servers, shows examples of interacting with other Internet services. In addition, you can amplify the power of gateways by using the forms interface to request a query or search string from the user to retrieve and display dynamic, or virtual, information. We will discuss these special documents next.
Virtual, or dynamic, document creation is at the heart of CGI. Virtual documents are created on the fly in response to a user's information request. You can create virtual HTML, plain text, image, and even audio documents. A simple example of a virtual document could be something as trivial as this:
Welcome to Shishir's WWW Server! You are visiting from diamond.com. The load average on this machine is 1.25. Happy navigating!
In this example, there are two pieces of dynamic information: the alphanumeric address (IP name) of the remote user and the load average on the serving machine. This is a very simple example, indeed!
On the other hand, very complex virtual documents can be created by writing programs that use a combination of graphics libraries, gateways, and forms. As a more sophisticated example, say you are the manager of an art gallery that specializes in selling replicas of ancient Renaissance paintings and you are interested in presenting images of these masterpieces on the Web. You start out by creating a form that asks for user information for the purpose of promotional mailings, presents a search field for the user to enter the name of a painting, as well as a selection list containing popular paintings. Once the user submits the form to the server, a program can email the user information to a certain address, or store it in a file. And depending on the user's selection, either a message stating that the painting does not exist or an image of the painting can be displayed along with some historical information located elsewhere on the Internet.
Along with the picture and history, another form with several image processing options to modify the brightness, contrast, and/or size of the picture can be displayed. You can write another CGI program to modify the image properties on the fly using certain graphics libraries, such as gd, sending the resultant picture to the client.
This is an example of a more complex CGI program using many aspects of CGI programming. Several such examples will be presented in this book.