CGI Application Modules for CPAN

if else block performs the required action using more CGI.pm calls, and ends by 

printing out the HTML for the page. Since I'm not trying to teach CGI program 

ming, I've left out the implementation of these functions.

This code employs a set of heuristics to determine which action to perform for 

the user. For example, it knows the user is saving a message by the presence of a 

CGI parameter called subject. If a user wants to read a message, then the msg_id

parameter will be included in the request and the script will know to act accord 

ingly. This approach has a major flaw it makes adding new features much harder 

than it should be.

For example, imagine adding support for replying to a message. This action 

will need only one parameter the message ID for the message being replied to. 

Thus, the elsif block might look like the following:

} elsif ($query >param("msg_id")) { # is the user replying to a message?

  do_reply();

}

But this won't work; the do_read() functionality is already using msg_id, and 

do_reply() will never be called! Instead you'd have to distort your parameter 

names to avoid clashing by adding a new parameter:

} elsif ($query >param("reply_msg_id")) { # is the user replying to a message?

  do_reply();

}

The problem here is that the CGI script has no way to know what the user is 

really doing it just looks at the incoming request and makes an educated guess. A 

small error on one form can lead your program down the wrong path with disastrous 

results. Also, understanding how the program works is unnecessarily difficult.

CGIs as State Machines

Many CGI programs, including the example shown earlier, can be viewed as finite 

state machines (or just state machines). A state machine is a system characterized 

by a series of discreet states and events that provide transitions between states. In a 

CGI like the BBS, the events are actions performed by the user, usually by clicking 

a button or following a link. In response, the application enters a particular state. 

In the preceding example, the do_save() subroutine is a state entered in response 

to the user saving a message. The output of this subroutine is displayed to the user. 

Thus, from a user's perspective, events are mouse clicks and states are screens of 

the application.

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