Single-Session State Maintenance AKA "Building a Pizza" Scenario

The wizened pizza parlor entrepreneur knows that a great many pizzas are ordered by computer jockeys, and that most computer jockeys prefer to do, well, anything from the keyboard. So, building a pizza parlor on the Web isn't as ludicrous an idea as it may sound.

To this end, we've conjured up a little pizza joint-in-a-CGI-script -- one script is all it takes, and in doing so we'll see how to maintain state while the hungry shopper hems and haws over various pizza options. Starting with a visual aid, the basic template for our parlor looks as follows:

The parlor's operation is straightforward: the customer fills in the text fields, selects pickup or delivery, and chooses toppings and a pizza size. When "Calculate" is clicked, the page reloads reflecting the current options and the current price. Any options can be changed, "Calculate" may be re-clicked, and an updated display will appear. This may go on infinitely, until one clicks "Finish Order", wherein a final invoice is generated and, presumably, the order will be sent onto the kitchen.

As we walk through the code to buildapizza.cgi, which is somewhat longer than our usual examples, you may want to consult a window or printout containing the entire script. Or test drive the live example of Build-A-Pizza (for fun only, no real pizzas are baked here!).

#build-a-pizza order form
#!/usr/local/bin/perl
use CGI;
$cgiobject=new CGI;
$cgiobject->use_named_parameters;

Beginning in familiar territory, the CGI environment is pumped and primed for the remainder of this Perl program.

@allparams=$cgiobject->param();
if ($#allparams>-1)
 {&get_state_variables} 
else 
 {&init}

The script must detect whether the user is submitting modified form fields or whether this is the beginning of a new order (no form fields submitted). Above, we grab an array of all parameters submitted and check whether there were any at all (a list length of -1 represents no parameters). If any parameters were submitted then we branch into the get_state_variables subroutine which will read the submitted values and populate the appropriate variables. For a new order, we branch to init which sets the initial values for the pizza building variables.

if ($orderComplete)
 { &complete_order }
else 
 { &build_order }

We'll see later that the variable $orderComplete represents whether the "Finish Order" submit button was clicked. If so, we want to branch to the invoice subroutine complete_order; otherwise it was the "Calculate" button which submitted the form and we wish to update the page via build_order.

Final Cookie Thoughts
The Perl You Need to Know
Building a Pizza: Taking the Order