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If the world has state, why doesn't the World Wide Web? It would seem reasonable for the Web to have some memory. After all, people do tend to hang around a site for awhile, exploring here and there. It would seem only polite for a Web site to remember the user who's been rattling around inside it for the past hour. But the HTTP protocol is stateless. Each request for a document is a new transaction; after the document is delivered the Web server wipes its hands of the whole affair and starts fresh.
The HTTP protocol was designed that way because a stateless model is appropriate for the bulk of a Web server's job: to listen for requests for HTML documents and deliver them without fuss, frills or idle chitchat. The fact that the implementors of the protocol saw fit to build this stateless protocol on top of the connection-oriented TCP network communications protocol, thereby ensuring that the Web is hobbled by the performance limitations of TCP without reaping any of its benefits, is a small irony that we won't discuss further.
CGI scripts, however, often fit the stateless model poorly. Many CGI scripts are search engines of some sort. People pose a question, the CGI script does a search, and returns an answer. The user looks at the results, refines or modifies his question, and asks again. Unfortunately, by the time the user has refined his query and wants to build on previous results, the original CGI script has terminated, and the search has to start all over again. Or consider the important class of CGI scripts called "shopping carts." A user browses around an online catalog for a while. Whenever something takes his fancy, he presses a button that adds it to his shopping cart. When he's ready, he reviews the contents of the cart and (the vendor hopes) presses a button that performs an online order.
In the absence of any stateful behavior in HTTP itself, CGI script writers have to keep track of state themselves. There are several techniques for doing this: most of them rely on tricking or cajoling the browser into keeping track of the state for you.
The listing on the next page is a sample state-maintaining CGI script called remember.cgi. When invoked, it displays a form containing a single text input field and two buttons labeled "ADD" and "CLEAR" (see the figure for a screen shot). The user may type a short phrase into the text field and press ADD. This adds the phrase to the bottom of a growing list of phrases displayed at the bottom of the page. When the user presses CLEAR, the list is emptied. You can try it out for yourself at
http://www.genome.wi.mit.edu/WWW/examples/ch9/remember.cgi.
01 #!/usr/bin/perl
02 # Collect the user's responses in a file and
# echo them back to him when requested.
03
# must be writable by 'nobody'
04 $STATE_DIR = "./STATES";
05
06 use CGI;
07
08 $q = new CGI;
09 $session_key = $q->path_info();
# get rid of the initial slash
10 $session_key =~ s|^/||;
11
# If no valid session key has been provided,
# then we generate one, tack it on to the end of our URL
# as additional path information, and redirect the user
# to this new location.
12 unless (&valid($session_key)) {
13 $session_key = &generate_session_key($q);
14 print $q->redirect($q->url() . "/$session_key");
15 exit 0;
16 }
17
18 $old_state = &fetch_old_state($session_key);
19
# Add the new item(s) to the old list of items
20 if ($q->param('action') eq 'ADD') {
21 @new_items = $q->param('item');
22 @old_items = $old_state->param('item');
23 $old_state->param('item',@old_items,@new_items);
24 } elsif ($q->param('action') eq 'CLEAR') {
25 $old_state->delete('item');
26 }
27
# Save the new list to disk
28 &save_state($old_state,$session_key);
29
# Now, at last, generate something for the user to look at.
30 print $q->header;
31 print $q->start_html("The growing list");
32 print <<END;
33 <h1>The Growing List</h1>
34 Type a short phrase into the text field below.
When you press <i>ADD</i>,
it will be added to a history of the phrases
35 that you've typed. The list is maintained on disk at
the server end, so it won't get out of order if you
36 press the "back" button. Press <i>CLEAR</i> to
clear the list and start fresh.
37 END
38
39 print $q->start_form;
40 print $q->textfield(-name=>'item',-default=>'',
-size=>50,-override=>1),"<p>";
41 print $q->submit(-name=>'action',-value=>'CLEAR');
42 print $q->submit(-name=>'action',-value=>'ADD');
43 print $q->end_form;
44 print "<hr><h2>Current list</h2>";
45
46 if ($old_state->param('item')) {
47 print "<ol>";
48 foreach $item ($old_state->param('item')) {
49 print "<li>",$q->escapeHTML($item);
50 }
51 print "</ol>";
52 } else { print "<i>Empty</i>" }
53
54 print <<END;
55 <hr><address>Lincoln D. Stein,
lstein\@genome.wi.mit.edu<br>
56 <a href="/">Whitehead Institute/MIT Center
for Genome Research</a></address>
57 END
58 print $q->end_html;
59
# Silly technique: we generate a session key from the
# remote IP address plus our PID. More sophisticated
# scripts should use a better technique.
60 sub generate_session_key {
61 my $q = shift;
62 my($remote) = $q->remote_addr;
63 return "$remote.$$";
64 }
65
# Make sure the session ID passed to us is valid
# by looking for pattern ##.##.##.##.##
66 sub valid {
67 my $key = shift;
68 return $key=~/^\d+\.\d+\.\d+\.\d+.\d+$/;
69 }
70
# Open the existing file, if any, and read the current state.
# We use the CGI object here, because it's straightforward to do.
# We don't check for success of the open(), because if there is
# no file yet, the new CGI(FILEHANDLE) call will return an empty
# parameter list, which is exactly what we want.
71 sub fetch_old_state {
72 my $session_key = shift;
73 open(SAVEDSTATE,"$STATE_DIR/$session_key");
74 my $state = new CGI(SAVEDSTATE);
75 close SAVEDSTATE;
76 return $state;
77 }
78
79 sub save_state {
80 my($state,$session_key) = @_;
81 open(SAVEDSTATE,">$STATE_DIR/$session_key") ||
82 die "Failed opening session state file: $!";
83 $state->save(SAVEDSTATE);
84 close SAVEDSTATE;
85 }
Figure 1: The Growing List
The script works by maintaining each session's state in a separate file. The files are kept in a subdirectory that is readable and writable by the Web server daemon. We keep track of the correspondence between files and browser sessions by generating a unique session key when the remote user first accesses the script. After the session key is generated, we arrange for the browser to pass the key back to us on each subsequent access to the script.
The technique this script uses to maintain the session key is to store it in the "additional path information" part of the URL. This is the part of the URL between the name of the script and the beginning of the query string. For example, in the URL
http://toto.com/cgi-bin/remember.cgi/202.2.13.1.117?item=hi%20there
the text "/202.2.13.1.117" is the additional path information. Although the additional path information syntax was designed for passing file information to CGI scripts, there's no reason it can't be used for other purposes, and it's often easier to keep the session key here than mixing it up with the other script parameters.
Lines 8 to 16 are responsible for generating a unique session key. After creating a new CGI object, the script fetches the additional path information and strips off the initial slash (lines 9-10). The session key is next passed to the subroutine valid() (lines 66-69). This subroutine performs a pattern match on the session key to ensure that it is a key generated by our program rather than something that the user happened to type in. Importantly, the valid() subroutine also returns false if the session key is an empty string, which happens the first time our script is called.
If the session key is blank or invalid we generate a new key (lines 12-16) using the subroutine generate_session_key(). This subroutine, located at lines 60-64, is responsible for generating something that won't conflict with other concurrent sessions. In this example we use the simple but imperfect expedient of concatenating the remote machine's IP address with the CGI script's process ID.
After creating a new session key we generate a redirect directive to the browser, incorporating the session key into the new URL. If our script's URL is http://toto.com/cgi-bin/remember.cgi and the newly-generated session key is 202.2.13.1.117, we redirect the browser to
http://toto.com/cgi-bin/remember.cgi/202.2.13.1.117.
The scripts exists after printing the redirect. It will be reinvoked almost immediately by the browser when it retrieves the new URL/session key combination.
The remainder of the script, from line 18 onward, contains the code that is invoked when the browser provides a valid session key. Line 18 calls fetch_old_state() to retrieve the current list of text lines. This subroutine, defined in lines 71-77, opens up a file that contains the saved state by using the session key directly as the name of the file. More sophisticated scripts will want to use the session key in more clever ways, such as the key to a record in a DBM file or a handle into a relational database session.
fetch_old_state() opens the file indicated by the session key, ignoring any "file not found" errors, and passes the filehandle to the CGI new() method. This creates a new CGI object with parameters initialized from data stored in the file. We create a new CGI object here solely because its param() method offers a convenient way to store multiple named parameters and because of its ability to save and restore these named parameters to a file. We don't check first whether the file exists. If the file doesn't exist already, the CGI new() method returns an empty parameter list, which is exactly what we want. We close the file and return the new CGI object.
We now have two CGI objects. The first object, stored in the variable $q, was initialized from the current query and contains the contents of the text field and information about which button the user pressed when he submitted the fill-out form. The second object, stored in the variable $old_state, is the CGI object initialized from the saved file, and contains cumulative information about the user's previous actions.
Lines 20 to 26 manipulate the saved state depending on the user's request. We find out which button the user pressed by examining the CGI query's "action" parameter (line 20). If equal to "ADD", we recover the contents of the text field from the query parameter "item" and add it to the cumulative list (line 23). If the "action" parameter is "CLEAR", then we clear the list completely (line 25). Otherwise, no button was pressed and we continue onward.
Next we save the updated list back to disk with save_state(), which reverses the process by opening up the file indicated by the session key and using the CGI save() method to dump out the contents of $old_state.
So far no text at all has been transmitted to the browser. It's a good idea to do all the back end work first, because network delays can make your CGI script hang during output. If the user presses the "stop" button during this period your CGI script will be terminated, potentially leaving things in an inconsistent state.
Lines 30 through 58 generate the HTML document. The script generates the HTTP header followed by the HTML preliminaries and some explanatory text (lines 30-37). Next it creates the fill-out form, using the start_form(), end_form(), and form element generating subroutines discussed in the previous column. The only trick in this section is the use of the -override parameter in the call to textfield(). We want the contents of this field to be blank each time the page is displayed. For this purpose we set the contents to an empty string and use -override to have CGI.pm suppress the usual sticky behavior of fields.
After closing the form, we print out the current list of phrases in lines 44-52. Because there's no control over what the user types into the text field, it's important to escape any special HTML characters (such as angle brackets and ampersands) before incorporating it into our own document. Otherwise the script might create a page that doesn't display properly. The escapeHTML() method accomplishes this.
Last, we end the page with end_html() and exit. This script doesn't save a vast amount of state information: only one parameter, and a short one at that. However, the same techniques can be used to store and manipulate the contents of hundreds of parameters. In order to turn this from an example into a real world script, you'll need to make a few refinements.
You might want to change the way session keys are chosen. Although this script chooses its keys in a way that minimizes the chances of conflict between two sessions, it isn't suitable for security-sensitive applications. Such scripts should make sure that the remote user is entitled to use the provided session key in order to prevent one user from "stealing" another user's state. Checking the IP address for consistency is a one way to do this; password-protecting the script and incorporating the encrypted password into the session key would be an even better technique.
In order to make this script useful in the real world you'll also need to remove state files when they've gone out of date. Otherwise the scripts' PIDs will eventually roll over and start using ancient state files that are no longer valid. (There's also the risk of proliferating state files filling up your disk!) The easiest way to handle this on a UNIX system is with a cron job that runs at regular intervals looking for old state files and deleting them.
Another thing you might want to change is the way the session key is maintained. Netscape browsers support a "magic cookie" field that is guaranteed to be maintained for the entire length of a browser/server session. You can set the browser's magic cookie when the script first accesses the script, using CGI.pm's set_cookie(), method, and retrieve it on subsequent invocations of the script using get_cookie(). Unfortunately only Netscape browsers support the magic cookie field, so it isn't useful for other browsers.
In the next column I discuss CGI:MiniSvr, a flexible and powerful way of saving state information at the server's side of the connection.
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