Return to the guestbook discussion

Subject: Re: Perl tutorial -- guestbook example
   Date: Thu, 20 Jan 2000 09:51:27 -0800
   From: John Callender 
     To: [name deleted]

[name deleted] wrote:
> Hello,
> I have just started learnign Perl online.  And I
> copied your guestbook.cgi for my guest book.  The
> problem is that after I add a comment to the
> guestbook, if I reload the page, my comment gets added
> once again, and, in fact, the comment gets re-added as
> many times as many times I reload the page.  I think
> that the reason is that, you have a condition that
> checks whether the submit button has been clicked, and
> in that case, the guestbook gets updated.  And,
> really, after you submit your comment, the button is
> clicked, and the condition is satisfied all the time
> since then.  So, I think, I should add a line that
> would, after I click the submit button and the
> guestbook.txt is updated, change the action back to no
> 'action' again.  Since I am so new to cgi (this is my
> second day), I don't really know how this can be done.
>  Could you help please?

When you "reload" a page that was shown to you as the result of
submitting an HTML form with method "POST", your Web browser is
actually re-submitting the form. In the HTTP spec, "POST"
transactions are different from "GET" transactions, in the sense
that a "GET" transaction is supposed to be locally cache-able and
bookmark-able; that is, it's always supposed to return more or
less the same thing, or at least, submitting a GET request
multiple times is not supposed to change anything on the server

POST transactions, though, are specifically intended for
transactions that may very well change something on the server
side. Since that's the case, re-submitting a form via method POST
(or hitting the Reload or Refresh button in your browser when
viewing a page that was created via a POST request), is not
guaranteed to deliver you the same page as the one you were
looking at before you reloaded. I guess what I'm trying to say is
that what you're describing is actually a feature of how the Web
works, rather than being a bug.

That said, this issue has come up before, when someone wanted a
way to prevent the situation where a user double-clicks (instead
of single clicking) on the guestbook form's submit button. The
way it's written right now, doing that will cause duplicate
entries to appear in the guestbook, just like it will when you do
the reload thing.

I didn't write the guestbook script to do anything fancy to
prevent that, in part because I wanted it to be as simple as
possible for learning purposes. But it wouldn't be too hard to
extend it to be smart enough not to post a duplicate entry.
Here's one way you could do that:

Every time the guestbook script delivers a form for the user to
look at, and possibly submit, it generates a unique ID number and
embeds it in the form in a hidden form field. Then, when it
writes an entry to the guestbook, the entry includes an HTML
comment that contains that unique ID. When the guestbook script
processes a form submission, it first checks to see if an entry
with that ID is already in the guestbook. If it is, the script
doesn't do anything, but just shows the guestbook in its current
form. Only if the ID accompanying the current submission is *not*
in the guestbook would the script actually add an entry.

Generating a unique ID is fairly easy; you could do something

$id = $$ . time; # current process ID, concatenated with Unix
                 # seconds

Checking for the current submission's ID in the existing
guestbook entries could be done quickly with a regular

I don't have time right now to enhance the guestbook in this way,
but if you take that on, and come up with a version that works,
I'd be interested in taking a look at it.

Good luck! I hope you find the tutorial useful!


Return to the guestbook discussion