CGI style -- alternatives to HIDDEN parms?

[John Evans]

On Thu, 2 Aug 2001, Tim Chambers wrote:

> I am writing a Perl CGI script that updates some information in a
> database. The field can be hundreds of lines of text. If the field
> hasn't been touched, then I don't want to update it in the database.
> (The database logs updates, and we don't want redudant log entries.)
> The user will press a Submit button, at which point I need to decide
> which fields to update in the database. I'm assuming there isn't a
> simple way of detecting which fields were modified from their initial
> values. Am I wrong? I see my options this way:
>
> 1. Query the database and compare before deciding whether to update.
> The downside is that database queries are relatively expensive in time
> and resources.
>
> 2. Use a HIDDEN parm to forward the original field contents, then
> compare it with the new value to see if it's changed. But I'm
> concerned about sending contents (remember -- the field could be
> hundreds of lines of text) back and forth between client to server.
> Should I be? I studied http://www.w3.org/TR/html4/interact/forms.html
> but found nothing talking about length limits on such data.
>
> 3. Cache it on the server somehow. This would just be a cache of the
> database, though.

You could get fancy and store a checksum of the field in a hidden field.
Take the checksum of the original, the checksum of the submitted field and
see if they match. If they do, then the field was not changed. If they
don't, you have new information to submit to the database.

I'm just not sure what the chances are of two totally different sets of
text generating the same checksum are. I know there are different methods,
but not sure what the advantages and disadvantages are of those methods.

I checked to see if there was a way to do it with Javascript, but there is
no method for doing something like:
if (FormName.FieldName.hasChanged()) { .... }

I'm thinking that the best way would be to go with the checksum method,
but since I don't know much about that, it's just a shot in the dark.


-- 
John Evans
http://evansj.kilnar.com/

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