Conditional http get for dynamic pages

[David Dick]

Scott Penrose wrote:

>
> On Monday, Nov 25, 2002, at 06:19 Australia/Melbourne, David Dick wrote:
>
>> Hmmmmm... Correct.  My problem is probably a specific one.  I'm 
>> building a inventory system (keep track of the amount of goods in a 
>> warehouse). This means that I can't use a cache for the volume of 
>> stock remaining type enquiries, cos it's vital that the application 
>> server is consulted for every enquiry.  However, it's quite likely 
>> that the level of stock only changes relatively slowly.
>>
>> But if I take a MD5 Digest (for example) of the final page just 
>> before actually sending it and send it with the page as a ETag, the 
>> next time the user requests a "level of stock remaining" page (for 
>> example :)), if the page has the same MD5 Digest, I can just send a 
>> 304 and save the network traffic of a full response.  Also extremely 
>> applicable to a "Search for a document" type pages, which i am using 
>> quite a lot.
>
>
> Oh I see, yeah that sounds kind of exciting. The search idea is kind 
> of cool. I was thinking about the fact that a whole bunch of students 
> in a class room may all click on the same search string to Google. Of 
> course the silly proxy won't cache it because it is a CGI - however... 
> in theory it could cache it and use your example above. In that case 
> instead of an ETag it would just be the query that it caches against.

I _think_ that would work.  The only difference between mine and your 
case is that you would also give it a TTL?  That way the proxy could 
validate it for a GET.  back i go to the rfc... :)

>
>> But if you do take the hash before the gzip, you may not need to 
>> gzip. :) Of course, if you use a MD5 Digest after any possible 
>> compression, you can also use it for a Content-MD5 field :)
>
>
> Umm... why not NEED to gzip ? What has the hash to do with 
> compression. Sure do the MD5 as you suggested above, bug compressions 
> still reduces bandwidth.

Due to poor english language skills, sending code instead.... and hoping 
early morning coding skills up to the job.... :)

sub send_output {
    my ($self, $reference_to_html) = @_;
    $self->{'etag'} = Digest::MD5::md5_base64($$reference_to_html);
    if ((exists $ENV{'HTTP_IF_NONE_MATCH'}) && 
($ENV{'HTTP_IF_NONE_MATCH'} eq $etag) && ($self->response() eq 'OK')) {
        # send 304
    } else {
        # compress if possible and send 200
    }
}

or

sub send_output {
    my ($self, $reference_to_html) = @_;

    # compress if possible

    my ($md5) = Digest::MD5::md5_base64($compressed_html);
    $self->{'etag'} = $md5;
    $self->{'content-md5'} = $md5;

    if ((exists $ENV{'HTTP_IF_NONE_MATCH'}) && 
($ENV{'HTTP_IF_NONE_MATCH'} eq $etag) && ($self->response() eq 'OK')) {
        # send 304
    } else {
        # send 200  
    }
}