[mod_python] Difference between filter.pass_on() and filter.disable()?

Graham Dumpleton graham.dumpleton at gmail.com
Wed Dec 10 22:25:27 EST 2008

2008/12/11 Jean Lagarde <jean.lagarde at gmail.com>:
> Thanks Graham,
> The difference is very clear now.
> Yes I agree that performance with mod_python is not optimal, but for now
> it's a compromise because I'm using an existing product
> ( http://www.infrae.com/products/tramline -- I know there's been other
> questions about this product on this list before). The question came up
> because there's a couple of branches of it around and I wanted to understand
> what was going on.

Hmmm, are they still claiming mod_python needs to be patched? I can't
remember the change they were saying needed to be made, but couldn't
see how that change fixed anything. A different change was made for
another issue in code which was found when investigating their change,
but not sure I ever got back any conclusive response as to whether
that addressed problem. There ticket related to that is:


Tramline was a good example of something that should have been a
Apache C module. I wouldn't be surprised actually if one already did
exist that does the same thing.


> -- Jean
> On Wed, Dec 10, 2008 at 2:13 AM, Graham Dumpleton
> <graham.dumpleton at gmail.com> wrote:
>> 2008/12/10 Jean Lagarde <jean.lagarde at gmail.com>:
>> > Hi all,
>> > The subject says it all. The documentation seems a little sparse on the
>> > topic. The description of pass_on is:
>> > Passes all data through the filter without any processing.
>> >
>> > The description of disable is:
>> > Tells mod_python to ignore the provided handler and just pass the data
>> > on.
>> > (which also states that mod_python uses that one internally).
>> > "passes all data through" and "just pass the data on" sound like the
>> > same
>> > thing to me. What is the additional functionality to "ignore the
>> > provided
>> > handler"? I assume that there is a difference between the two methods
>> > and I
>> > would like to know what it is no matter how subtle. I'm looking for the
>> > most
>> > CPU efficient way to decide early on in the execution of a filter that
>> > we
>> > don't want to filter a particular request in any way, but maybe
>> > understanding the difference would help me to make a better use of
>> > mod_python filters in other ways.
>> > I did try to use both in code, and on one server with one version of
>> > Apache
>> > 2 (and other components) it seemed to make no functional difference
>> > (although I did not attempt to closely measure CPU usage), whereas on
>> > another server with an earlier version of Apache 2 (and other
>> > components),
>> > using disable() caused no error, but dropped values from POST forms
>> > passed
>> > on to PHP scripts.
>> > Thanks for any info. I did search the lists, but only found one comment
>> > on
>> > someone's code from Graham that "Normally 'filter.pass_on()' would be
>> > used
>> > here, not 'filter.disable()'", but without explaining why.
>> If you use disable(), your filter will never be called for that
>> request (input or output as appropriate) again.
>> If you use pass_on(), then it is passing on data available to that
>> specific call to your filter function.
>> The distinction exists because your filter function can be called
>> multiple times for a request. The latter is allowing you to pass on
>> the data for that call, but still allowing you the option to process
>> data in a subsequent call of the filter function.
>> The disable() call is used internally to mod_python to ensure that
>> filter function is never called again for a specific request if the
>> filter function itself raises an exception of some sort. If it doesn't
>> do this you could end up with a loop if the error in filter function
>> actually generates output, for which it would then call the filter
>> function again, etc etc.
>> FWIW, if you are concerned about performance in any way, then you
>> shouldn't be using mod_python to implement Apache input/output
>> filters. Yes you can write filters with mod_python, but it will impact
>> performance. For best performance you should write filters in C code
>> as proper Apache module.
>> Graham
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