Caching and XPointer - how is validity determined?

I have a question about the way OPS handles caching; I'm trying to
figure out whether my pipeline will do what I want it to do.

Here's some background. I'm internationalizing an existing OPS app, and
trying to take advantage of caching for the "static" elements in the
page (labels, page headers, etc.). Here's a simplified version of the
pipeline in question:

<p:config xmlns:p="http://www.orbeon.com/oxf/pipeline" etc...>

     <p:param type="input" name="request"/>
     <p:param type="output" name="view"/>

     <!-- The "request" input is a processed version of the output
     from the OPS request generator. The important thing here is that
     it contains a subtree at "/req/site" that tells me what version of
     the site to serve, and a subtree at "/req/locale" that contains
     the desired locale for this request, like "en_US". There is only a
     small set of possible sites, and a small set of locales. -->

     <!-- Fill in the menu, subnav, etc -->
     <!-- This transform populates tags in the template like <twc:menu/>
          with a list of the menu items that should appear. This can vary
          depending on the site and the locale. -->

     <p:processor name="oxf:unsafe-xslt">
         <p:input name="config" href="oxf:/framework/view/insert-nav.xsl"/>
         <p:input name="data" href="oxf:/a-file-from-disk-somewhere"/>
         <p:input name="site" href="#request#xpointer(/req/site)"/>
         <p:input name="locale" href="#request#xpointer(/req/locale)"/>
         <p:output name="data" id="raw-view-with-nav"/>
     </p:processor>

     <!-- Localize the completed template -->
     <!-- This transform replaces <twc:i18n>some-text</twc:i18n> tags
          with the appropriate text for the locale. -->

     <p:processor name="oxf:unsafe-xslt">
         <p:input name="config" href="oxf:/framework/view/localize.xsl"/>
         <p:input name="data" href="#raw-view-with-nav"/>
         <p:input name="site" href="#request#xpointer(/req/site)"/>
         <p:input name="locale" href="#request#xpointer(/req/locale)"/>
         <p:output name="data" ref="view"/>
     </p:processor>

</p:config>

My question has to do with the "site" and "locale" inputs to those two
transforms. I'm using XPointer to peel off just the "site" and "locale"
elements, and I want the results of this pipeline to be cached using
those elements to determine the validity: in other words, for any given
combination of template file, site, and locale, I only want to run this
pipeline once.

Will this pipeline have the desired result? Or will the cache validity
be determined by the whole "request" input, which will of course be
different with every request? Is there something else I can do that will
  give me the desired caching behavior?

Thanks in advance for any help you can offer.

Jeff Jones
Sr. Internet Appl. Developer
The Weather Channel Interactive

Sr Internet Appl Dev



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Jeff,

 > My question has to do with the "site" and "locale" inputs to those
 > two transforms. I'm using XPointer to peel off just the "site" and
 > "locale" elements, and I want the results of this pipeline to be
 > cached using those elements to determine the validity: in other
 > words, for any given combination of template file, site, and locale,
 > I only want to run this pipeline once.
 >
 > Will this pipeline have the desired result? Or will the cache
 > validity be determined by the whole "request" input, which will of
 > course be different with every request? Is there something else I
 > can do that will give me the desired caching behavior?

An XPointer expression within p:input/@href is transformed into an
internal XPathProcessor. The output of the processor (which is
connected to the "site" and "locale" inputs of the XSLT processor)
depends, from a caching perspective, on the value of the XPath
expression *and* on the document on which the XPath epxression
applies, that is #request.

On the XSLT side, caching of the output will occur only if you refer
to the "site" and "locale" inputs statically, e.g. by using
doc('input:site') and doc('input:locale'). If you compute the
parameters to the doc() or document() function, caching will not
occur.

So I think the answer to your question is that the output of the XSLT
transformation will be cacheable based on all the XSLT
transformation's dependencies, including your entire #request
document.

If that document depends in turn on the output of an oxf:request
processor that contains the *whole* request, then as you say whenever
anything in the request changes caching will be invalidated. On the
other hand, if your oxf:request processor produces only what you need
(using the include and exclude elements in its configuration), then
caching should occur, because oxf:request correctly digests its output
for cache validation.

One bottom line for this message: caching is tricky! Maybe somebody
could write an OPS pipeline cache analyser in XForms? That would
surely be quite welcome ;-)

-Erik

--
Orbeon - XForms Everywhere:
http://www.orbeon.com/blog/




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