Here's what I hope to be a less "dogmatic" rewrite, thanks to input from Conal and Nicola Ken. I added the term "may" in the first sentence of the opening "A." paragraph. I also included advice for those who need to use of the document() function in Cocoon toward the end.
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Q. What's "wrong" with use of the document() function in Cocoon?
A. Using the document() function for aggregation in Cocoon may break Separation of Concerns (SoC). That is, the designers of Cocoon view inclusion and transformation as different functions, best handled by separate Cocoon components. Treating them separately allows you to achieve performance gains and increases the resusability of your pipelines.
Alternatives to the document() in the Cocoon environment include aggregation or the use of a multi-stage transformation using the XInclude Transformer. This involves transforming a list of documents (generated dynamically or statically) by adding xinclude elements which reference (via xpointer) specific document content, and then transforming again via the XInclude Transformer, to obtain the desired result.
For an example of this, see: http://marc.theaimsgroup.com/?l=xml-cocoon-users&m=102617106411067&w=2
You'll achieve better performance if you aggregate content prior to transformation.
This allows you to take full advantage of Cocoon's pipeline caching. In contrast,
making dynamic document() calls inside an XSLT within a cached pipeline is problematic.
At this time, Cocoon does not recognize changes in documents (called by the document() function)
until the requested page expires from cache.
Understand that the document() function was designed *before* xinclude
with xpointer facilities existed. Had such capabilities been available,
perhaps the document() function, which essentially mimics xinclude and xpointer,
would have never been added to XSLT.
Please note that if you must work with your XML files outside of the
Cocoon environment, you may need to use the document() function
in order to utilize the limited capabilities of other pipeline engines.
This includes engines which are not xinclude-capable or which
lack a predefined way to indicate document processing steps. If you
are working with legacy code from non-pipelined engines, you may need to use
the document() function as well, at least initially.
If you do use the document() function in Cocoon, you can still observe SoC by
having separate XSLT stylesheets perform inclusion and transformation functions.
For example, you can put multiple XSLT transforms in a pipeline and have the
first one perform inclusion and the second one perform transformation. However,
be mindful of some unresolved caching issues in Cocoon related to the document() function.
At this time, Cocoon is unable to check validity of content included via the document()
function. In addition, the document() function implemented by Xalan is inefficient. See:
http://nagoya.apache.org/bugzilla/show_bug.cgi?id=4257
Until this bug is fixed, consider using Saxon instead for document() function-related parsing
needs.
For other aggregation/inclusion approaches, please stay tuned for XpathDirectoryGenerator (2.1 scratchpad),
as well as Forrest's Libre (currently alpha in the Forrest cvs).
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If this is acceptable to everyone, I'll commit it in the XSLT FAQs.
Thanks again to Nicola Ken and Conal.
-- Diana
