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Re: Cannot write more than one result document to the
Subject: Re: Cannot write more than one result document to the same URI
From: Dan Vint <dvint@xxxxxxxxx>
Date: Thu, 04 Apr 2013 20:22:20 -0700
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Thanks, I can see that argument. but now you open a can of worms for
me. So say your template for b just writes output to the standard
result tree (not separate files). Why doesn't the same argument for
parallelism apply there?
Probably answering my own question here, I'm guessing the difference
is writing to a file more or less serially vs writing to a tree. The
processor can write to multiple locations in the tree and keep track
of where multiple b outputs should be created, but not as well
supported writing to a file from multiple sources (unless your
willing to live with jumbled results in the file).
Thanks again.
..dan
At 07:56 PM 4/4/2013, you wrote:
At 2013-04-04 19:37 -0700, Dan Vint wrote:
I can live with the rule, just would like to understand the logic.
Consider the following scenario. An XML document has two elements <b>:
<a>
<b id="1">...</b>
<b id="2">...</b>
</a>
An XSLT stylesheet uses the built-in template rule for <a> and has a
template rule for <b>:
<xsl:template match="b">
<xsl:result-document href="output.xml">
<xsl:copy-of select="."/>
</xsl:result-document>
</xsl:template>
If the specification allowed this, then without considering the
opportunities of parallelism, one might come to the conclusion that
the file "output.xml" would always contain:
<b id="1">...</b><b id="2">...</b>
The problem is that the specification does not require the XSLT
processor to complete the processing of the first <b> before
starting or even ending the processing of the second <b>. Sure a
single-process implementation "X" likely would. But a parallelized
(is that a word?) implementation "Y" running on multiple CPUs could
very well fully process the second <b> before the first <b> if it
chose to do so. Its only obligation is to arrange the resulting
tree with the result of processing the first <b> before the result
of processing the second <b>. This obligation ensures that the
result of processing by "X" is identical to the result of processing
by "Y". But there is no obligation on what the processor does to
get to that result.
When using <xsl:result-document> the processor is not building the
result tree. It is creating a completely separate result. If the
instruction required "re-opening" of the file for append, processor
"X" likely would produce the expected result, but processor "Y" in
the situation above would produce an unexpected result. Two
processors would produce two results.
And this is also why one cannot assert that the writing to the file
is even finished before the next attempt to write to the file
starts. The file handle could very well still be left open by one
parallel process when the other is ready to open it for itself. So
it can't be used even if the file is opened for write and not for append.
Note that some of my students have come to class thinking that you
have to fully complete an <xsl:result-document> before starting
another one to another URI, but I tell them that is not the
case. You can nest <xsl:result-document> instructions to different
URI target locations, and the nested <xsl:result-document> will
complete the nested file and resume the "outer" file output when
done, without having to close and re-open the outer file. This has
been a very handy feature when fragmenting files. And this would be
another reason not to allow the same URI to be used.
I know the what now, even though I don't understand why the rule exists ;-)
I hope the explanation above has helped.
. . . . . . . . . Ken
--
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http://www.dvint.com
voice: 619-938-3610
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