Hi,
I cannot resist writing color commentary to Ken's excellent explanation.
On Wed, Nov 20, 2013 at 8:38 AM, G. Ken Holman
<gkholman(_at_)cranesoftwrights(_dot_)com> wrote:
If my XSLT program accesses ancestor nodes, that seems to require the XSLT
processor to back up. And isn't that a violation of the fundamental law of
streaming, "The XSLT processor shall not back up"?
I don't think so ... you haven't finished with that ancestor yet, so you
really haven't left it. It is still "open" in that you are still in the
process of acting on that ancestor. I think "backing up" is the issue only
once you have left the node and moved on. In the case of ancestors, you
haven't left them yet.
Indeed. Remember that the XPath 'preceding' axis includes only nodes
that have started *and finished* (opened and closed) before the
context node.
(As Ken also follows up to remark.)
Or, perhaps my XSLT program can access ancestor nodes because, as the XSLT
processor descends the XML tree it keeps a record of each node through which
it descends (the node's name, its attributes, its namespaces). Yes, that
must be what the XSLT processor does. Suppose that my XML tree is very deep,
then the XSLT processor will have to remember a lot of stuff, right? In the
extreme case, every node in the XML document has no siblings, just one
child. Thus, the XSLT processor would have to remember the entire XML
document, right?
I think you are putting too much into "remembering" for ancestors. And XML
documents are typically flat (though my course material happens to go quite
deep).
I think it would be very interesting to see a survey of how deep XML
documents go in the wild. Except for pathological cases, I think they
would rarely go beyond 20 deep. Of course this will vary a great deal
by document type.
Ken, what does "quite deep" mean in your case?
I think streaming is a benefit for flatter XML documents ... I certainly
would not think that a skinny and deep XML document with only one leaf node
and the thousands of branches would be seen in the wild, nor would it
benefit from streaming.
Or one leaf on one very long branch :-> if you prefer.
This is related to the rule of thumb that streaming will help with
memory management, but will not (perhaps contrary to expectations)
have much impact on processing speed. Streaming is useful when
branches of the document can be processed without reference to other
branches. The smaller your branches (each of which offers a discrete
processing context), the more you benefit. Note that in this way of
thinking, every "branch" goes all the way to the root.
Cheers, Wendell
Wendell Piez | http://www.wendellpiez.com
XML | XSLT | electronic publishing
Eat Your Vegetables
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