[xsl] Incremental Update: change a portion of the XML and the XSLT operates on just that portion

["Costello, Roger L." <costello@xxxxxxxxx>]

Hi Folks,

On page 987 of the book, "XSLT 2.0 and XPath 2.0" the author Michael Kay
discusses the advantages of functional programming. One of the main benefits,
he argues, is the ability to do incremental updates. Here is a paragraph that
really captured my interest:

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We want to get away from static pages; if you're showing a map of traffic
congestion hotspots in your area, then when the data for a particular road
junction changes, you want the map updated in real time, and it should be
possible to do this without recalculating and redrawing the whole map. This is
possible if there's a direct relationship--a function--between whats shown at
a particular place on the map display and a particular data item in the
underlying database. So if a program is decomposed into a set of smaller,
independent functions, each relating one piece of the output to one piece of
the input, then we have the potential to do this on-the-fly updating.
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Wow!  I want to do that.  But how?

Suppose that I have an XML document and an XSLT transform. The XSLT transform
processes the entire document and outputs, say, an HTML document. Later, I
update one portion of of the XML document. How do I get the XSLT to operate on
just the updated portion? And how does the XSLT update just the relevant
portion of the HTML document?

/Roger