[xsl] Dynamic pipelining in XSLT 2.0 w/ Saxon extensions

[Wendell Piez <wapiez@xxxxxxxxxxxxxxxx>]

Hi,

One of the niftier extensions available in the Saxon library is one 
that allows a stylesheet to be compiled and applied dynamically.

As its designer undoubtedly anticipated, it can even provide the 
basis for at least a lightweight pipelining infrastructure. The 
stylesheet below allows the specification of a set of stylesheets to 
be run in sequence over the input. Presently the stylesheets are 
named in a top-level variable, though of course this spec could also 
be external (and more fully-featured). Given the variable

  <xsl:variable name="processes">
    <m:path>stylesheets/</m:path>
    <m:step>one.xsl</m:step>
    <m:step>two.xsl</m:step>
    <m:step>three.xsl</m:step>
    <m:done/>
  </xsl:variable>

the stylesheets "stylesheets/one.xsl", "stylesheets/two.xsl" and 
"stylesheets/three.xsl" (relative to the pipelining stylesheet) are 
run in series over the input document, until either a "done" element 
is encountered, or there are no more steps. (The "path" element may 
be omitted; if used it must currently appear first.)

My questions:

* This runs (tested under the current Saxon 8.9), but how will it 
scale? In particular, Mike Kay may be able to say whether compiled 
stylesheets are be cached when this is run over a set of documents. 
If not, wouldn't compiling each stylesheet anew for each input 
document be an impediment?

* Are there any obvious pitfalls or problems with this approach? (Or 
any not so obvious?) How does it compare to other methods?

(So far I can say only that it's really easy to use, but again I'm 
not sure it will scale as well as other methods. It does have the 
advantage, it appears, of avoiding serialization altogether until the 
last step.)

The stylesheet:

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform";
  xmlns:saxon="http://saxon.sf.net/";
  xmlns:m="http://mulberrytech.com/local/xslt";
  version="2.0"
  extension-element-prefixes="saxon"
  exclude-result-prefixes="m">

   <xsl:output method="xml"/>

  <!-- Saxon pipelining stylesheet:

       Takes document input and passes it through a chain of transformations

       The steps will run in order until a "done" element is 
encountered or they run out

       -->

  <xsl:variable name="processes">
    <m:path>./</m:path>
    <m:step>one.xsl</m:step>
    <m:step>two.xsl</m:step>
    <m:step>three.xsl</m:step>
    <m:done/>
  </xsl:variable>

  <xsl:template match="/">
    <xsl:apply-templates select="$processes/m:step[1]">
      <xsl:with-param name="in" select="/"/>
    </xsl:apply-templates>
  </xsl:template>

  <xsl:template match="m:step">
    <xsl:param name="in"/>
    <xsl:param name="stylesheet" select="concat($processes/m:path,.)"/>
    <xsl:variable name="out">
      <xsl:variable name="process" 
select="saxon:compile-stylesheet(doc($stylesheet))"/>
      <xsl:apply-templates select="saxon:transform($process,$in)" 
mode="annotate">
        <!-- 'annotate' mode will provide the result with local 
annotations -->
        <xsl:with-param name="stylesheet" select="$stylesheet"/>
      </xsl:apply-templates>
      <!-- If you don't want the result annotated, simply use
           <xsl:copy-of select="saxon:transform($process,$in)"/> -->
    </xsl:variable>
    <xsl:if test="empty(following-sibling::*[1])">
      <xsl:copy-of select="$in"/>
    </xsl:if>
    <xsl:apply-templates select="following-sibling::*[1]">
      <xsl:with-param name="in" select="$out"/>
    </xsl:apply-templates>
  </xsl:template>

  <xsl:template match="m:done">
    <xsl:param name="in"/>
    <xsl:copy-of select="$in"/>
  </xsl:template>

  <xsl:template match="/" mode="annotate">
    <xsl:param name="stylesheet" select="'not given'"/>
    <xsl:comment>
      <!-- adding a comment to the top of the result -->
      <xsl:text> process run: </xsl:text>
      <xsl:value-of select="$stylesheet"/>
    </xsl:comment>
    <xsl:text>&#xA;</xsl:text>
    <xsl:copy-of select="node()"/>
  </xsl:template>

</xsl:stylesheet>

For testing, my one.xsl, two.xsl and three.xsl stylesheets all look 
like this (with adjustments):

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0"
 xmlns:xsl="http://www.w3.org/1999/XSL/Transform";>

<xsl:template match="*">
  <xsl:element name="{local-name()}-one">
    <xsl:copy-of select="@*"/>
    <xsl:apply-templates/>
  </xsl:element>
</xsl:template>

  <xsl:template match="comment() | processing-instruction()">
    <xsl:copy-of select="."/>
  </xsl:template>

</xsl:stylesheet>

Comments welcome,
Wendell



======================================================================
Wendell Piez                            mailto:wapiez@xxxxxxxxxxxxxxxx
Mulberry Technologies, Inc.                http://www.mulberrytech.com
17 West Jefferson Street                    Direct Phone: 301/315-9635
Suite 207                                          Phone: 301/315-9631
Rockville, MD  20850                                 Fax: 301/315-8285
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