xsl:fork

Description

xsl:fork solves a fundamental streaming constraint: you can only read a stream once. If you need to compute two different aggregations (say, a count and a sum) over the same streaming input, you would normally need two passes — but streaming prohibits that. xsl:fork allows both operations to happen in a single pass by running multiple sequence constructors over the same streamed input simultaneously.

Each direct child sequence constructor of xsl:fork is evaluated independently against the same input. The results of all branches are concatenated in document order. Branches must be individually streamable — they cannot share state with each other except via accumulators.

xsl:fork is only meaningful in a streaming context. Outside streaming it works but is equivalent to simply writing the branches sequentially. The real value is inside xsl:stream or inside a streamable template where the input can only be consumed once.

A common pattern is xsl:fork containing two or more xsl:sequence children, each with a streamable select expression (such as count(//element) or sum(//amount)). Each select is computed in parallel over the single stream.

Attributes

xsl:fork has no element-specific attributes. Its children are sequence constructors forming the independent branches.

Examples

Computing count and sum in one streaming pass

Stylesheet:

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="3.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xsl:output method="xml" indent="yes"/>

  <xsl:param name="source-uri" select="'transactions.xml'"/>

  <xsl:template name="xsl:initial-template">
    <xsl:stream href="{$source-uri}">
      <xsl:variable name="stats">
        <xsl:fork>
          <xsl:sequence select="count(//transaction)"/>
          <xsl:sequence select="sum(//transaction/@amount)"/>
        </xsl:fork>
      </xsl:variable>
      <summary>
        <count><xsl:value-of select="$stats[1]"/></count>
        <total><xsl:value-of select="$stats[2]"/></total>
      </summary>
    </xsl:stream>
  </xsl:template>
</xsl:stylesheet>

Fork with apply-templates branches

Stylesheet:

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="3.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:output method="xml" indent="yes"/>
  <xsl:mode name="count-mode" streamable="yes"/>
  <xsl:mode name="list-mode" streamable="yes"/>

  <xsl:param name="source-uri" select="'catalog.xml'"/>

  <xsl:template name="xsl:initial-template">
    <xsl:stream href="{$source-uri}">
      <report>
        <xsl:fork>
          <xsl:apply-templates select="." mode="count-mode"/>
          <xsl:apply-templates select="." mode="list-mode"/>
        </xsl:fork>
      </report>
    </xsl:stream>
  </xsl:template>

  <xsl:template match="/" mode="count-mode">
    <total-items><xsl:value-of select="count(//item)"/></total-items>
  </xsl:template>

  <xsl:template match="/" mode="list-mode">
    <xsl:for-each select="//item">
      <item><xsl:value-of select="@name"/></item>
    </xsl:for-each>
  </xsl:template>
</xsl:stylesheet>

Notes

• Each branch of xsl:fork must be independently streamable. The branches cannot share variables or communicate except through accumulators.
• The results of all branches are concatenated in the order the branches appear.
• xsl:fork can contain xsl:fallback to handle non-streaming processors gracefully.
• Using xsl:fork outside a streaming context is valid but adds no value — branches execute sequentially.
• Processors may or may not truly parallelise the branches; the specification only requires that the result is as-if they ran simultaneously.

See also

• xsl:stream
• xsl:merge
• xsl:accumulator