Streaming with xsl:merge
Saxon (since 9.6) allows several streamed inputs to be merged using the new XSLT 3.0 xsl:merge instruction. For this to work, there are a number of rules to follow:
-
Streaming must be requested by specifying
streamable="yes"
on the xsl:merge-source element. -
When streaming is requested, the
for-each-source
attribute ofxsl:merge-source
must be present, and must be a single string. -
The
select
attribute on thexsl:merge-source
element must take the form of a motionless pattern.
For each node selected by the select
expression, Saxon takes an implicit
snapshot (in the sense of the XSLT 3.0 fn:snapshot() function). The merge keys are evaluated in
relation to this snapshot, and it is this snapshot that is presented within the
xsl:merge-action
construct as the result of the fn:current-merge-group() function.
Here is an example of streamed merging of two log files:
<xsl:merge> <xsl:merge-source streamable="yes" for-each-source="'log-file-1.xml'" select="events/event"> <xsl:merge-key select="xs:dateTime(@timestamp)"/> </xsl:merge-source> <xsl:merge-source streamable="yes" for-each-source="'log-file-2.xml'" select="log/day/record"> <xsl:merge-key select="dateTime(../@date, time)"/> </xsl:merge-source> <xsl:merge-action> <group> <xsl:copy-of select="current-merge-group()" /> </group> </xsl:merge-action> </xsl:merge>