Logging with XSLT

Question

How and where could I output log messages for debug and performance purposes during an XSLT transformation?

I guess the simplest method is using expressions like this:

<xsl:text>message text</xsl:text>

here and there in the code, using xsl:value-of if needed.

But this method prints the messages in a lot of places in the output file (HTML page in my case), that is where it is called, and not always in the same place (like a log file).

Is this the only way or is there a better solution? Thanks!

Answer 1

Modifying your XSLT itself for the purposes of logging is inevitably going to impact on performance, you're probably better off using an external tool. There are a few available depending on what you're working with:

• Stylus Studio
• Visual Studio
• Altova XMLSpy

Answer 2

You should be able to use <xsl:message> I think, although where the logging goes is implementation-dependent.

Answer 3

This is exactly what <xsl:message> is designed for. However, the output location is entirely dependent on the processor. I only have a Mac handy but, sadly, both Firefox and Safari suppress the <xsl:message> output. I expect MSIE will do the same.

Given that, I think your best bet is to use <xsl:comment> to generate your logs. Something like the below should do the trick:

<xsl:template match='my-element'>
   <xsl:comment>Entering my-element template</xsl:comment>
   <p class='my-element'><xsl:apply-templates/></p>
   <xsl:comment>Leaving my-element template</xsl:comment>
</xsl:template>

That would give you something like this in the output:

<!-- Entering my-element template -->
<p class='my-element'>...</p>
<!-- Leaving my-element template -->

Clearly, you can put whatever logging you want into that that output. I would consider creating something like the following and using it to run your logging. This references a global param called 'enable-logging' to determine if logging should occur or not.

<xsl:template name='create-log'>
   <xsl:param name='message'/>
   <xsl:if test="$enable-logging = 'yes'">
       <xsl:comment><xsl:value-of select='$message'/></xsl:comment/>
   </xsl:if>
</xsl:template>

Use this in your stylesheet as:

<xsl:template match='my-element'>
   <xsl:call-template name='create-log'>
     <xsl:with-param name='message'/>Entering my-element template</xsl:with-param>
   </xsl:call-template>
   <p class='my-element'><xsl:apply-templates/></p>
   <xsl:call-template name='create-log'>
     <xsl:with-param name='message'/>Leaving my-element template</xsl:with-param>
   </xsl:call-template>
</xsl:template>

One benefit of doing it this way is you can change that <xsl:comment> to <xsl:message> when in a more complete environment. It is more verbose but more general.

Answer 4

I would suggest using xsl:message if you are in a development environment such as oXygen or Stylus Studio, and using xsl:comment if you are running in the browser. You shouldn't really be debugging your XSLT code in the browser - the browsers I know about are lousy as XSLT debugging tools.

Answer 5

By looking for a solution to this problem, I ended up implementing a logging mechanism in XSLT in similar fashion to log4j, using mostly xsl:message and pure XSLT 2.x. I took some of the answers in this page as inputs. The library is available here: https://github.com/ukuko/log4xslt

Answer 6

A simple hack would be to create a variable with xsl:variable and to either just concat() new values to it or to set up a xsl:template which does the same thing. Then you just need to output this variable at the end of the execution and you can explicitly choose where to show the log.