Meta-stylesheets: Exploring the Provenance of XSL Transformations
Copyright © 2012 by the author. Used with permission.
Table of Contents
Copyright © 2012 by the author. Used with permission.
Table of Contents
Provenance records are records for trust and authenticity, records to prove that the object is what it is claimed to be. Provenance has its roots in the archival world, where the goal is to see objects survive with minimal weathering as they pass from one caretaker to the next (Sweeney, 2008). Similarly, data provenance answers the question "How did this come to be here, in this state?", and it does so by identifying the agents, transformation events, and other artifacts associated with that object.
Computer usage is so prevalent in humanities and sciences research that it is all but guaranteed that data will be changed in some way during its lifetime. This has the advantage that tools can automatically generate provenance information, recording important details about the transformation process. Later, a provenance record of this sort may be used for error-checking, to independently reproduce a result, or simply for results verification (Küster, Ludwig, Al-Hajj, & Selig, 2011). Few e-humanities software tools offer provenance capabilities, likely due to time constraints and a lack of common practice. However, one reason may be in terms of nomenclature. Humanities research often involves documents, and humanists may bristle to think of their documents as the 'data' part of data provenance (Fear, 2011).
Still, more and more documents are becoming - or are born as - data, thanks to mark-up languages (Buneman, Khanna, & Tan, 2000). XSLT is one possible step in document transformation, and as such, is subject to the same provenance needs as other processes. XSL transformations can be examined and, with a little effort, used to generate provenance information about the creation of documents. The goal of this project is to examine methods of gathering provenance information about XSL transformations, and to explore the possibilities of automatically generating such records.
A handful of provenance ontologies exist, and each identifies three basic classes of provenance: information about processes, or transformation events; about agents, or the people (or software) which catalyze or carry out the event; and artifacts, or 'things' which may be acted on, generated by, or otherwise associated with a transformation event. The below figure gives the terminology of two such ontologies, the Open Provenance Model (Moreau et al., 2010) and the W3C PROV-O (Belhajjame et al., 2012).
Figure 1: Provenance classes of OPM and W3C PROV-O
|Open Provenance Model (OPM)||Artifact||Process||Agent|
My first attempts at provenance documentation were founded on the XML binding of OPM. OPM was designed primarily to capture information about scientific data workflows, and loosely fit the information needed to document XSLT provenance. Current work makes use of the Open Provenance Model Vocabulary, a modified version of OPM which includes a module for XSLT provenance (Zhao, 2010). As such, this paper makes use of OPM and OPMV XSLT module terminology.
Gathering XSLT provenance information is already possible through a number of methods. The simplest method (though not necessarily the easiest for understanding) is to simply examine the artifacts associated with an XSL transformation - mainly the input, output, and stylesheets involved in the transformation. In particular, comments left on a stylesheet could reveal the author agent's identity, thought process, and understanding of what the XSLT code does or is supposed to do. One might be able to determine the templates' firing order, but the process may just as well be laborious in terms of piecing together provenance.
A better method is to use a debugger to step through a transformation, which grants an in-depth look at the way an output document is constructed from input and stylesheet. Parameter and variable values may also be shown, which may explain choices made by the XSLT processor. This step-through method is aimed for human understanding, but no record of the steps is available for consultation. Provenance information would be easier to identify with this method as opposed to the first, but it would still have to be documented by hand.
In contrast, the Saxon processor has a trace function which provides a tree view of the steps within an XSL transformation. It shows what input nodes are matched to templates, and includes the file name and line numbers of each processed node. The trace log cannot reference the specific elements in the output that came into existence due to any one transformation step. Instead, one must infer the structure of the output from the steps taken. Still, when used as a roadmap to examining the input documents and stylesheets, the trace log is explicit and exact in its references - a good representation of the transformation which created the output, so long as one has access to the files referenced.
Figure 2: Sample Saxon trace output (with other error output removed)
<trace saxon-version="188.8.131.52" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"> <source node="/" line="0" file="test1.xml"> <xsl:template match="/" line="16" module="testRegStylesheet.xsl"> <LRE name="html" line="17" module="testRegStylesheet.xsl"> <LRE name="head" line="18" module="testRegStylesheet.xsl"> <LRE name="title" line="19" module="testRegStylesheet.xsl"> <xsl:text line="19" module="testRegStylesheet.xsl"> </xsl:text> </LRE> </LRE> <xsl:apply-templates line="21" module="testRegStylesheet.xsl"> <source node="/test/compute" line="3" file="test1.xml"> <xsl:template match="compute" line="26" module="testRegStylesheet.xsl"> <LRE name="body" line="27" module="testRegStylesheet.xsl"> <xsl:apply-templates line="28" module="testRegStylesheet.xsl"> <source node="/test/compute/message" line="3" file="test1.xml"> <xsl:template match="message" line="33" module="testRegStylesheet.xsl"> <xsl:call-template line="34" module="testRegStylesheet.xsl"> <xsl:value-of line="36" module="testRegStylesheet.xsl"> </xsl:value-of> <xsl:template name="summon" line="42" module="testRegStylesheet.xsl"> <xsl:param name="sayeth" line="43" module="testRegStylesheet.xsl"> </xsl:param> <LRE name="b" line="44" module="testRegStylesheet.xsl"> <xsl:text line="45" module="testRegStylesheet.xsl"> </xsl:text> <xsl:value-of line="46" module="testRegStylesheet.xsl"> </xsl:value-of> <xsl:text line="47" module="testRegStylesheet.xsl"> </xsl:text> <xsl:value-of line="48" module="testRegStylesheet.xsl"> </xsl:value-of> </LRE> </xsl:template> </xsl:call-template> </xsl:template> </source><!-- /test/compute/message --> </xsl:apply-templates> </LRE> </xsl:template> </source><!-- /test/compute --> </xsl:apply-templates> </LRE> </xsl:template> </source><!-- / --> </trace>
XSLT itself also provides the capability to capture some provenance information, through functions such as
base-uri(). With these functions, the output of a stylesheet could include information on the XSLT version used; the processor agent which controlled the XSL transformation; and the file names of an input document and the primary stylesheet. However, the use of this method is dependent on those who created or who maintain the stylesheet.
The oXygen XML Editor has a documentation generation tool for XSLT stylesheets. The tool creates a log giving information about the stylesheet and children of the
xsl:stylesheet node (most notably, templates). Interestingly enough, the documentation tool also assumes that if a node has a comment placed immediately before it, then the comment is documentation for the contents of that node. While this assumption could be wrong, it certainly rewards those who provide descriptions of stylesheets and templates through comments. It is worth noting that the documentation tool may not contain any provenance information at all, since it only describes the stylesheet artifact and does not guarantee any references to agents, process instances, or other artifacts. On the other hand, the documentation is an excellent example of the types of information that should be included when a provenance record references a stylesheet or template artifact.
What kinds of information, then, can we find about an XSL transformation?
XSLT processor name, version, vendor
input document URI
template matches or calls
literal result element (LRE) insertion
input node transformation steps
authorship (dependent on programmer)
copyright information (dependent on programmer)
explanation of coding choices (dependent on programmer)
When I started looking at XSLT provenance, I hoped to find a method that would be:
comprehensive, combining as many types of the above provenance information as possible;
easy to use - either fitting right into existing workflows, or able to be run and used with minimum fuss; and
in a format built for shareable provenance.
Wendell Piez suggested a possible solution - to build a "meta-stylesheet." The meta-stylesheet will take stylesheet A as input, and output a stylesheet B, which in turn will approximate the XSLT processes of running stylesheet A with the same input document. Instead of the output expected from stylesheet A, the "meta'd" transformation scenario will output an OPMV-based RDF graph, providing provenance information on multiple levels of granularity. Currently, only two levels of provenance are captured: first, the file-level transformation process; second, the firing of templates as they are matched or called.
The steps of the XSLT provenance workflow are currently as follows:
Use the meta-stylesheet to create stylesheet B (for generating provenance) and, optionally, stylesheet C (for assigning
rdf:IDs to template code)
Use stylesheet B on input.xml to create an OPMV RDF graph representing the provenance of step 2.
Figure 3: An overview of the meta-stylesheet method
Figure 4: Provenance at the file level
Figure 5: Sample file-level provenance documentation (provenance.rdf)
<!-- Provenance for the transformation of file:/C:/Users/Ashley/Desktop/global_001/input.xml using stylesheetC.xsl: --> <xslt:Stylesheet rdf:about="stylesheetC.xsl"> <xslt:version>1.0</xslt:version> </xslt:Stylesheet> <xslt:Processor rdf:ID="proc01"> <xslt:vendor rdf:resource="http://www.saxonica.com/"/> <xslt:productName>SAXON</xslt:productName> <xslt:productVersion>PE 184.108.40.206</xslt:productVersion> </xslt:Processor> <xslt:Binding rdf:ID="alpha"> <xslt:value>Y</xslt:value> </xslt:Binding> <xslt:Transformation rdf:ID="overall"> <xslt:source rdf:resource="file:/C:/Users/Ashley/Desktop/global_001/input.xml"/> <xslt:stylesheet rdf:resource="stylesheetC.xsl"/> <xslt:processor rdf:resource="#proc01"/> <xslt:binding rdf:resource="#alpha"/> </xslt:Transformation>
Like the overall transformation process, template-level provenance heavily uses the XSLT module for OPMV. Here, each
xslt:Transformation points to an
xslt:Template artifact, which could be of either subtype
xslt:NamedTemplate. The edge between the two is
xslt:used. If the template firing was triggered by
xsl:apply-templates, then it also contains a reference to the source node to which the template was matched. Any template, and thus any transformation, might contain parameters or variables, and thus, bindings. Unlike stylesheet bindings, a template parameter of the same name - but different
rdf:ID! - can carry a different value every time the template is run.
Figure 6: Provenance at the template level
Figure 7: Sample template-level provenance documentation (provenance.rdf)
<!-- Template-level provenance: --> <xslt:Template rdf:about="stylesheetC.xsl#art_t1"> <rdfs:label>mode='#default', match='/', name=''</rdfs:label> <rdfs:comment><xsl:template match='/'> <html> <head> <meta http-equiv='Content-Type' content='text/html; charset=UTF-8'></meta> <title>Provenance</title> </head> <xsl:apply-templates></xsl:apply-templates> </html> </xsl:template></rdfs:comment> <opmv:wasEncodedBy rdf:resource="stylesheetC.xsl"/> </xslt:Template> <xslt:Template rdf:about="stylesheetC.xsl#art_t2"> <rdfs:label>mode='#default', match='compute', name=''</rdfs:label> <rdfs:comment><xsl:template match='compute'> <body> <xsl:apply-templates></xsl:apply-templates> </body> </xsl:template></rdfs:comment> <opmv:wasEncodedBy rdf:resource="stylesheetC.xsl"/> </xslt:Template> <xslt:Transformation rdf:ID="pr_d1-1"> <rdfs:label>Template art_t1 applied</rdfs:label> <xslt:source rdf:resource="file:/C:/Users/Ashley/Desktop/global_001/input.xml#"/> <xslt:used> <xslt:MatchedTemplate rdf:about="stylesheetC.xsl#art_t1"/> </xslt:used> <xslt:processor rdf:resource="#proc01"/> <opmv:wasTriggeredBy rdf:resource="#overall"/> </xslt:Transformation> <xslt:Transformation rdf:ID="pr_d1e3-2"> <rdfs:label xmlns="">Template art_t2 applied</rdfs:label> <xslt:source rdf:resource="file:/C:/Users/Ashley/Desktop/global_001/input.xml#/test/compute"/> <xslt:used> <xslt:MatchedTemplate rdf:about="stylesheetC.xsl#art_t2"/> </xslt:used> <xslt:processor rdf:resource="#proc01"/> <opmv:wasTriggeredBy rdf:resource="#pr_d1-1"/> </xslt:Transformation>
The job of stylesheet B is to carry the same structure as stylesheet A, so that for any input document, an XSLT processor will choose the same transformation step for either stylesheet. However, the structure needed to accurately mirror stylesheet A depends on the granularity expectations. If stylesheet B only needs to capture file-level granularity, the XSLT code only requires information about the input document artifact and the processor agent. On the other hand, template-level granularity would require stylesheet B to maintain the same templates with the same attributes and the same template calls or matches. Currently, the stylesheet B keeps only those elements from stylesheet A which are essential to the structure of the stylesheet (such as
xsl:template), or which are needed to determine parameter or variable bindings. The more provenance required, the more stylesheet B will come to resemble stylesheet A. However, stylesheet B keeps this structure only to accurately represent the process of obtaining output.xml in step 2.
The meta-stylesheet is simply a means to create stylesheet B. The meta-stylesheet passes the OPMV base structures to the meta'd stylesheet. The stylesheet artifact, and template artifacts must be generated at this step. The meta-stylesheet also sets up the XSLT code for the transformation processes and artifacts which must be generated when stylesheet B is used with input, so as to accurately represent the transformation at step 2.
As an example, the next figure shows a simple comment taken from the beginning of the provenance output. The two figures after that show the XSLT code needed to generate that comment. The meta-stylesheet must layer
xsl:comment within an
xsl:element so that the comment appears in provenance.rdf and not stylesheet B. The meta-stylesheet also passes the name of stylesheet A to stylesheet B, since stylesheet B otherwise would have no knowledge of the parallel stylesheet. Stylesheet B, however, must find the
base-uri() of the input document and generate the comment for provenance.rdf.
Figure 8: Selection from provenance.rdf
<!-- Provenance for the transformation of file:/C:/Users/Ashley/Desktop/global_001/input.xml using stylesheetC.xsl: -->
Figure 9: Selection from stylesheet B.xsl
<xsl:comment> Provenance for the transformation of <xsl:value-of select="base-uri()"/> using stylesheetC.xsl: </xsl:comment>
Figure 10: Selection from the meta-stylesheet (metaRDF.xsl)
<xsl:element name="xsl:comment"> <xsl:text> Provenance for the transformation of </xsl:text> <xsl:element name="xsl:value-of"> <xsl:attribute name="select">base-uri()</xsl:attribute> </xsl:element> <xsl:text> using </xsl:text> <xsl:value-of select="$regularStylesheet"/> <xsl:text>: </xsl:text> </xsl:element>
Essentially, the meta-stylesheet has knowledge of stylesheet A; the version of XSLT and the files it uses; and the number of and identifying information regarding the templates. The stylesheet B will be able to grab information about the input document, the specifics of an individual transformation, and the XSLT processor. The meta-stylesheet sets up stylesheet B to take that kind of information and funnel it into the provenance output.
Before it does anything else, the meta-stylesheet matches the root of its input and creates an copy of stylesheet A, only with
rdf:IDs attached to the templates. "Stylesheet C" is unnecessary for the provenance workflow above, but it may be useful for linking relevant template code to the provenance template artifacts.
After stylesheet C is taken care of, the meta-stylesheet starts on stylesheet B. However, stylesheet A might fall into one of three categories:
there is no template matching root (default template rules apply);
there is a template which matches root and only root;
or, there is a template which matches root as well as other nodes.
rdf:Graphcontainer element must be carefully placed. In the first case, a root template is created. In the second case, stylesheet A's root template can be reduced to provenance-ready structure, but with the template node's descendants wrapped in the
rdf:Graph. The third case is slightly more difficult. The original template must have the root node stripped from
@matchbefore it can be copied forward into stylesheet B, and a new template created to match only root, with the same instructions wrapped in the
rdf:Graph. It's also worth noting that each template in stylesheet B will be prepared with an
rdf:aboutwith a link to a template in stylesheet C. In the third case, both template artifacts derived from the original will point to the same template in stylesheet C.
Placing the root element is also important because there is no better place to record the file-level provenance. When the processor matches the root node to any template (or built-in rule, as the case may be), the stylesheet and input file artifacts are already known, as are the processor agent and stylesheet-level parameters and variables.
Figure 11: Code for the overall transformation process (metaRDF.xsl)
<!-- Set up the xslt:Transformation for $regularStylesheet and its input. --> <xslt:Transformation rdf:ID="overall"> <xslt:source> <xsl:element name="xsl:attribute"> <xsl:attribute name="name">rdf:resource</xsl:attribute> <xsl:element name="xsl:value-of"> <xsl:attribute name="select">base-uri()</xsl:attribute> </xsl:element> </xsl:element> </xslt:source> <xslt:stylesheet> <xsl:attribute name="rdf:resource"> <xsl:value-of select="$regularStylesheet"/> </xsl:attribute> </xslt:stylesheet> <xslt:processor> <xsl:attribute name="rdf:resource"> <xsl:text>#proc01</xsl:text> </xsl:attribute> </xslt:processor> <xsl:for-each select="/(xsl:transform | xsl:stylesheet)/xsl:param"> <xslt:binding> <xsl:attribute name="rdf:resource"> <xsl:text>#</xsl:text> <xsl:value-of select="@name"/> </xsl:attribute> </xslt:binding> </xsl:for-each> </xslt:Transformation>
xsl:for-each, so long as one is willing to accept that template artifacts may be listed that are not referenced in any transformation.
The processes for templates firing should be generated during the transformation of stylesheet B, thus guaranteeing a chain of template-level processes. A template-level process is triggered by another process - either the file-level process, or by an
xsl:call-template during another template's firing. If a template is called or matched, OPMV contains two Template subclasses -
xslt:CalledTemplate. For a process to correctly reference the use of a template artifact, the meta-stylesheet must set up stylesheet B to pass along information about the 'parent' template. I used tunneled parameters to accomplish this - each template in stylesheet B gains two parameters, called 'provTemplate' and 'provTrigger'. 'provTemplate' identifies the XSLT instruction which initiated the current template process, and 'provTrigger' passes along the identifier for the 'parent' template. The meta-stylesheet must assign
xsl:with-param instructions to each
Figure 12: Code to track template parentage with xsl:call-template (metaRDF.xsl)
<xsl:template match="xsl:call-template"> <xsl:copy> <xsl:copy-of select="@*"/> <xsl:element name="xsl:with-param"> <xsl:attribute name="name">provTemplate</xsl:attribute> <xsl:attribute name="tunnel">yes</xsl:attribute> <xsl:text>called</xsl:text> </xsl:element> <xsl:element name="xsl:with-param"> <xsl:attribute name="name">provTrigger</xsl:attribute> <xsl:attribute name="tunnel">yes</xsl:attribute> <xsl:element name="xsl:value-of"> <xsl:element name="xsl:text">#pr_</xsl:element> <xsl:element name="xsl:value-of"> <xsl:attribute name="select">generate-id()</xsl:attribute> </xsl:element> <xsl:element name="xsl:text">-</xsl:element> <xsl:number count="xsl:template" level="any"/> </xsl:element> </xsl:element> <xsl:apply-templates/> </xsl:copy> </xsl:template>
The meta-stylesheet has been successfully created, and it can provide file- and template-level provenance documentation for XSL transformations, as shown in the examples above. Still, there are a number of questions still left to explore regarding the meta-stylesheet method and XSLT provenance documentation. For example, how much provenance collection is too much? Further work can be done to include finer levels of granularity, but it probably isn't necessary to document every XSLT instruction. What about coarser granularity? If XSLTs are used in a workflow, what kinds of provenance documentation can be generated, and what should it look like?
Unfortunately, the meta-stylesheet method does not entirely fulfill the assumptions listed previously, especially in ease of use. No matter what version of XSLT is used by stylesheet A, both the meta-stylesheet and stylesheet B currently require the use of an XSLT 2.0 processor. This certainly limits the number of processors able to make use of this method without fuss. Further, the meta-stylesheet has only undergone testing using the Saxon HE and PE processors shipped with oXygen. More testing is needed to ensure that the approach works in broader contexts, with other tools and XSLT 2.0 processors.
But the biggest problem with this approach is the meta-stylesheet's inability to say anything concrete about the output of stylesheet A. Neither the meta-stylesheet nor stylesheet B has knowledge of the actual contents of output.xml, although a good estimation can probably be made by modifying the structure of stylesheet B. It is unclear whether this is a beneficial side-effect (as in workflows where intermediate outputs may not be available) or something that should be fixed.
Besides attempting to solve these questions, I continue to test the meta-stylesheet against a greater number of input stylesheets, and to include other types of provenance information, such as the template descriptions given by the oXygen Editor. Even at this date, this project shows that an impressive amount of provenance information can be generated with just XSLT.
Many, many thanks to Wendell Piez for coming up with the idea for a meta-stylesheet, as well as providing guidance and technical help along the way. Thanks also to Allen Renear, Megan Senseney, and my colleagues at CIRSS for their advice and unflagging support.
This project was supported by DCEP-H, an initiative to extend the Data Curation Education Program to the humanities. Funded by IMLS Grant RE-05-08-0062-08, DCEP-H was based at the Center for Informatics Research in Science and Scholarship at the University of Illinois at Urbana-Champaign.
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 Currently a work in progress, the W3C PROV is an attempt at a provenance ontology general enough to be used for many fields, but flexible enough to handle the specific provenance needs of any organization or community. Once completed, it will likely prove more useful than OPM for provenance markup. For example, OPM has no versioning capabilities, while PROV does.
 Append "-T" when transforming via Java platform.
 Read: those who likely already know the files and processor being used, and who may not see the use in including such information.
 Since A and C have the same output, I refer to "A" when talking about the characteristics of both, and "C" when specifically talking about stylesheet C.
 The input and output can, of course, be in formats other than XML.
 As provenance becomes increasingly fine-grained, other elements will be left in as well.