XML for Configuration: Background
One of the consequences of the rapid development and dissemination of an ecosystem of XML technologies, including free and open source XML parsers, XSLT engines, and binding tools for various programming languages, was the widespread adoption of XML as a meta-format for the specification of application configuration information. The XML ecosystem obviated the need to write custom parsers for one-off configuration formats. This was true in part because, at least at the syntactic level, there was a tool-chain at hand to warrant the well-formedness and validity of those files. XML also facilitated the use of richly structured configuration information. Coincident with an increasing community of practice in architectural idioms such as abstract factories (see [gamma et al]), this capacity for rich configuration made XML the norm for configuration of such applications as Apache’s Tomcat server for Java servlets and Java Server Pages (JSP), Hibernate’s object/relational mapping framework, the Ant build tool, and the Spring application framework and inversion-of-control (IOC) container.
These rich configuration files forward many diverse ends, ranging from (at least the possibility of) more cleanly engineered code, to hot-swappable web applications. However, as even a cursory view of these projects’ listservs indicates, configuration files often are the cause of hiccups in application deployment. Some of these problems can be alleviated by the application of standard XML validating parsers in the deployment process. Other problems however do not yield themselves to the standard XML tool chain.
Widely used applications such as the ones mentioned here often have interactive development environment (IDE) support for the creation of configuration files. The IDE might make use of template files, for example, and provide hints when creating and populating configuration instances. Such IDE support however is not typically robust in validating the content entered, whether or not the hints are taken.
The information models of which these XML configuration files are instantiations (see [abrams]) entail constraints more complex than those enforced by XML well-formedness and validity. These constraints are not expressed, and are perhaps inexpressible, in a configuration file’s document type definition, whether that definition is a DTD, or an XSD schema, or a RelaxNG specification. How might we categorize at least some of these constraints? What techniques can we employ to enforce them before, or as part of, deployment?
XML for Configuration at Portico: Issues
What is Portico?
Portico is a digital preservation service for electronic journals, books, and other content. Portico is a service of ITHAKA, a not-for-profit organization dedicated to helping the academic community use digital technologies to preserve the scholarly record and to advance research and teaching in sustainable ways. As of April 2012, Portico is preserving more than 17.7 million journal articles, nearly 17,000 books, and nearly 1.5 million items from digitized historical collections (for example digitized newspapers of the 18th century).
Content comes to Portico in approximately 300 different XML and SGML vocabularies.
These XML and SGML documents are accompanied by page image (PDF) and other
supporting files such as still and moving images, spreadsheets, audio files, and
others. Typically content providers do not have any sort of manifest or other
explicit description of how files are related (which ones make up an article, an
issue of a journal, a chapter of a book). This content is batched and fed into a
Java workflow that is driven by XML configuration files, which Portico calls
profiles (about 190, one for each publisher content stream), and
registries (shared across all content streams).
The Portico workflow maps the publisher-provided miscellany of files into bundles that comprise an article or book or other content item.
Publisher-provided XML and SGML files are normalized to the Portico profile of the National Library of Medicine’s Journal Archiving DTD. The workflow identifies the format of each of the component files, and, where a format specification and validation tool is available, validates each file against its format specification. It generates metadata considered important for preservation (descriptive, or bibliographic, metadata; technical metadata about files and their formats; provenance and event metadata, detailing the tool chain, including hardware and software information, used in processing the content). These metadata are formatted as XML, and are stored with the preserved digital object.
Some of the sub-steps in this workflow are explicit QA checks of the XML content – both that provided by the publishers, and that produced by Portico in the workflow itself. This QA includes XML validation, the assertion (via Schematron) of other constraints on content values, and visual inspection of sample content. We have written about some of the QA techniques and challenges associated with these content files for Balisage and other venues (see, for example, [morrissey et al] and [morrissey 2011]). In this paper we would like to focus on the QA challenges associated with those XML registry files that drive our workflow.
Portico XML Configuration Files: A Description
The Portico workflow is a pluggable framework. At each step, or activity, in the workflow, the particular tool to be employed is dynamically selected, based on the format or mime type of the file or files being processed at that step. Thus, for example, the “de-layer” activity would invoke standard tar, gzip, or zip tools to expand and separate out the content of .tar, .gzip, or .zip files. The same activity would invoke an XSL transform to split a publisher XML file containing bibliographic metadata for all the articles in an issue of a journal into separate XML files for each article (and would invoke a different XSL transform for each different publisher XML format).
So, at the root of all the XML configuration files that drive and parameterize the
Portico workflow is the format registry file: FormatRegistry.xml. There is a
<Format> element for each distinct format for which the archive contains at least one instance.
This, for example, is part of the
<Format> element for one publisher’s profile of one version of the NLM Journal Publishing
<Format FormatId="XXX_NLM_Journal_Publishing_DTD_2.1" CreationTimestamp="2006-06-13T13:00:00-05:00"> <PorticoDefinedName>XXX Journal Publishing DTD v2.1 20050630</PorticoDefinedName> … <Format>
For each workflow step or activity, the tool registry file, ToolRegistry.xml, maps each format to the Java class to be plugged in, configured, and executed at that step. So, for example, in the tool registry, we have entries such as:
<TransformationSet> <ToolStrategy SupportingFormatId="XXX_NLM_Journal_Publishing_DTD_2.1"> <Script Rid="scrxxx"/> </ToolStrategy> ... </TransformationSet>
This entry indicates that in the workflow
Transform Files step, if
the Portico identifier (defined in the format registry) for the format of the file
to be transformed is
XXX_NLM_Journal_Publishing_DTD_2.1, then we should
look for plug-in information about what tool to employ, and how to parameterize it,
in a subsequent
ScriptInfo element with a
attribute value of
scrxxx. In that element, we will see specified such
things as a list of relative file paths to XSL transforms that comprise the
transformation pipeline for instances of this format, along with (relative)
directory names where those files are located; the Portico identifier for the format
of the output of this transformation (also defined in the format registry); a
fully-qualified Java class name for a filter through which the input file is to be
passed; and an
Rid attribute referring to yet another subsequent
element containing full information about the Java tool class that will invoke the
filters and the XSL pipeline.
<ScriptInfoSet> <ScriptInfo ScriptId="scrxxx" ScriptType="transformation" ScriptDir="xxx"> <Tool Rid="BaseTransform_1.0"> <Parameters> <Parameter> <Name>StyleSheetList</Name> <ValueOrderedList> <ValueOrderedListItem> <Number>10</Number> <Value>xxx2ptc_1_10.xsl</Value> </ValueOrderedListItem> <ValueOrderedListItem> <Number>20</Number> <Value>fix-data_1.xsl</Value> </ValueOrderedListItem> ... </ValueOrderedList> </Parameter> .... <Parameter> <Name>outputFormatId</Name> <Value>PTC_Article_DTD_2.1</Value> </Parameter> <Parameter> <Name>InputFilterClass</Name> <Value> org.portico.threadedtool.tool.transform.filter.XmlPrologTransformFilter </Value> </Parameter> ... </Parameters> </Tool> </ScriptInfo> ... </ScriptInfoSet>
Later in the tool registry, information about the
BaseTransform_1.0 tool is specified, including its Java class name, and information about the parameters
to be passed when instantiating that class (as we did in the
ScriptInfo element above), including whether or not the parameter is required, and what its
type should be:
<ToolInfoSet> <ToolInfo Id="BaseTransform_1.0"> <Name>BaseTransformTool:1.0:2007-05-01</Name> <Description> Tool for transformation of XML files via XSL stylesheets. </Description> <Status>ACTIVE</Status> <ClassName> org.portico.threadedtool.tool.transform.BaseTransformTool </ClassName> <ToolParameters> <ToolParameter Name="StyleSheetList" Required="true" ParameterType="ValueOrderedList"> <Description> This is the list of XSL stylesheets that are to be processed and the order in which they should be processed. </Description> </ToolParameter> <ToolParameter Name="outputFormatId" Required="true" ParameterType="Value"> <Description> This identifies the output format ID of this set of transforms. </Description> </ToolParameter> <ToolParameter Name="InputFilterClass" Required="false" ParameterType="Value"> <Description> This identifies the filter to be applied to the file before transforms. It should contain the fully qualified class name of the filter. If this Parameter is not supplied, the default BaseFilter class is used. Legal values come from package org.portico.threadedtool.tool.transform.filter. </Description> </ToolParameter> ... </ToolParameters> </ToolInfo> ... <ToolInfoSet>
Portico XML Configuration Files: Categories of Configuration Issues
There are a lot of
moving parts in even this condensed description of the semantics of these two configuration files.
As more and more publisher streams were added to Portico’s workflow, more and more
configuration information was added to these files, by more and more developers working
at the same time to add new content streams and their accompanying Java tool and filter
classes, and associated XSL transformations. And these additions had to be made in
several different workflow environments: a developer environment; an integration
environment where profiles and registries for new publisher streams are first worked
out; a QA environment for regression testing of tool, transform, workflow, and configuration
changes; and, finally, to the production environment.
Perhaps to no one’s surprise, Portico began to experience deployment glitches. None of these glitches occurred because the XML registry files were either not well formed or invalid, as developers consistently validated the files against their respective schemas before committing to Portico’s source control system, and the deployment scripts also invoked a parser to validate the files. The workflow was paused; new configuration files and other resources were deployed; the workflow was cranked back up. Then it would hum along through several workflow steps, before encountering what effectively was a configuration error that would bring one or more batches to a halt.
So the first question we asked ourselves was, what is it about the semantics of the content of the elements in these files – and the relationship among elements in the same and in different XML configuration files, and the relationship between the content in those elements and other components of the workflow software and other resource files – that enabled configuration errors to pass undetected through the sieve of standard XML validation tools?
At run time, every tool specified in the tool registry verifies the presence or absence of various required and optional parameters, and checks to see, when present, that they are of the required type before proceeding to execute the tool. Just as the workflow that invokes the tools is a pluggable framework, so too are the individual tools themselves pluggable (See Figure 3). This enables Portico to use a single generic XML transformation Java tool on input that requires slightly varying processing.
Since the tool is in some sense generic, some parameters for the tool are
optional. However, it can be the case that if one of these optional parameters
is present, and if the parameter has a particular value, then other optional
parameters must be provided as well. For example, the
InputFilterClass parameter is optional, but if it is present, and if its value is
then the tool registry must also provide the additional otherwise optional
If these constraints are not met, the tool registry file will pass schema validation, but the workflow step attempting to transform an instance of this file format will fail at runtime, because the tool it invokes requires more configuration information to perform the transformation. So we have a need to check for consistency between the (variable, and complexly dependent) input expectations of the Java tool, and the configuration values provided in the XML registry.
Referential Integrity Issues
As mentioned above, the Portico workflow is "format driven.” The choice of
tool to be plugged in at different steps in the workflow is determined by the
format of the object to be processed at the step (indeed, the sequence of
workflow steps itself is driven by the expected collection of format instances
in a content stream). And, as noted, we encounter many formats – the
FormatRegistry.xml file contains, at the time of writing, 545
Various constructs for defining an XML vocabulary (DTD, Schema, and RelaxNG) have provisions for specifying a constrained list of values for, for example, attributes. These provisions typically are employed for a smaller number of values than would be required to cover the ever-growing list of formats Portico encounters. Nor would such a constrained list of identifiers include the other information about the format that is associated with the identifier in the format registry. For our purposes, we would categorize that list of format identifiers (along with associated format information) as “data” rather than “structure”. Further, if the list of constrained values were to be maintained in the document type definition itself – at “compile time”, so to speak, – we would be injecting what would be for us an unwanted level of complexity in the versioning of our schema.
Nevertheless, we have the requirement to ensure a sort of “referential integrity check” among XML files – that is, between the unique Portico format identifiers in the format registry, and the format identifiers employed in the tool registry and other Portico configuration files – to ensure successful runtime interaction between the workflow tools and the registries that drive them.
As seen above, the tool registry refers to many objects that are assumed to exist at run time: Java tool and filter classes, XSL files, and other supporting files. The existence or non-existence of such objects, even if their names are specified in a document type definition, is extrinsic to the kind of structural information a document type definition can provide, and which standard parsers can validate. Yet a successful deployment of these XML configuration files depends on verifying the actual existence of these objects in the total deployment package. This was in fact the most frequent cause of configuration-file-dependent deployment errors. An updated tool registry would be deployed, but the new XSL files, or a new JAR file containing new Java tool or filter classes specified in the updated registry, or the DTD or XSD files associated with an XML or SGML format, were not deployed along with the new tool registry. When the workflow was restarted, a (Java) workflow step would look to the tool registry to determine which Java tool or filter class it should employ, or, if the workflow step was an XSL transformation, what list of XSL makes up the transformation pipelene, or against which DTD or schema a format instance was to be validated. If the classes or files specified in the tool registry had not also been deployed, the workflow step would raise a fatal error and halt processing.
XML for Configuration at Portico: Solutions
The second question we asked ourselves was whether we could devise some automated solutions to these consistency, referential integrity, and existence issues in order to avoid, or at least minimize, costly cycles of stop/deploy/restart/fail/stop/correct/redeploy/restart in our release deployments. We found that we could, and that we could do so fairly simply with XSL transforms, assisted, in some cases, with Java extension functions. We run these transform as part of deployment script before the stop/restart of the workflow.
The tool registry’s
ToolInfo element documents all the possible calling parameters that can be passed to the Java
tool class it specifies, indicating whether they are required or optional, and specifying
the type of each parameter. Assuming correct documentation (for ensuring which, to
probably no one’s surprise, we have not yet invented a completely automated tool),
this meant that we had sufficient information in the tool registry itself to perform
consistency checks. We use an XSL transform (Schematron is another obvious candidate
for this) to compare the various
Paramter elements in the
ScriptInfo element that configures each invocation of a particular tool for a particular format,
with the information in the
ToolInfo elements. If an error message is created, the registry is not deployed until it
“Referential Integrity” Checker
Again, we used an XSL transform. The transform extracts format and mime type information from the format registry being deployed with the tool registry, and uses that extracted information to verify that any referenced format id or mime type in the tool registry has been declared in the format registry.
Our existence checker also uses XSL, aided by extension functions, to look outside
the XML box to determine that expected directories and XSL and other files exist in
the target deployment directories, and that classes referenced in the tool registry
exist in the .jar files also on the deployment path. In the case of XSL and other
XML files, it would of course be possible to use
xsl-document to check
for the existence of necessary resources. For non-XML file resources whose existence
we wished to confirm, we could have used the
interpreting an empty string result as a non-existent file. As we were using Java
extension functions to confirm the existence (i.e. deployment) of Java classes on
what would be the workflow's runtime Java classpath, and as the workflow uses the
classpath to resolve the location of XSL and other file resources, we used Java
extension functions to confirm the existence of these resources at deployment.
<xsl:stylesheet version="2.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:jexist="java/org.portico.conprep.util.deployment.validatetoolregistry.ExistenceChecker" > <xsl:variable name="jExistInstance" select="jexist:new()"/> <!-- ==================================================== --> <!-- name="check-class-exists" --> <!-- ==================================================== --> <xsl:template name="check-class-exists"> <xsl:param name="className"/> <xsl:param name="Path"/> <xsl:variable name="result" select="jexist:classExistsOnClasspath($jExistInstance, $className)"/> <xsl:if test="not($result)"> <xsl:message terminate="no" > Class <xsl:value-of select="$className"/> does not exist. <xsl:value-of select="$Path"/> </xsl:message> </xsl:if> </xsl:template> <!-- ==================================================== --> <!-- name="check-file-exists" --> <!-- ==================================================== --> <xsl:template name="check-file-exists"> <xsl:param name="scriptType"/> <xsl:param name="scriptDir"/> <xsl:param name="fileName"/> <xsl:param name="Path"/> <xsl:variable name="result" select="jexist:fileExistsOnClasspath($jExistInstance, $scriptType, $scriptDir, $fileName)"/> <xsl:if test="not($result)"> <xsl:message terminate="no" > File <xsl:value-of select="$fileName"/> does not exist. ScriptType = <xsl:value-of select="$scriptType"/>, ScriptDir = <xsl:value-of select="$scriptDir"/> Path to script = <xsl:value-of select="$Path"/> </xsl:message> </xsl:if> </xsl:template>
Not all configuration files -- not even all Portico workflow configuration files -- are XML files, of course, and not all are likely to be amenable to an XML-based QA solution. XML would be overkill for the sort of name-value pairs of configuration information more compactly expressed in conventional properties files. As the category name of one class of deployment issues suggests, a different implementation choice (relational database in lieu of XML registry files for configuration) would provide a different mechanism for enforcing at least one sort of consistency in deployment.
There are some limitations to these tools, even as applied to XML configuration files.
Our consistency checker, which validates the number, name, type, and compulsoriness
parameters passed to our Java tools, depends on a manual process of updating the
ToolInfo section whenever we update our Java tools.
We could conceivably employ Java annotations, and a meta-process to generate appropriate
ToolInfo elements, and test to see if those generated elements matched
the actual elements in the ToolRegistry.xml file (though this approach perhaps only
pushes back our "manual" dependency on developer discipline to ensuring consistent
annotation in the Java code).
A warrant of existence of a resource at deployment time, while reassuring, is not necessarily a warrant of existence at run time, particularly for resources outside of one's institutional domain or control (though this has not been an issue in Portico's deployment process). We have found a simple syntactic surrogate (a non-empty return from a method call) that reliably signifies the existence or non-existence of an artifact "out there" in the world extrinsic to the XML document we are interrogating. That surrogate is satisfactory for our purposes, but it might not be sufficient for all purposes. And it is certainly no magical solution to the general ontological problem of assessing an XML document's assertion about external reality.
As mentioned above in the Background section, users of other applications with rich XML configuration files experience deployment problems. Many of these are caused by these same consistency, referential integrity, and existence issues experienced by Portico with its registry files. Spring, Hibernate, and Tomcat configuration files all contain elements whose contents are intended to be fully-qualified Java class names, for classes that are presumed to be on the application’s classpath at runtime, and hence available for instantiation via inversion of control. The XML configuration files for any of the applications mentioned above can contain elements whose content refers to files and directories, both local and non-local, assumed to exist at runtime (for example, the location for JSP files used by Apache, as well as “welcome” files and login pages, can be specified). Nothing in the schema for Ant files ensures that a developer using Ant to build and deploy a Java servlet populates the Ant configuration file so as to follow the Java servlet conventions for relative deployment location for servlet and other files. A Tomcat web.xml file specifying both type and values for JNDI can be well-formed and valid, and yet specify a value that cannot be instantiated as an instance of the specified type, or specify a type that in fact does not exist.
What we have found is that, for our XML configuration files, where standard XML validation tools will not resolve these deployment issues, we can use XML-based tools for at least these categories of non-structural, “extrinsic” validation requirements. Further, because they are XML based, they can be employed in both in simple deployment scripts and in continuous integration tools, reducing many common deployment problems.
XML-based tools, though not the only possible solution for solving XML configuration file-related deployment issues, were a natural choice for Portico. In part this was a pragmatic choice: such XSL and Java-based solutions were well-supported by the skill set in our programming team, and the categories of deployment problems we experienced where amenable to solution by the means described. Standard XML parsers and XSL transform engines made it easy to "get at" the content of XML elements and attributes to test assertions about that content beyond the document's well-formedness and validity (Do certains things exist "out there" on the file system? Have we provided all the parameters that an optional Java filter plugin to a Java plugin tool to a Java framework requires at runtime?) . And they made it easy to "get at" them in different contexts --both a manual context (a developer updating an XML registry in Oxygen, for example) and an automated one (a deployment script that is part of a configuration management system).
In part however this was also an aesthetic choice: there is a certain "turtles all the way down" appeal to applying XML-based QA tools to XML configuration files that drive a workflow for processing digital artifacts whose key components are XML files. The aesthetic shaded into the pragmatic, as it so often does in software engineering. The consistent, conventional reuse of large-scale structure and coding idioms across such a large application as the Portico workflow system helps in making that system comprehensible to the developers who support and extend it -- an approach, so to speak, toward the possibly asymptotic, reader-centric rather than process- or processor-centric goal of "Read once, understand everywhere" [reflection] in complex systems.
What precisely comprises the "beauty" or aesthetic appeal of a piece of software (both data structures and algorithms, and their instantiation in concrete formalisms such as languages) is a topic of much discussion. To what extent does a software instance provide, as mathematicians might say of a proof, an "elegant" or "economical" solution to the problem it was created to solve? The extent to which the XML community has grappled with the ways in which, and the degree to which, an XML document type definition can or might comprise a model homomorphic to some domain, suggests that such homomorphism is a key criterion. It might be argued that the choice of XML as a configuration language for these large-scale frameworks was in some sense opportunistic, or "merely pragmatic." The ecosystem of XML parsers provided a "good enough" solution to the expression of richly hierarchical configuration information, even if it did not provide a solution to the problem of the more complex relationships among components of these hierarchies, and to extrinsic realities. The necessity of performing QA on these files can be viewed as an indicator of the limits of the economy of XML as a solution to the original problem. Whether other software constructs would provide a more "beautiful" solution is an open question. That a partial solution to the limits of XML as a configuration language can be found in XML-based tools has, at least for Portico, its own economical appeal.
[abrams] Abrams, Stephen.
File Formats in DCC Digital Curation Manual. S.Ross, M.Day (eds), (October 2007), Retrieved April
20, 2012, from http://www.dcc.ac.uk/sites/default/files/documents/resource/curation-manual/chapters/file-formats/file-formats.pdf.
[gamma et al] Gamma, Erich, Richard Helm, Ralph Johnson, and John Vlissides. Design Patterns: Elements of Reusable Object-Oriented Software.Reading, MA: Addison-Wesley, 1995.
[morrissey et al] Morrissey, Sheila, John Meyer, Sushil Bhattarai, Sachin Kurdikar, Jie Ling, Matthew Stoeffler and Umadevi Thanneeru. "Portico: A Case Study in the Use of XML for the Long-Term Preservation of Digital Artifacts." Presented at International Symposium on XML for the Long Haul: Issues in the Long-term Preservation of XML, Montréal, Canada, August 2, 2010. In Proceedings of the International Symposium on XML for the Long Haul: Issues in the Long-term Preservation of XML Balisage Series on Markup Technologies, vol. 6 (2010). doi:https://doi.org/10.4242/BalisageVol6.Morrissey01. Retrieved April 20, 2012, from http://www.balisage.net/Proceedings/vol6/html/Morrissey01/BalisageVol6-Morrissey01.html.
[morrissey 2011] Morrissey, Sheila M.
‘More What You’d Call ‘Guidelines’ Than Actual Rules' : Variation in the Use of Standards. Journal of Electronic Publishing 14, no. 1: 14. doi:https://doi.org/10.3998/3336451.0014.104. Retrieved April 20, 2012, from http://quod.lib.umich.edu/cgi/t/text/text-idx?c=jep;view=text;rgn=main;idno=3336451.0014.104
[reflection] Morrissey, Sheila
JHOVE2 and Java Reflection. (February 2009), Retrieved August 05, 2012, from https://bitbucket.org/jhove2/main/wiki/Background_Papers.
1 Portico filters incoming XML and SGML files for a variety of reasons. The format validation step, for example, makes use of the JHOVE tool, which is not able to make use of catalogs. The DOCTYPE statement will therefore sometimes require filtering to cause the parser to resolve to a local copy of the DTD or schema. Sometimes a filter is used to correct syntactic errors in the DOCTYPE statement, such as white space or comments before the XML declaration, encoding declarations inconsistent with actual encoding used by the publisher (see morrissey et al for other examples)