XML Matters: Kicking back with RELAX NG, Part 2

Tools and special issues

RELAX NG schemas provide a more powerful, concise, and semantically straightforward means of describing classes of valid XML instances than do W3C XML Schemas. In this installment, David continues the discussion of RELAX NG begun in part 1 of this series by addressing a few additional semantic issues and looking at tools for working with RELAX NG.

David Mertz (mertz@gnosis.cx), Facilitator, Gnosis Software, Inc.

Photo of David MertzDavid Mertz, in his gnomist aspirations, wishes he had coined the observation that the great thing about standards is that there are so many to choose from. But then, he is also fuzzy on OS design. David may be reached at mertz@gnosis.cx; his life pored over at http://gnosis.cx/dW/. Suggestions and recommendations on this, past, or future, columns are welcomed.

26 March 2003

Also available in Japanese

In the last installment I gave you a fairly complete overview of both the syntax and semantics of RELAX NG schemas. However, a few issues were glossed over, and are worth looking at more closely.

Both DTDs and W3C XML Schemas allow for infoset augmentation, while RELAX NG does not. James Clark, one of the creators of RELAX NG (and many other widely used XML tools), argues vehemently that infoset augmentation violates modularity in the roles of XML instance documents and schemata. In other words, for Clark, RELAX NG has a feature where DTDs and W3C Schemas have a bug. My own feelings on the matter are mixed, but I can understand his intuition.

Let's backtrack a little and look at what this infoset stuff is about. Basically, you can ask an XML instance what data it contains. If you parse the instance without validation, the answer depends solely on what values occur in its attributes and element bodies. If you value modularity, a schema should only tell you whether an instance is valid or not; it should not change the actual information in a document. However, such modularity is violated if you use a DTD or W3C XML Schema for validation. For example, consider the following DTD:

Listing 1. curious.dtd
<!ATTLIST foo bar CDATA "curious"
              baz CDATA #FIXED "curiouser">

And this XML instance:

Listing 2. curious.xml
<?xml version="1.0"?>
<!DOCTYPE foo SYSTEM "curious.dtd">

A non-validating parser will find a different set of information in this document than a validating parser would. Contrast the non-validating utility xmlcat with the validating 4xml (both echo whatever they encounter back to the console):

Listing 3. Infosets with validating and non-validating parsers
% ./xmlcat curious.xml
<?xml version="1.0" encoding="iso-8859-1"?>
% 4xml -p curious.xml
<?xml version="1.0" encoding="utf-8"?>
<foo bar="curious" baz="curiouser"/>

In a W3C Schema, default and fixed attributes have similar effects for both <xsd:attribute> and <xsd:element> tags.

The argument in favor of defaulting is that it allows XML instance minimization. I have used defaults (or more likely #FIXED attributes) for this very purpose. But I can also see dangers -- both of malice and of debugging nightmares -- if the very content of a local XML document depends upon a remote (and perhaps spoofable) URI , and even upon an absence of network interruptions during parsing.

RELAX NG does not perform any infoset augmentation. Well, almost -- I think Clark overstates this point. If you impose a data type on an element or attribute, you still change the content of the value in an important way. The value of the string "1.0" is different from the value of the float "1.0", even though the two are represented in exactly the same way in an XML instance.

Stating cardinality

W3C XML Schemas have better means of requiring occurrence cardinalities than do DTDs or RELAX NG schemas. If you want a <foo> element to occur between 5 and 30 times within the <bar> element, you can declare this in W3C Schemas with a straightforward rule:

Listing 4. W3C XML Schema cardinality rule
<xsd:element name="bar">
  <xsd:element name="foo" minOccurs="5" maxOccurs="30"/>

The same cardinality rule can be stated in a DTD, but very clumsily:

Listing 5. DTD cardinality rule
    (foo, foo, foo, foo, foo, foo?,foo?,foo?,foo?,foo?
     foo?,foo?,foo?,foo?,foo?,foo?,foo?,foo?,foo?,foo?) >

What I would like for RELAX NG would be an explicit <cardinality> tag, so that you could (hypothetically) write something like:

Listing 6. Hypothetical RELAX NG 2.0 cardinality rule
<element name="bar" xmlns="http://relaxng.org/ns/structure/1.0>
  <cardinality min="5" max="30">
    <element name="foo"/>

Unfortunately, in the current version of RELAX NG, the only cardinalities you get are <zeroOrMore>, <oneOrMore>, and <optional>. However, named patterns can at least be used to make spelling out cardinalities slightly less painful. In compact syntax, for example:

Listing 7. Actual RELAX NG compact syntax cardinality rule
start = element bar { fivefoo, upto25foo }
fivefoo = element foo { empty }, element foo { empty },
          element foo { empty }, element foo { empty },
          element foo { empty }
maybefoo = element foo { empty }?
upto25foo =
  fivefoo?, fivefoo?, fivefoo?, fivefoo?,
  maybefoo, maybefoo, maybefoo, maybefoo, maybefoo

I confess that this sort of naming is not perfect, but at least it is possible to name large numbers by effectively raising to powers through repetition of named patterns.

Transformations and validations

A variety of tools are available for working with RELAX NG schemas. These tools are predominantly implemented in the Java language, but some tools and libraries have been written in Python, C#, and Visual Basic. Surprisingly, I have not found any libraries written in other languages -- such as Perl, Ruby, or C/C++ -- that seem to be good fits.

One obvious class of RELAX NG application is validators. Just as with validating parsers that work with DTDs or W3C XML Schemas, a number of command line, online, and library parsers are available for RELAX NG. A slightly less obvious class of application is tools to transform schemas into each other. Sun's RELAX NG Converter and James Clark's trang and DTDinst let you convert among RELAX NG (XML and compact syntax), DTDs, and W3C XML Schemas. I plan to write a less ambitious Python tool (compact2xml.py) in time for the next installment of this column, which will allow 4Suite and xvif to utilize the RELAX NG compact syntax (the authors of each have expressed an interest in including such a tool).

Transformations are worth looking at in a bit more detail. Part 1 looked at ways in which RELAX NG is strictly more powerful than W3C XML Schemas, and looking at some best-effort transformations helped illustrate this point. For example, the previous installment presented a schema for a library patron, which is expressed in compact syntax as:

Listing 8. Library patron compact syntax
element patron {
  element name { text }   &
  element id-num { text } &
  element book {
    attribute isbn { text } |
    attribute title { text }

See Part 1 for the XML syntax version, which is semantically identical but more verbose. trang makes a good attempt at turning this into a W3C XML Schema. The file extensions of the input and output file are used to guess types (or may be overridden with switches):

Listing 9. Transforming RELAX NG to W3C XML Schema
% java -jar trang.jar patron.rnc patron.xsd
% cat patron.xsd
<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
           elementFormDefault="qualified" version="1.0">
  <xsd:element name="patron">
      <xsd:choice minOccurs="0" maxOccurs="unbounded">
        <xsd:element ref="name"/>
        <xsd:element ref="id-num"/>
        <xsd:element ref="book"/>
  <xsd:element name="name">
    <xsd:complexType mixed="true"/>
  <xsd:element name="id-num">
    <xsd:complexType mixed="true"/>
  <xsd:element name="book">
      <xsd:attribute name="isbn"/>
      <xsd:attribute name="title"/>

To the credit of trang, I think this W3C Schema is the best that can be done for the situation. Every XML instance that's accepted by the RELAX NG schema is also accepted by the W3C XML Schema, and many errors are rejected by both. The problem is that there is a distinct class of XML instances that are not really valid according to the desired rule, but that pass validation with the W3C Schema. For example:

Listing 10. Limits of W3C XML Schema discernment
% cat patron-i1.xml
<?xml version="1.0" encoding="UTF-8"?>
  <book isbn="0-528-84460-X"/>
  <name>John Doe</name>    <!-- repeats name subelement -->
  <name>Second Name</name>
  <book title="Why RELAX is Clever"/>
% cat patron-i2.xml
<?xml version="1.0" encoding="UTF-8"?>
  <name>John Doe</name>
  <!-- Too many and too few attributes of book element -->
  <book title="Why RELAX is Clever" isbn="0-528-84460-X"/>
% cat patron-i3.xml
<?xml version="1.0" encoding="UTF-8"?>
<patron/>        <!-- No required subelements -->

Of course, even though the three examples above validate falsely, W3C XML Schema still rejects XML instances with entirely disallowed elements/attributes, or ones that nest elements in improper ways (for example, <book> inside <name> rather than as a sibling).

As far as validation tools go, I find that jing does a good job of producing useful error messages when validation fails. The Python XML library 4Suite incorporates a version of the xvif library, and also performs validation (the latter is also accessible online -- see Resources). But compare the errors:

Listing 11. Validation error messages with jing
% java -jar ../trang/jing.jar patron.rng patron-i3.xml
Error at URL "file:/.../patron-i3.xml",
line number 2: unfinished element
% java -jar ../trang/jing.jar patron.rng patron-i1.xml
Error at URL "file:/.../patron-i1.xml",
line number 5: element "name" not allowed in this context
Listing 12. Validation error messages with 4Suite
% 4xml --rng=patron.rng patron-i1.xml
Traceback (most recent call last):
  File "/.../site-packages/Ft/Xml/_4xml.py", line 89, in Run
    raise RngInvalid(result)
Ft.Xml.Xvif.RngInvalid: Qname {None}name not exected
% 4xml --rng=patron.rng patron-i3.xml
Traceback (most recent call last):

Of course, in an application context, the choice of the programming language that will utilize the libraries outweighs differences in the messages produced.

Compiled validators

One category of tool that I have not seen much of outside of RELAX NG contexts is a single-schema validator. Take a look at the RELAX NG home page for links to such tools, including Bali and RelaxNGCC. These frameworks automatically emit code for specialized validation of a particular RELAX NG schema. Presumably, such a specialized validator runs faster than a general purpose one. Such tools are possible -- or at least much more straightforward than the same thing would be relative to W3C XML Schemas -- because the design of RELAX NG is extremely well grounded in algorithmic analysis.

RELAX NG-enhanced XML editors

Unfortunately, XML editors do not yet support RELAX NG as widely as they do W3C XML Schemas. Of course, DTDs remain much more widely supported than either of these schema styles. This is a shame because it would actually be far easier to include customizations around RELAX NG in an editor because of the simple conceptual framework of RELAX NG validation. Ideally, a custom XML editor would utilize a RELAX NG schema to direct and assist a user in the insertion of attributes and elements in ways that maintain validity.

One compromise would be to use a tool like trang to convert a RELAX NG schema into a W3C XML Schema or DTD that approximates it, then use those within a GUI XML editor. But doing so would help only to a limited extent.

One XML editor is built around RELAX NG -- the Java technology-based XML Operator (see the RELAX NG home page in Resources). I played with it a little, and found that it could be potentially useful, but it would fall on the low end of the XML editors I have previously reviewed (see Resources); XML Operator implements just a few features here and there, and provides neither the huge array of tools of XML Spy, or the simple elegance of oXygen. XML Operator implements just a few features here and there, and provides neither XML Spy's huge array of tools, or oXygen's simple elegance.

Until next time

In part 1 and here in part 2, I have looked at most of the elements of RELAX NG, and included a summary of tools for working with it. The third and final installment will touch briefly on how RELAX NG lets you include external schemas in your schema, and selectively merge the specifications of different schemas. But part 3 will primarily look at the RELAX NG compact syntax in more detail, and explain the exact correspondences between compact syntax and XML syntax.


  • Participate in the discussion forum.
  • Check out the home page for RELAX NG, which contains numerous useful links to articles, tools, and so on. Of particular note is the excellent tutorial written by two great luminaries of XML technologies, James Clark and Murata Makoto (Oasis, December 2001).
  • James Clark wrote a discussion of the algorithmic principles behind RELAX NG validation. Interestingly, his sample code is provided in Haskell, which has advantages that I've described in my XML Matters installment on the HaXml library (developerWorks, October 2001).
  • Use this online tool to validate an XML instance document against a RELAX NG schema. A RELAX NG schema itself is validated during the process, as well. This tool is based on Eric van der Vlist's xvif tool, which is written in Python. This online validator lets you select from a set of test cases, as well as check your own examples. The test cases are also available (and briefly discussed).
  • Download the xvif library. Alternatively, 4Suite is a somewhat more polished tool that incorporates xvif for RELAX NG validation. The command-line tool 4xml validates against both RELAX NG and DTDs, with various options. 4Suite includes many other tools and libraries for working with many XML-related technologies.
  • For background and comparison, use this online tool to validate an XML instance document against a W3C XML Schema, and try this tool to check W3C XML Schemas against the Approved Recommendation.
  • trang and jing are complementary tools for transformation between schemata and validation against RELAX NG schemas. The former depends on the latter, but both can be downloaded.
  • You will need to obtain an implementation of the Java API for XML Processing (JAXP) to use trang. If you run a Java 1.4 JVM, you are fine; otherwise, you can download crimson.
  • DTDinst is a Java technology-based tool for converting DTDs to XML instance document format, including handling of parametric entities. The DTDinst XML format is of limited utility by itself, since nothing else works with it. However, an XSLT style sheet is available to transform this format into RELAX NG (with a few caveats). You will need an XSLT tool to utilize it.
  • Find a collection of schemata and test XML instance documents for the library patron example discussed in this article.
  • Read David Mertz's roundup of XML editors: Part 1 (developerWorks, August 2002) examines Java and MacOS applications, while Part 2 looks at Windows-based products (developerWorks, September 2002). You'll find all of the previous installments of the XML Matters column.
  • Find more XML resources on the developerWorks XML zone.
  • IBM trial software: Build your next development project with trial software available for download directly from developerWorks.
  • IBM XML certification: Find out how you can become an IBM-Certified Developer in XML and related technologies.


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