XML schema languages

An XML schema is a description of a type of XML document, typically expressed in terms of constraints on the structure and content of documents of that type, above and beyond the basic syntax constraints imposed by XML itself. There are several different languages available for specifying an XML schema. Each language has its strengths and weaknesses.

Note: the W3C defined schema language is called, "XML Schema". However, this name can be confusing in the context of referring to a number of XML schema languages. As such, throughout this document, references to the term "XML Schema" will be any XML schema language where the meaning might be ambiguous, while the term, "W3C XML Schema" (abbreviated to WXS) will be used for the W3C-defined XML schema language.

Overview

Though there are a number of schema languages available, the primary three languages are Document Type Definitions, W3C XML Schema, and RELAX NG. Each language has its own advantages and disadvantages.

This article also covers a brief review of other schema languages.

The primary purpose of a schema language is to specify what the structure of an XML document can be. This means which elements can reside in which other elements, which attributes are and are not legal to have on a particular element, and so forth. A schema is somewhat equivalent to a grammar for a language; a schema defines what the vocabulary for the language may be and what a valid "sentence" is.

Document Type Definitions

Main article: Document Type Definition

Advantages

Of the primary three languages, DTDs are the only ones that can be defined inline. That is, the DTD can actually be embedded directly into the document.

DTDs can define more than merely the content model. It can define data elements that can be used in the document, much like a C or C++ preprocessor may have #defines that are used internally.

The DTD language is compact and highly readable, though it does require some experience to understand.

Disadvantages

The primary disadvantage to DTDs is their weakness of specificity. The content models for DTDs are very basic, particularly compared to the other two languages.

Overuse of DTD-defined elements may make a document illegible or incomprehensible without the associated DTD. Additionally, there are several XML processors that, typically for ease-of-implementation reasons, do not understand DTDs. As such, if DTD-defined entities are being used, these XML processors will not recognize them.

The language that DTDs are written in is not XML. Therefore, DTDs cannot use the various frameworks that have been built around XML. XML editors that support writing DTDs must do so by parsing an additional language, for example. Some XML processors, typically for economy of implementation or execution, simply ignore DTD information, including DTD data elements.

The DTD concept for XML was borrowed from the SGML DTD concept. As such, the construct could not be changed when XML was extended with namespaces. As such, DTDs are namespace unaware.

There is limited support for defining the type of the contained data. DTDs are primarily structural in nature. They do not have the ability to specify that an element contains an integral number, real number, a date, or anything of that nature.

Tool Support

DTDs are perhaps the most widely supported schema language for XML. Because DTDs are one of the earliest schema languages for XML, defined before XML even had namespace support, they are widely supported. Internal DTDs are often supported in XML processors; external DTDs are less often supported, but only slightly. Most large XML parsers, ones that support multiple XML technologies, will provide support for DTDs as well.

W3C XML Schema

Main article: XML Schema (W3C)

Advantages over DTDs

Compared to DTDs, W3C XML Schemas are exceptionally powerful. They provide much greater specificity than DTDs could. They are namespace aware, and provide support for types.

W3C XML Schema is written in XML itself, and therefore has a schema of its own (appropriately, written in W3C XML Schema).

W3C XML Schema has a large number of built-in and derived data types. These are specified by the W3C XML Schema specification, so all W3C XML Schema validators and processors must support them.

Due to the nature of the schema language, after an XML document is validated, the entire XML document, both content and structure, can be expressed in terms of the schema itself. This functionality, known as Post-Schema-Validation Infoset (PSVI), can be used to transform the document into a hierarchy of typed objects that can be accessed in a programming language through a neutral interface.

Commonality with RELAX NG

Both RELAX NG and W3C XML Schema allow for similar mechanisms of specificity. Both allow for a degree of modularity in their languages, going so far as to being able to split the schema into multiple files. And both of them are, or can be, defined in an XML language.

Advantages over RELAX NG

Depending on point of view, each of these advantages can be considered misfeatures.

RELAX NG lacks any analog to PSVI.

Additionally, RELAX NG has slightly poorer specificity in certain respects. For example, it is not possible to define a specific number or range of repetitions of patterns under RELAX NG; under W3C XML Schema, this is possible.

Also, RELAX NG has no ability to apply default attribute data to an element's list of attributes, while W3C XML Schema does.

RELAX NG has only two built-in data types (string and token), while W3C XML Schema has far more.

W3C XML Schema has a formal mechanism for attaching a schema to an XML document.

Disadvantages

W3C XML Schema does not allow for the definition of which element, or elements, may be valid root elements of a document. As such, all elements defined in the schema can be a root element. A well-defined list of root elements (and equally importantly, elements that cannot be roots) is a useful feature for a schema, particularly for languages that are intended to be split into multiple files and included from one to another. It allows easy validation of the separate parts, as well as not validating invalid root elements.

It is difficult to define that a particular set of elements may occur in any order relative to one another. This is a valuable construct, particularly for formats pertaining to documents.

Although being written in XML is an advantage, it is also a disadvantage in some ways. The W3C XML Schema language in particular can be quite verbose, while a DTD can be terse and relatively easily editable.

W3C XML Schema does not implement most of the DTD ability to provide data elements to a document. While technically a comparative deficiency, it also does not have the problems that this ability can create as well, which makes it a strength.

Although W3C XML Schema's ability to add default attributes to elements is an advantage, it is a disadvantage in some ways as well. It means that an XML file may not be usable in the absence of its schema, even if the document would validate against that schema. In effect, all users of such an XML document must also implement the W3C XML Schema specification, thus ruling out minimalist or older XML parsers. It can also dramatically slow down processing of the document, as the processor must potentially download and process a second XML file (the schema).

Although W3C XML Schema has an extensive number of built-in data types, it provides no mechanism for the user to add more. This means that a user who needs to define a type not supported by WXS will need to provide post-Schema validation services for that kind of data.

Tool Support

WXS support exists in a number of large XML parsing packages. Xerces and the .NET Framework's Base Class Library both provide support for WXS validation.

RELAX NG

Main article: RELAX NG

Advantages over W3C XML Schema

RELAX NG provides for most of the advantages that W3C XML Schema does over DTDs.

While the language of RELAX NG can be written in XML, it also has an equivalent form that is much more like a DTD, but with greater specifying power. This form is known as the compact syntax. Tools can easily convert between these forms with no loss of features or even commenting. Even arbitrary elements specified between RELAX NG XML elements can be converted into the compact form.

RELAX NG provides very strong support for unordered content. That is, it allows the schema to state that a sequence of patterns may appear in any order.

RELAX NG also allows for non-deterministic content models. What this means is that RELAX NG allows the specification of a sequence like the following:

   <zeroOrMore>
     <ref name="odd"/>
     <ref name="even"/>
   </zeroOrMore>
   <optional>
     <ref name="odd"/>
   </optional>

When the validator encounters something that matches the "odd" pattern, it is unknown whether this is the optional last "odd" reference or simply one in the zeroOrMore sequence without looking ahead at the data. RELAX NG allows this kind of specification. W3C XML Schema requires all of its sequences to be fully deterministic, so mechanisms like the above must be either specified in a different way or omitted altogether.

RELAX NG allows attributes to be treated as elements in content models. In particular, this means that one can provide the following:

   <element name="some_element">
     <choice>
       <attribute name="has_name">
         <value>false</value>
       </attribute>
       <group>
         <attribute name="has_name">
           <value>true</value>
         </attribute>
         <element name="name"><text/></element>
       </group>
     </choice>
   </element>

This block states that the element "some_element" must have an attribute named "has_name". This attribute can only take true or false as values, and if it is true, the first child element of the element must be "name", which stores text. If "name" did not need to be the first element, then the choice could be wrapped in an "interleave" element along with other elements. The order of the specification of attributes in RELAX NG has no meaning, so this block need not be the first block in the element definition.

W3C XML Schema cannot specify such a dependency between the content of an attribute and child elements.

RELAX NG's specification only lists two built-in types (string and token), but it allows for the definition of many more. In theory, the lack of a specific list allows a processor to support data types that are very problem-domain specific.

Most RELAX NG schemas can be algorithmically converted into W3C XML Schemas and even DTDs (except when using RELAX NG features not supported by those languages, as above). The reverse is not true. As such, RELAX NG can be used as a normative version of the schema, and the user can convert it to other forms for tools that do not support RELAX NG.

Lack of W3C XML Schema Misfeatures

Depending on point of view, the advantages of W3C XML Schema over RELAX NG can be seen as misfeatures of W3C XML Schema which makes the lack thereof advantages of RELAX NG.

RELAX NG lacks any analog to PSVI. PSVI couples type annotation with validation. This is considered a misfeature, because it couples processing of the document with validation giving different result depending on whether validation has been performed.

W3C XML schema allows repetitions to be constrained by arbitrary minimum and maximum numbers of repetitions. This violates the Zero One Infinity Rule. RELAX NG defines repetitions as one, one or more or zero or more true to the Zero One Infinity Rule.

The core feature set of RELAX NG has no ability to apply default attribute data to an element's list of attributes, while W3C XML Schema does. Attribute defaulting is considered a misfeature. Attribute defaulting makes a consuming application see different values depending on whether validation was performed.

RELAX NG has only two built-in data types (string and token), while W3C XML Schema has far more. However, the W3C XML Schema set of datatypes is not extensible. The RELAX NG datatype system is extensible and RELAX NG validators commonly ship with built-in support the W3C XML Schema datatype library.

W3C XML Schema has syntax for attaching a schema to an XML document. By design, RELAX NG doesn't involve putting schema-related syntax into the XML document that will be validated. Instead, the RELAX NG validation process takes two independent inputs: the document and the schema. It follows that the document cannot tamper with the grammar that is being used to check it. Such tampering is a potential security problem.

Disadvantages

Most of RELAX NG's disadvantages are covered under the section on W3C XML Schema's advantages over RELAX NG.

Though RELAX NG's ability to support user-defined data types is useful, it comes at the disadvantage of only having two data types that the user can rely upon. Which, in theory, means that using a RELAX NG schema across multiple validators requires either providing those user-defined data types to that validator or using only the two basic types. In practice however, most RELAX NG processors support the W3C XML Schema set of data types.

Tool Support

RELAX NG's tool support is significant, but it is slightly less widespread than W3C XML Schema. The Mono Project's implementation of the .NET Framework includes a RELAX NG validator. The C programming language library LibXML2 provides RELAX NG support as well. Sun Microsystems's Multiple Schema Validator for Java also provides RELAX NG support.

Schematron

Main article: Schematron

Schematron is a fairly unique schema language. Unlike the main three, it defines an XML file's syntax as a list of XPath-based rules. If the document passes these rules, then it is valid.

Advantages

Because of its rule-based nature, Schematron's specificity is very strong. It can require that the content of an element be controlled by one of its siblings. It can, also, request or require that the root element, regardless of what element that happens to be, have specific attributes. It can even specify required relationships between multiple XML files.

Disadvantages

While Schematron is good at relational constructs, its ability to specify the basic structure of a document, that is, which elements can go where, results in a very verbose schema.

The typical way to solve this is to combine Schematron with RELAX NG or W3C XML Schema. There are several schema processors available for both languages that support this combined form. This allows Schematron rules to specify additional constraints to the structure comprised of W3C XML Schema or RELAX NG.

Tool Support

Schematron's reference implementation is actually an XSLT transformation that transforms the Schematron document into an XSLT that validates the XML file. As such, Schematron's potential toolset is any XSLT processor, though libxml2 provides an implementation that does not require XSLT. Sun Microsystems's Multiple Schema Validator for Java has an add-on that allows it to validate RELAX NG schemas that have embedded Schematron rules.

Namespace Routing Language (NRL)

Main article: Namespace Routing Language

This is not technically a schema language. Its sole purpose is to direct parts of documents to individual schemas based on the namespace of the encountered elements. An NRL is merely a list of XML namespaces and a path to a schema that each corresponds to. This allows each schema to be concerned with only its own language definition, and the NRL file routes the schema validator to the correct schema file based on the namespace of that element.

This XML format is schema-language agnostic and works for just about any schema language.

See Also

• Document Type Definition
• Document Structure Description
• W3C XML Schema
• RELAX NG
• Schematron
• Namespace Routing Language