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Source: XML schema

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 syntactical constraints imposed by XML itself. These constraints are generally expressed using some combination of grammatical rules governing the order of elements, Boolean predicates that the content must satisfy, data types governing the content of elements and attributes, and more specialized rules such as uniqueness and referential integrity constraints.
There are languages developed specifically to express XML schemas. The document type definition (DTD) language, which is native to the XML specification, is a schema language that is of relatively limited capability, but that also has other uses in XML aside from the expression of schemas. Two more expressive XML schema languages in widespread use are XML Schema (with a capital S) and RELAX NG.
The mechanism for associating an XML document with a schema varies according to the schema language. The association may be achieved via markup within the XML document itself, or via some external means.
The XML Schema Definition is commonly referred to as XSD.

Validation

XML

Languages

= Document Type Definitions

= W3C XML Schema

or ).

Disadvantages

W3C XML Schema is complex and hard to learn, although that is partially because it tries to do more than mere validation (see PSVI).
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.
Likewise, WXS's formal mechanism for associating a document with a schema can pose a potential security problem. For WXS validators that will follow a URI to an arbitrary online location, there is the potential for reading something malicious from the other side of the stream.
W3C XML Schema does not implement most of the DTD ability to provide data elements to a document.
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 slow down the processing of the document, as the processor must potentially download and process a second XML file (the schema); however, a schema would normally then be cached, so the cost comes only on the first use.

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

=

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

Advantages over W3C XML Schema

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:

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:

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.

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.

= Schematron

=

Schematron is a fairly unusual 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 defined by 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)

=

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.

Terminology

Capitalization in the schema word: there is some confusion as to when to use the capitalized spelling "Schema" and when to use the lowercase spelling. The lowercase form is a generic term and may refer to any type of schema, including DTD, XML Schema (aka XSD), RELAX NG, or others, and should always be written using lowercase except when appearing at the start of a sentence. The form "Schema" (capitalized) in common use in the XML community always refers to W3C XML Schema.

Schema authoring choices

The focus of the schema definition is structure and some semantics of documents. However, schema design, just like design of databases, computer program, and other formal constructs, also involve many considerations of style, convention, and readability. Extensive discussions of schema design issues can be found in (for example) Maler (1995) and
DeRose (1997).

Consistency
One obvious consideration is that tags and attribute names should use consistent conventions. For example, it would be unusual to create a schema where some element names are camelCase but others use underscores to separate parts of names, or other conventions.
Clear and mnemonic names
As in other formal languages, good choices of names can help understanding, even though the names per se have no formal significance. Naming the appropriate tag "chapter" rather than "tag37" is a help to the reader. At the same time, this brings in issues of the choice of natural language. A schema to be used for Irish Gaelic documents will probably use the same language for element and attribute names, since that will be the language common to editors and readers.
Tag vs attribute choice
Some information can "fit" readily in either an element or an attribute. Because attributes cannot contain elements in XML, this question only arises for components that have no further sub-structure that XML needs to be aware of (attributes do support multiple tokens, such as multiple IDREF values, which can be considered a slight exception). Attributes typically represent information associated with the entirety of the element on which they occur, while sub-elements introduce a new scope of their own.
Text content
Some XML schemas, particularly ones that represent various kinds of documents, ensure that all "text content" (roughly, any part that one would speak if reading the document aloud) occurs as text, and never in attributes. However, there are many edge cases where this does not hold: First, there are XML documents which do not involve "natural language" at all, or only minimally, such as for telemetry, creation of vector graphics or mathematical formulae, and so on. Second, information like stage directions in plays, verse numbers in Classical and Scriptural works, and correction or normalization of spelling in transcribed works, all pose issues of interpretation that schema designers for such genres must consider.
Schema reuse
A new XML schema can be developed from scratch, or can reuse some fragments of other XML schemas. All schema languages offer some tools (for example, include and modularization control over namespaces) and recommend reuse where practical. Various parts of the extensive and sophisticated Text Encoding Initiative schemas are also re-used in an extraordinary variety of other schemas.
Semantic vs syntactic
Except for a RDF-related one, no schema language express formally semantic, only structure and data-types. Despite being the ideal, the inclusion of RDF assumptions is very poor and is not a recommendation in the schema development frameworks.

References

Comparative Analysis of Six XML Schema Languages by Dongwon Lee, Wesley W. Chu, In ACM SIGMOD Record, Vol. 29, No. 3, page 76-87, September 2000
Taxonomy of XML Schema Languages using Formal Language Theory by Makoto Murata, Dongwon Lee, Murali Mani, Kohsuke Kawaguchi, In ACM Trans. on Internet Technology (TOIT), Vol. 5, No. 4, page 1-45, November 2005

External links

Comparing XML Schema Languages by Eric van der Vlist (2001)
Application of XML Schema in Web Services Security by Sridhar Guthula, W3C Schema Experience Report, May 2005
March 2009 DEVX article "Taking XML Validation to the Next Level: Introducing CAM" by Michael Sorens