XML Schema (W3C)

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This article is about the W3C's XML Schema language. For information on XML schema languages in general, see XML schema.

XML Schema, published as a W3C Recommendation in May 2001, is one of several XML schema languages. It was the first separate schema language for XML to achieve Recommendation status by the W3C.

Like all XML schema languages, XML Schema can be used to express a schema: a set of rules to which an XML document must conform in order to be considered 'valid' according to that schema. However, unlike most other schema languages, XML Schema was also designed with the intent of validation resulting in a collection of information adhering to specific datatypes, which can be useful in the development of XML document processing software, but which has also provoked criticism.

An XML Schema instance is an XML Schema Definition (XSD) and typically has the filename extension ".xsd". The language itself is sometimes informally referenced as XSD. It has been suggested that WXS (for W3C XML Schema) is a more appropriate initialism^[1] though this acronym has not been in a widespread use and W3C working group rejected it. XSD is also an initialism for XML Schema Datatypes, the datatype portion of XML Schema.

1 History
2 Post-Schema-Validation Infoset
3 Example
4 XML Schema Definition
5 See also
6 References
7 External links

[edit] History

In its appendix of references, XML Schema acknowledges the influence of DTD and other early XML schema efforts such as DDML, SOX, XML-Data, and XDR. It appears to have picked pieces from each of these proposals, but is also a compromise between them. Of those languages, XDR and SOX continued to be used and supported for a while after XML Schema was published. A number of Microsoft products supported XDR until the release of MSXML 4.0 (which dropped XDR in favor of XML Schema) in October 2001. Commerce One, Inc. supported its SOX schema language until declaring bankruptcy in late 2004. In December of that year, Novell, Inc. purchased the company's patents, including those related to SOX, reportedly in an effort to keep them from being exploited by unrelated companies whose primary business is filing patent-related lawsuits.^[2]

[edit] Post-Schema-Validation Infoset

After XML Schema-based validation, it is possible to express an XML document's structure and content in terms of the data model that was implicit during validation. The XML Schema data model includes:

the vocabulary (element and attribute names)
the content model (relationships and structure)
the data types.

This collection of information is called the Post-Schema-Validation Infoset (PSVI). The PSVI gives a valid XML document its "type" and facilitates treating the document as an object, using object-oriented programming (OOP) paradigms.

This particular OOP approach to XML data access was primarily advocated by Microsoft, a major contributor to the development of XML Schema. Converting an XML document to a datatype-aware object can be beneficial in some parts of computer software design, but critics contend that it also undermines openness, a key feature of XML, and that it is biased toward compatibility with the datatypes native to Microsoft's favored programming languages.^[3]

In addition, the limitations inherent to (and caused by) XML Schema datatypes, the restrictive coupling of those datatypes with the rest of XML Schema, and dependencies on those datatypes in other W3C specifications are points of contention among a number of XML software developers.^[4]

[edit] Example

An example of a very simple XML Schema Definition to describe a country is given below.

<xs:schema
 xmlns:xs="http://www.w3.org/2001/XMLSchema">
 <xs:element name="country" type="Country"/>
 <xs:complexType name="Country">
  <xs:sequence>
   <xs:element name="name" type="xs:string"/>
   <xs:element name="population" type="xs:decimal"/>
  </xs:sequence>
 </xs:complexType>
</xs:schema>

An example of an XML document that conforms to this schema is given below (assuming the schema file name is "country.xsd" and both files are in the same directory).

<country
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:noNamespaceSchemaLocation="country.xsd">
  <name>France</name>
  <population>59.7</population>
</country>

[edit] XML Schema Definition

An XML Schema Definition (XSD) is an instance of an XML schema written in XML Schema. An XSD defines a type of XML document in terms of constraints upon what elements and attributes may appear, their relationship to each other, what types of data may be in them, and other things. It can be used with validation software in order to ascertain whether a particular XML document is of that type, and to produce a PSVI.

XSDs were the first W3C-recommended XML schemas to provide a namespace and datatype aware alternative to using XML's native Document Type Definitions (DTDs).

XML Schema Definition files usually have the filename extension ".xsd". A unique Internet Media Type is not yet registered for XSDs, so "application/xml" or "text/xml" should be used, as per RFC 3023.

[edit] See also

XML Schema Language Comparison - Comparison to other XML Schema languages.
RELAX NG - another XML schema language (an ISO international standard) that is often used with XML Schema datatypes

[edit] References

^ The W3C Working Group that developed the language deliberately avoided creating an initialism so that people would refer to the language simply as XML Schema. However, some people in the technology industry have a fondness for three- and four-letter initialisms, and "DTD" was already in general use to refer to both a Document Type Definition and the "language" — actually an unnamed portion of the markup languages XML or SGML — in which a DTD is written. Of the two most common XML Schema initialisms, XSD and WXS, XSD is relatively ubiquitous. However, the W3C has not given its blessing to either one, and a number of XML professionals, in postings on electronic mailing lists such as xml-dev and in articles published in online trade journals like XML.com, advocate referring to the language as WXS or W3C XML Schema.
^ http://www.iht.com/articles/2005/05/02/business/novell.php
^ http://www.biglist.com/lists/xsl-list/archives/200302/msg00821.html
^ http://www.oreillynet.com/pub/wlg/2511?wlg=yes