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.

Citation

Since other XML schema languages exist, it is important to cite the W3C-recommended language as either XML Schema or W3C XML Schema, always with the word Schema capitalized.

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, even though WXS (for W3C XML Schema) is a more appropriate initialism.That is, XSD is appropriate for referring to a schema written in the XML Schema language, but is somewhat less appropriate for referring to the XML Schema language itself, since 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, 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 leading 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. As Simon St. Laurent explains in an xml-dev post from August 12, 2002, "WXS seems to make a lot of sense to people, especially once they've encountered alternatives to WXS. I'm starting to apply the same terminology to XQuery — W3C XML Query is just one option among many. It's not my problem that the W3C uses a naming scheme to effectively declare itself as the sole authority on these subjects." XSD is also an initialism for XML Schema Datatypes, the datatype portion of XML Schema.

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.http://www.iht.com/articles/2005/05/02/business/novell.php

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)
and 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.http://www.biglist.com/lists/xsl-list/archives/200302/msg00821.html

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.http://www.oreillynet.com/pub/wlg/2511?wlg=yes

Example

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

An example of an XML document that conforms to this schema is given below.

France 59.7

References

External links

W3C XML Schema Specification: Primer, Structures, Datatypes, and Miscellaneous
W3C XML Schema Patterns for Databinding: Working Group
W3C Workshop on User Experiences of XML Schema 1.0 Chairs' Summary Report
W3C XML Schema Versioning Use Cases
Eclipse XSD Model, an open source Java implementation of the XML Schema model.
Zvon XML Schema Reference
XMLPatterns.com - Design patterns for developing XML Schema documents
XSD tutorial from XMLzoo
Brief XSD Schema Tutorial
W3Schools XML Schema Tutorial
Schema made simple Advantages of Schema over Doctype.
A Simple Overview of W3C Schema
xFront Tutorials on XML Schema and XML
XML Schema Standard Type Library A collection of universally-useful data types defined in W3C XML Schema

W3C standards | XML-based standards