XML Schema (W3C)

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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 that determination of a document's validity would produce a collection of information adhering to specific data types. Such a post-validation infoset can be useful in the development of XML document processing software, but the schema language's dependence on specific data types has 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.

[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 among 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.

[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.^[2]

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.^[3]

[edit] Data Types

Data types in XML Schema are classified into built-in data types and user-defined data types.

There are 19 built-in primitive data types and 25 built-in derived data types specified. They represent common data types such as string, integer, decimal and date.

[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").

<country
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:noNamespaceSchemaLocation="country.xsd">
    <name>France</name>
    <population>60,144,000</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
• List of XML Schemas - list of XML schemas in use on the Internet sorted by purpose

[edit] References

[edit] External links

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Wikibooks has a book on the topic of

XML Schema

• 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
• Liquid XML Studio - Free Graphical XSD Editor
• Eclipse XSD Model, an open source Java implementation of the XML Schema model.
• Zvon XML Schema Reference
• Interactive XML Schema tutorial from XMLzoo
• XSD Tutorial
• Brief XSD Schema Tutorial
• W3Schools XML Schema Tutorial
• 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
• XSDBench XML Schema Benchmark Compares the performance of validating XML parsers
• NetBeans XML Schema Editor, free, open source XML tools, including an advanced XML Schema editor

The external links in this article may not follow Wikipedia's content policies or guidelines. Please improve this article by removing excessive or inappropriate external links.