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. It has been suggested that W3C Markup Validation Service be merged into this article or section. ... Year 2001 (MMI) was a common year starting on Monday (link displays the 2001 Gregorian calendar). ... 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. ... The Extensible Markup Language (XML) is a general-purpose markup language. ...

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. On computer science, a datatype (often simply type) is a name or label for a set of values and some operations which can be performed on that set of values. ...

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. Look up acronym, initialism, alphabetism in Wiktionary, the free dictionary. ...

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. Document Type Definition (DTD), defined slightly differently within the XML and SGML (the language XML was derived from) specifications, is one of several SGML and XML schema languages, and is also the term used to describe a document or portion thereof that is authored in the DTD language. ... Document Definition Markup Language (DDML) is an XML schema language proposed in 1999 by various contributors from the xml-dev electronic mailing list. ... Schema for Object-Oriented XML, or SOX, is an XML schema language developed by Commerce One. ... Microsoft Corporation, (NASDAQ: MSFT, HKSE: 4338) is a multinational computer technology corporation with global annual revenue of US$44. ... Microsoft XML Core Services (MSXML) is a set of services that allow applications written in JScript, VBScript, and Microsoft development tools to build Windows-native XML-based applications. ... Commerce One was one of the pioneering e-commerce companies. ...

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: A data model is a model that describes how data are represented and used in an abstract way. ...

• 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 article needs cleanup. ... Object-oriented programming (OOP) is a programming paradigm that uses objects and their interactions to design applications and computer programs. ...

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] Microsoft Corporation, (NASDAQ: MSFT, HKSE: 4338) is a multinational computer technology corporation with global annual revenue of US$44. ... In computer science, a datatype or data type (often simply a type) is a name or label for a set of values and some operations which one can perform on that set of values. ...

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]

Data Types

There are 19 primitive datatypes specified. There are several proposed solutions on embedding binary data into xml (such as an image). Binary data could be considered an "opaque data" type. "An opaque type gets its name from the fact that the data model knows nothing about the internal representation of the type." (IBM Article) As seen here hexadecimal text and base64 text are both primitive types according to the W3C standard. A data model is a model that describes how data are represented and used in an abstract way. ...

• string
• boolean
• decimal
• float
• double
• duration
• dateTime, time, date, gYearMonth, gYear, gMonthDay, gDay, gMonth
• hexBinary
• base64Binary
• anyURI
• QName
• NOTATION

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>59.7</population> </country> 

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. This article needs cleanup. ...

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). An XML namespace is a W3C standard for providing uniquely named elements and attributes in an XML instance. ... Document Type Definition (DTD), defined slightly differently within the XML and SGML (the language XML was derived from) specifications, is one of several SGML and XML schema languages, and is also the term used to describe a document or portion thereof that is authored in the DTD language. ...

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.

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

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. ... In computing, RELAX NG (REgular LAnguage for XML Next Generation) is a schema language for XML, based on Murata Makotos RELAX and James Clarks TREX. A RELAX NG schema specifies a pattern for the structure and content of an XML document. ... This is a list of XML schemas in use on the Internet sorted by purpose. ...

References

Document Type Definition (DTD), defined slightly differently within the XML and SGML (the language XML was derived from) specifications, is one of several SGML and XML schema languages, and is also the term used to describe a document or portion thereof that is authored in the DTD language. ... An electronic mailing list, a type of Internet forum, is a special usage of e-mail that allows for widespread distribution of information to many Internet users. ...

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.
• W3Schools XML Schema Tutorial