W3C XML Schema, commonly called XSD, is a language for defining the structure and allowed values of XML documents. An XSD schema describes a class of documents by setting rules for elements, attributes, content, and datatypes; XML applications can then check whether a document conforms to those rules.
What does W3C XML Schema mean?
W3C XML Schema Definition Language (XSD) is a formal language for describing and constraining classes of XML documents. A schema is itself written in XML, using the XML Schema vocabulary and namespaces. It describes components and relationships in a document, including which elements and attributes may appear, how content is organized, and what values are allowed. The W3C defines this purpose in its XSD 1.1 Structures specification.
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“XML Schema” names the W3C language and its specifications; “XSD” is the common abbreviation for XML Schema Definition. In practice, people also use “an XSD” to mean a schema document written in that language.
What is an XSD schema used for?
An XSD schema gives XML-producing and XML-consuming applications a shared set of rules. A processor can assess whether an XML document conforms to the schema, helping systems detect structural or datatype problems before using the document. Schemas can also document the intended meaning and use of document components, and can specify normalization and default values.
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XSD is not a guarantee that a document meets every business or application requirement. Some rules need checks beyond what the schema language expresses, so software may need an additional validation layer.
What can an XSD describe?
- Elements and content: which elements a document may contain and the relationships or content constraints that apply.
- Attributes: which attributes are allowed and what constraints apply to their values.
- Datatypes and values: the kinds of values components may hold and rules for checking them.
- Document meaning and use: documentation of how components are intended to be used, alongside formal constraints.
How XSD 1.1 is organized
The W3C XSD 1.1 Recommendation is split into two parts. Part 1: Structures defines schema components, how schemas are represented and composed, and how documents are assessed for schema validity. Part 2: Datatypes defines datatype concepts and the rules used to check datatype validity.
For datatypes, XSD distinguishes a value space—the values themselves—from a lexical space—the text literals used to represent those values. The specification also defines mappings between these spaces and selected relations and operations needed when processing schemas.
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Which XSD version is the standard?
The W3C identifies XML Schema 1.1 as a Recommendation intended to remain mostly compatible with version 1.0 while correcting errors and making improvements. The W3C records the original XSD 1.0 Recommendation as 2 May 2001 and its second edition as 28 October 2004 in its XML Schema overview. XSD 1.1 Parts 1 and 2 became Recommendations on 5 April 2012, according to the W3C publications listing.
A Recommendation establishes the W3C specification; it does not by itself establish that a particular XML tool supports every feature of that version. If a project depends on a specific XSD feature, check that its schema processor supports the required version.
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The detailed Structures specification is primarily a precise language-definition reference, not a tutorial. The W3C recommends the non-normative XML Schema Primer as a step-by-step introduction. After learning the basics, use the Structures and Datatypes specifications when you need authoritative detail about schema components or datatype rules.
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