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The W3C XML Schema Language (schemas) is a declarative language used to describe the allowed contents of XML documents by assigning types to elements and attributes. The schema language includes several dozen standard types and allows you to define your own custom types. The combination of the information in an XML document instance and the types in that document as defined by the schema is sometimes called the Post Schema Validation Infoset (PSVI).
A schema processor reads both an input XML document and a schema (which is itself an XML document because the W3C XML Schema Language is an XML application) and returns a Boolean result specifying whether the document adheres to the constraints in the schema. A document that satisfies all the schema's constraints and in which all the document's elements and attributes are declared is said to be schema-valid, though in this chapter we will mostly just call such documents valid. A document that does not satisfy all of the constraints is said to be invalid.
All standard schema elements are in the http://www.w3.org/2001/XMLSchema namespace. In this chapter, we assume that this URI is mapped to the xs prefix using an appropriate xmlns:xs declaration. This declaration is almost always placed on the root element start-tag:
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
In addition, several attributes are used in instance documents to associate schema information with them, including schemaLocation and type. These attributes are in the http://www.w3.org/2001/XMLSchema-instance namespace. In this chapter, we assume that this URI is mapped to the xsi prefix with an appropriate xmlns:xsi declaration on either the element where this attribute appears or one of its ancestors.
In a few cases, schema elements may contain elements from other, arbitrary namespaces or no namespace at all. This occurs primarily inside xs:appinfo and xs:documentation elements, which provide supplementary information about the schema itself or the documents the schema describes to systems that are not schema validators or to people reading the schema.
Finally, most schema elements can have arbitrary attributes from other namespaces. For instance, this allows you to make an xs:attribute element a simple XLink by giving it xlink:type and xlink:href attributes or to identify the language of an xs:notation using an xml:lang attribute. However, this capability is not much used in practice.
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| 20.5. XML Document Grammar |  | 21.2. Schema Elements |
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