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 applied to that information 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 determines 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 , although 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, 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 used much in practice.