XML in a Nutshell, 3rd Edition

Overview

An XML Schema is an XML document containing a formal description of what comprises a valid XML document. A W3C XML Schema Language schema is an XML Schema written in the particular syntax recommended by the W3C.

Tip

In this chapter, when we use the word “schema” without further qualification, we are referring specifically to a schema written in the W3C XML Schema language. However, there are numerous other XML Schema languages, including RELAX NG and Schematron, each with their own strengths and weaknesses.

An XML document described by a schema is called an instance document . If a document satisfies all the constraints specified by the schema, it is considered to be schema-valid . The schema document is associated with an instance document through one of the following methods:

• An xsi:schemaLocation attribute on an element contains a list of namespaces used within that element and the URLs of the schemas with which to validate elements and attributes in those namespaces.

• An xsi:noNamespaceSchemaLocation attribute contains a URL for the schema used to validate elements that are not in any namespace.

• A validating parser may be instructed to validate a given document against an explicitly provided schema, ignoring any hints that might be provided within the document itself.