Chapter 11. Referencing Schemas and Schema Datatypes in XML Documents

So far, we have seen how W3C XML Schemas can be written outside XML documents without touching the actual instance documents. In this chapter, we will introduce a new namespace to be used inside XML documents to provide information for use by schema processors. This information may identify the location of the associated schemas, as well as further identify the schema types used, which opens a new level of flexibility and interaction between schemas and instance documents in the design of XML applications.

This namespace (which is the same for all W3C XML Schema meta-information located in the instance documents themselves) is http://www.w3.org/2001/XMLSchema-instance. The prefix usually used to designate this namespace is xsi. This namespace uses only four attributes, which are considered valid in any element of any instance document without being declared in the schema.

Associating Schemas with Instance Documents

The first piece of information that may be useful for a schema processor is some hints about the locations where the schema processor might find schemas relevant to the instance document. This feature is similar to the SYSTEM identifier of the XML doctype declaration, but with some important differences. The first difference is that a schema may not be enough to describe a document, since each schema might describe only one namespace (or lack of a namespace), and the composition of the schemas can be ...

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