XML Schema

Chapter 10. Controlling Namespaces

The W3C released Namespaces in XML about a year after XML 1.0. Namespaces provide a URI-based mechanism that helps differentiate XML vocabularies. Rather than update XML 1.0’s DTDs to provide explicit namespace support, the W3C chose to implement namespace support in W3C XML Schema. Support of namespaces was eagerly awaited by the XML community and, thus, are especially well-polished by the W3C XML Schema editors.

Namespaces caused two problems to DTDs. One was how to recognize namespaces defined using different prefixes in instance documents. The other was how best to facilitate the definition of schemas with multiple namespaces. The problem of open schemas tightly controlling some namespaces while keeping the flexibility to add unknown elements and attributes from unknown namespaces, was especially difficult.

W3C XML Schema has gone beyond these expectations for its use of namespaces by associating a namespace to all the objects (elements and attributes, but also simple and complex types as well as groups of elements and attributes) defined in a schema, allowing the use of namespaces to build modular libraries of schemas.

Namespaces Present Two Challenges to Schema Languages

Namespace prefixes should only be considered to be local shortcuts to replace the URI references that are the real identifiers for a namespace. The following documents should, therefore, be considered strictly equivalent by namespace-aware applications:

<?xml version="1.0"?> ...