XML in a Nutshell, 3rd Edition

Controlling Qualification

One of the major headaches with DTDs is that they have no explicit support for namespace prefixes since they predate the “Namespaces in XML” recommendation. Although “Namespaces in XML” went to great pains to explain that prefixes were only placeholders and only the namespace URIs really matter, it was painful and awkward to design a DTD that could support arbitrary prefixes. Schemas correct this by validating against namespace URIs and local names rather than prefixed names.

The elementFormDefault and attributeFormDefault attributes of the xs:schema element control whether locally declared elements and attributes must be namespace-qualified within instance documents. Suppose the attribute attributeFormDefault is set to qualified in the schema, like this:

<xs:schema xmlns:xsi="http://www.w3.org/2001/XMLSchema"
  targetNamespace="http://namespaces.oreilly.com/xmlnut/address"
  xmlns:addr="http://namespaces.oreilly.com/xmlnut/address"
  attributeFormDefault="qualified">

Now, if addressdoc.xml is validated against the schema, the validator reports the following error:

Attribute "language" must be declared for element type "fullName".

Since the default attribute form has been set to qualified, the schema processor doesn’t recognize the unqualified language attribute as belonging to the same schema as the fullName element. This is because attributes, unlike elements, don’t inherit the default namespace from the xmlns="..." attribute. They must always be explicitly ...