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Most of the time a
single XML application has a single namespace URI for all its elements,
though a few applications use multiple namespaces to subdivide different
parts of the application. For instance, XSL uses different namespaces
for XSL Transformations (XSLT) and XSL Formatting Objects (XSL-FO).
Still a little strong for me, /unless I've failed to grasp what you mean by
an XML application/. Mixed vocabularies are becoming increasingly common,
particularly in the various e-gov XML projects, where there is a focus on
integration between the vast array of public organisations. This has lead to
e.g. the creation of nationally standardised core vocabularies to represent
such key data structures as address, person-ID. Usually the public body
responsible for registering and storing a specific piece of information will
be responsible for creating and maintaining a namespace and a set of schemas
for defining it. Even something as simple as a name and address can then
include namespaces for several different public registries and bodies.
Various layers are then built up on top of this in the various sectors, each
adding to namespace proliferation. Lastly, the practice of using dates in
the namespace for versioning adds even more to the namespace soup. Many of
the XML formats I currently work with contain 5 or 6 namespaces, just to
define the information architecture for fairly simple documents.
Whether or not similar things are occuring in the private sector I couldn't
say, thgouht I note that the GML schemas contain references to XLink, and
are also presupposed to be a generic layer that can be accessed by an
application layer (in its own namespace). So your average GML app will be
dealing with at least 3 namespaces, and thats prior to the XML protocols it
could use to fetch/post data from/to a server.