XHTML and the Three Namespaces

Andrew Layman andrewl at microsoft.com
Thu Sep 23 20:43:34 BST 1999

Since there have been several recent misunderstandings of my position
regarding namespaces and schemas, I will attempt to short-cut speculation
and spell-out explicitly what I believe to be the correct relation between
the two:

1.	Namespaces serve to associate universally-unique identifiers with
qualified names.
2.	Anything so identified can have, at most, one definition.
3.	All distinctions between identified items must be reflected by a
difference in the respective identifiers.
4.	An item's definition might be supplied by a human-readable document,
or a machine-readable one; by one document or perhaps several taken in
aggregate, combined in some determinate fashion.
5.	Discovery of similarity between two distinct items may require
reading their respective definitions, either directly at the time of
processing, or indirectly at an earlier time, and may be effected either by
direct processing of the defining documents, or indirect reflection of the
contents of the defining documents, for example by a human reading the
document and incorporating some of its information into a program. 
6.	Schemas may be associated with namespaces, and may serve the role of
defining the items in the namespace.
7.	If a schema is associated with a namespace, there will be mechanisms
for locating that schema given the namespace URI. (Note that this
formulation expressly does not require that the URI of the schema is the
same as the URI of the namespace.)

I believe that the point of greatest misunderstanding here is the belief
that point two requires conflating namespaces with schemas.  (I conclude
this because several of my arguments in favor of point two have met with
replies arguing not directly against point two, but instead arguing that
namespaces and schemas are distinct.)

I understand that there are other viewpoints.  This mail is not intended to
argue in favor of my position, nor to argue against other positions, but to
make clear statements of the several issues involved and clarify my position
on each.

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