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RE: Using fragment identifiers with URNs

From: Stephen Cranefield <SCranefield@infoscience.otago.ac.nz>
Date: Thu, 27 Sep 2001 13:10:23 +1200
Message-ID: <B57613845A50D211864C0000F8FA5C2804207612@mars.otago.ac.nz>
To: "'uri@w3.org'" <uri@w3.org>
Roy Fielding wrote:

> It is possible to use a URN to perform a retrieval action, so I don't
> follow your argument.  Just because the action isn't implied 
> doesn't mean
> that the identifier can't be used in performing the action explicitly.

Yes, but that would be making an application-level assumption, whereas
the generic syntax and meaning of URIs and URI references should be
independent of any particular application.  Basically, my point is that
perhaps the definition of URI references should be updated to explicitly
state that the notion of retrieval is specific to the URI scheme, and
that URN schemes are not required to have any retrieval mechanism defined.
To add a fragment identifier to a URN that represents an abstract name
would be misleading - it would be making an assumption that there *is* some
retrieval mechanism defined for that URN scheme.

To clarify my previous post, consider two different types of URN.
First, consider Digital Object Identifiers (which will
presumably one day become a registered type of URN):

  The DOI .... is simply one element of a complex system. ...
  A user can use a DOI to do something. The simplest action that a user
  can perform using a DOI is to locate the entity that it identifies.  
  (From http://www.doi.org/handbook_2000/what_is_a_doi.html)

URLs have this property too: they are defined in the context of a
distributed system architecture (the infrastructure of the Web) which
defines what it means to retrieve information using a URL.

Now, suppose I develop a URN scheme that I intend to use for naming
classes and properties in an RDF schema, and I intend URIs in this
scheme to be unique identifiers of concepts, but not references to
their definition in a schema (I can use rdfs:isDefinedBy to convey that
information).  Given the current definition of a fragment identifier,
it would not be appropriate to use a fragment identifier with one of
these URNs.  The very use of a fragment identifier is making an
assumption that the URN is intended as a reference that can be used
for retrieval - which is not true in this case.

I am not denying that the URI reference syntax is convenient
and could be usefully extended to apply to names as well as
locators, but that would require broadening the current definition.

- Stephen
Received on Wednesday, 26 September 2001 21:07:57 UTC

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