W3C home > Mailing lists > Public > www-rdf-interest@w3.org > August 2001

RE: Using urn:publicid: for namespaces

From: Danny Ayers <danny@isacat.net>
Date: Fri, 10 Aug 2001 14:35:11 +0100
To: "Stephen Cranefield" <scranefield@infoscience.otago.ac.nz>, <www-rdf-interest@w3.org>
Sorry for jumping in at this point (my mail folders are all over the place,
the URI/URN thread likewise), but I would have thought a URL would make a
particularly appropriate candidate for identifying an RDF schema - without
wishing to revive old debates about what's at a URL (shoot me down), if the
schema is identified by a url then my machine can go and get it. There is a
scheme that RDF schema designers can use now - wouldn't it make more sense
for http://www.w3.org/TR/rdf-schema/ to point to a chunk of RDF (perhaps
embedded in an informative html doc) rather that to something little more or
less than human-readable?

Again, with apologies for the arguments I've missed (bang bang), I would
have thought that the role of URNs would primarily be in the places where
the thing named is unavailable online - e.g. urn:WilliamShakespeare. This is
enough to be able to make assertions etc about the thing named, including
presumably to specify links to online representations of the work of said
gent. When there is an option to put something of substance ((meta-)data)
online, I would expect that a URL would always be preferred (boom splash).
In this context I personally don't think that the protocol is relevant - ok,
so it's http now, but when protocol X comes along, http://stuff can still
have validity (glug glug). There is always the fallback position (e.g. on an
intermittently connected device) of using the url as just a urn - and it is
usable if there is local knowledge of the thing identified (e.g. a cached
page, or a matching RDF schema within the kb of a local agent). Which isn't
dissimilar to the human using of names, as it happens (gasp).


Danny Ayers

>-----Original Message-----
>From: www-rdf-interest-request@w3.org
>[mailto:www-rdf-interest-request@w3.org]On Behalf Of Stephen Cranefield
>Sent: 10 August 2001 03:15
>To: www-rdf-interest@w3.org
>Subject: Using urn:publicid: for namespaces
>I agree with the comments that have been made on this mailing list
>about the inappropriateness of using the http URL scheme for names -
>especially those representing RDF schemas.  While proposed URL schemes
>such as tag and ark are promising, it would be nice if there was an
>existing scheme that RDF schema designers could use right now.  I've
>been looking around for possibilities and wondered if it would be
>appropriate to use the urn:publicid: scheme for RDF schema namespaces
>(this is defined by http://www.ietf.org/rfc/rfc3151.txt and is also
>listed as officially registered at
>I have two proposals: the first based on SGML "formal public
>(http://www.oasis-open.org/cover/petersonFPI-TAG7030101.html) and the
>other based on an informal syntax for public identifiers.
>Using formal public identifiers
>Suppose I am defining a schema for tourist information.  A formal
>public identifier for its namespace might look like this:
>  -//University of Otago//NONSGML Tourism ontology v1.0//EN
>This would be encoded as a URN according to IETF RFC 3151 as follows:
>  urn:publicid:-:University+of+Otago:NONSGML+Tourism+ontology+v1.0:EN
>This isn't pretty, but at least no one is going to expect a Web
>browser to find a document using this URI.  In theory it should be
>resolvable to a document via an XML Catalog
>(http://www.oasis-open.org/committees/entity/spec.html) but I think
>this is less of a problem than the expectations caused by using http
>URLs for namespaces.
>The major problem is that the existing mechanism in RDF for mapping
>QNames to URIs (via concatenation) would produce a URI that doesn't
>correspond to a formal public ID (which must end with a language
>indicator).  For example, the RDF class BungyJump in the above
>namespace would have the following odd-looking URI:
>One answer to this would be to make the QName to URI mapping be
>dependent on the URI scheme used for the namespace.  For urn:publicid
>the algorithm might be to insert ";" and the local name before the
>language specifier (":EN").  This would give:
>which corresponds to the public identifier:
>  -//University of Otago//NONSGML Tourism ontology v1.0::BungyJump//EN
>It would be nice if the "public text class" (which can only be one of
>a few specified values) could be something more useful than NONSGML,
>such as NAMESPACE.  However, adding this as a possibility to the XML
>specification is probably not easily achieved.  It would also
>complicate the QName to URI mechanism which would need to replace
>NAMESPACE with something like NAME.
>Using informal public identifiers
>Public identifiers don't have to follow the above syntax.  They can
>comprise any upper and lower case letters, digits, space characters
>and line breaks (white space is normalised to a single space), and any
>of the following characters: ()+,-./:=?
>Therefore an unofficial syntax could be used to identify namespaces.
>One possibility is to use the formal public ID syntax but with the
>language part omitted and a convention to end the ID with "::" (so
>that concatenation will work for QName to URI mapping):
>  urn:publicid:-:University+of+Otago:NONSGML+Tourism+ontology+v1.0;
>corresponding to:
>  -//University of Otago//NONSGML Tourism ontology v1.0::
>Alternatively some other syntax could be used, such as the one
>proposed for the tag scheme (http://www.taguri.org/):
>  urn:publicid:infoscience.otago.ac.nz,2001-08-10:TourismOntology;
>which corresponds to the following public identifier:
>  infoscience.otago.ac.nz,2001-08-10//TourismOntology::
>Are there any reasons why SGML public identifiers shouldn't be used to
>identify namespaces in this way?  Although they were intended
>specifically to identify SGML external entities rather than conceptual
>things such as namespaces, the urn:publicid scheme has the distinct
>advantage of being an officially sanctioned URI scheme that I can
>start using today.
>- Stephen
>Stephen Cranefield
>Department of Information Science
>University of Otago                               Phone: 64 3 479 8083
>PO Box 56, Dunedin		                  Fax:   64 3 479 8311
>New Zealand	           E-mail: scranefield@infoscience.otago.ac.nz
Received on Friday, 10 August 2001 08:37:35 UTC

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