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

Re: URIs vs. URNs vs. URLs

From: Roy T. Fielding <fielding@ebuilt.com>
Date: Fri, 3 Aug 2001 20:44:38 -0700
To: "Sean B. Palmer" <sean@mysterylights.com>
Cc: uri@w3.org, www-rdf-interest@w3.org
Message-ID: <20010803204438.I1122@waka.ebuilt.net>
> Often, in RDF, you don't often expect a network entity to be returned
> from a particular term, because the semantics of it are coded into the
> processing tools (e.g. the built-ins in CWM). Thus, the persistence is
> still only as good as the implementation, but there is no particular
> benefit to using a URL, i.e. something that will give you a data
> representation of the resource upon dereferencing. Indeed, my argument
> is that because the DNS system is not all that stable (with domain
> takeovers, and money-crises a-plenty), a set of names that aren't
> based upon this is useful. Names that aren't necessarily expected to
> resolve to something could be useful too.

So what?  Names that are not used are not useful, period.  Names that are
not useful are never created.  Names that are never created cannot be the
basis of a formal reasoning system like RDF.  So, unless this is yet another
exercise in navel gazing, using anything other than the same old names that
everyone else uses (commonly known as URI) is a total waste of time.

People call me "Roy" because there exists a socially-significant binding
between that name and myself as a conceptual entity within a reasonable
scope of my existance.  To increase scope, people add some more identifiers
(like my last name, my last known address, the institutions where I worked,
etc.).  They don't add syntax.

> > Persistence is not, and never has been, a function of the
> > syntax used to create the name.
> Apart from where the authority component for that syntax can be
> delegated away to other entities by "mistake", "force majure", "lack
> of money", or whatever? Also, I think that persistence is reflected in
> the syntax once it has been put to use for a while. People know that
> if they dereference an HTTP URL, 90%+ of the time, they're going to
> get a 200 O.K. response.

Yes, which is what gives the name a valuable association.  The fact that
some DNS names are not permanent is simply not relevant to my scope.  If
I want better scope, I use more names or pay for more resources to keep
that name around a lot longer.

> > If you want persistent names, ask a library to
> > create one
> Would the W3C and PURL count as libraries? I trust these two
> organizations to provide persistent names, but only because I know
> their backgrounds, and that if someone took away their domain names,
> there would be an uproar.

PURL is maintained by the OCLC, which is essentially a library (albeit
an experimental one).  W3C is not.  However, I am quite certain that if
you give MIT a million dollar trust fund, they will give you an mit.edu
URL that will be permanent.

> > [...] the syntax only determines how often the name will be
> > used.  Making it a URN only reduces the name's scope of use.
> Surely the syntax that one uses for a URI scheme is independent of the
> scope of its implementation? I mean that just because I use a URN for
> a new scheme, that doesn't preclude me from building a successful
> range of tools that use the new scheme, does it?

No, but no one can use that scheme until you do build those tools and deploy
them to the extent that the new name is more significant than some other
name.  Nobody has done that for URN without an additional layer of
indirection --- a resolution service that interprets the URN --- which
cannot avoid the problem of domain names changing any better than a
DNS-based naming authority.  Therefore it doesn't get deployed, and no
association exists between URNs and things people want to name, and thus
it is not useful as a naming system.

ISBN names are an excellent example.  They are not legitimate URNs, since
publishers are allowed to reuse them for multiple books (once the previous
ones are out of print).  And yet they are useful, so useful in fact that
folks commonly try to claim that <urn:isbn:0-201-56317-7> is somewhow
magically better than <isbn:0-201-56317-7>.  What makes an ISBN useful
is that it is used on the back of every book, not the fact that you can
squeeze it into a URL or URN syntax.

> them needing to use a URL for a name. I don't know of any particular
> reason why an XML namespace cannot be a URN, and the only reason for
> it not being a URL is that it is more difficult to find URLs with a
> good quality of service.

As opposed to finding a URN with any quality of service?

Received on Friday, 3 August 2001 23:48:30 GMT

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