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

Re: URI etymology

From: Sean B. Palmer <sean@mysterylights.com>
Date: Mon, 11 Jun 2001 17:50:31 +0100
Message-ID: <000a01c0f297$2b07f860$b9dc93c3@z5n9x1>
To: "Pierre-Antoine CHAMPIN" <champin@bat710.univ-lyon1.fr>
Cc: <www-rdf-interest@w3.org>
> It may that the server is unreachable or to busy... how would I
> know ?? Answer: 204 code ! Which I don't think purl.org returns.

At the moment, the PURL resolver sends back a 403 temporary redirect.
That may lead to a URI-Reference, or some 204 "no content"
representation of a resource.

> > You just ask for a URL, you don't tell the server what should
> > be there.
> I don't tell ! I only guessed the "http:"prefix tells me... But you
> it, I changed my mind :)

Cool :-) Insofar as HTTP is concerned, a guess is just that - a guess.
You can't tell what is going to be returned until it is retured,
unless you can "read the tea leaves" as Jon put it. But you can't, and
you shouldn't attempt to try - that's where the problems come from.

> I would say that a URI may resolve to different *instances* of
> the same resource.

No. A request on a URL always brings back a *representation* of the
resource that it is identifying. That is the correct term. Using
"instance" makes it sound as if you are actually sending back the
resource in some light, but you're not.

> But OK, would http://www.w3.org/1999/xlink/ return nothing
> with a 2xx code, I would admit it identifies the namespace
> unambiguously [...]

No, then you should admit that there is no available representation of
that resource available as a network entity at that time.

You can't identify a "namespace" because a namespace isn't a resource
in the RDF sense of the word - it just is. A namespace in RDF is a
syntactic thing, not something that you can retrieve over HTTP; just a
string (that happens to be a URI/URI-Reference to be globally unique,
and to allow the SW to remain decentralized) that when you attach
other strings to it, comes up with URIs or URI-References that
identify terms that your processor may or may not understand to some
certain context. A namespace is not a "thing" that you need to query,
it's just a string.

What we're talking about here is using URLs to identify concepts, not
URLs to be used as namespaces (although one obviously leads to the
other). These "concepts", i.e. RDF Resources (or as TimBL calls them,
"things"), are just things that Semantic Web processors use in certain
contexts. The context part is the key part that people seem to be
forgetting here.

Kindest Regards,
Sean B. Palmer
@prefix : <http://webns.net/roughterms/> .
:Sean :hasHomepage <http://purl.org/net/sbp/> .
Received on Monday, 11 June 2001 12:53:07 UTC

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