W3C home > Mailing lists > Public > www-validator@w3.org > August 2004

Re: suggest validator prefer URI to FPI

From: Nick Kew <nick@webthing.com>
Date: Tue, 17 Aug 2004 15:02:41 +0100 (BST)
To: Dominique Haza√ęl-Massieux <dom@w3.org>
Cc: www-validator@w3.org
Message-ID: <Pine.LNX.4.53.0408171446030.2690@hugin.webthing.com>

On Tue, 17 Aug 2004, Dominique [ISO-8859-1] HazaŽl-Massieux wrote:

> Le mar 17/08/2004 ŗ 13:42, Nick Kew a ťcrit :
> > There are also serious drawbacks to that.  URIs are used by W3C for
> > two different and mutually-incompatible purposes:
> >
> >  (1) As addresses that become meaningful only when dereferenced
> >      (e.g. HTTP).
> >  (2) As unique identifiers that are NOT dereferenced (e.g. RDF).
>
> Hmm... I don't see how they are mutually incompatible; a URI is an

They are not intrinsically incompatible.  But they cannot be treated
as interchangeable.

> identifier; depending on the URI scheme, the said identifier may or may
> not be dereferenceable; in some URI scheme, there is an authoritative
> representation of the URI that can be obtained following a well-defined
> protocol. For instance, http: URIs can be obtained through the HTTP
> protocol, which also defines a caching mechanism.

But the HTTP protocol is perfectly clear that content may be dynamic.
As soon as you dereference a URI, you lose the guarantee of uniqueness
that is fundamental to the vocabulary/RDF usage.

> > This leads to a lot of confusion: take for example Annotea, which
> > treats URLs as unique (the RDF sense) yet requires them to be
> > dereferenced (the HTTP sense), and thus fails spectacularly to deal
> > with dynamic, negotiated or updated contents.
>
> Hmm... We're drifting a long way off the initial discussion :) To reply
> shortly, Annotea is indeed better used on stable resources rather than
> changing ones - but stable resources doesn't mean static; also, I think
> Annotea now deals well with content negotiation, using the
> Content-Location header as it should. But I guess this should be rather
> discussed on www-annotations :)

Content-Location doesn't make the dereferenced content unique or
invariant.  Content-MD5 - or full HTTP support - can verify or deny it,
but niether of those is the simple URI-vocabulary.

The bottom line is confusion.  Annotea is fundamentally broken because
it assumes invariance (and XML well-formedness - but that's another story)
of dereferenced content.  I submit that this is entirely relevant to the
current discussion, because it demonstrates what's wrong with W3C's usage
of URI.

Not that either usage of URI is intrinsically wrong, or even problematic.
Not even that both usages can't in principle work together.
But that *in practice*, they are often confused in the W3C's work,
because each usage has its own 'community', and not everyone appreciates
the difference.  There are ivory towers - particularly in semweb-land -
constructed on this confusion.  And too much historical baggage to
fix it as W3C would like.

-- 
Nick Kew

Nick's manifesto: http://www.htmlhelp.com/~nick/
Received on Tuesday, 17 August 2004 14:03:14 UTC

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