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Re: Last minute input to discussion re 'on the boundaries of content negotiation in the context of the Web of Data'

From: Richard Cyganiak <richard@cyganiak.de>
Date: Thu, 19 Feb 2009 15:53:18 +0000
Cc: "www-tag@w3.org" <www-tag@w3.org>, <timbl@w3.org>
Message-Id: <E0FE6795-CA85-4C92-9D5D-8D0E8218C65C@cyganiak.de>
To: Michael Hausenblas <michael.hausenblas@deri.org>

The original question you posed when starting the thread was this: If  
a1 and a2 are representations of resource a, then can we say anything  
about how “equivalent” they should be?

This question is unrelated to httpRange-14, only minimally related to  
RDFa, and only somewhat related to fragment ID semantics. Your  
question can be considered independently of all those other issues,  
and I think it would be a disservice to the TAG to broaden the  
question to include all those other areas at once.

Your axioms are concerned with the semantics of fragment IDs, that is,  
what does a URI of the form <xyz#foo> identify? I think this questions  
is adequately answered by RFC 3986, RFC 2616, and AWWW. It's well- 
trodden ground and I didn't see anyone articulate a reason for  
revisiting this part of the architecture. (You might well have such a  
reason (media fragments?), but don't make us speculate.)

Your axioms do not say anything new about

* httpRange-14 (which is concerned only with URIs of the form <xyz>,  
not <xyz#foo>),

* RDFa (which requires an update of the HTML/XHTML MIME type  
declarations, but no changes or clarifications of web architecture),

* the representation-equivalence question (which remains the same very  
thorny a question even if none of the representations define any  
fragment IDs).

Hence I really don't know what you are trying to achieve with your  
proposal :-(


On 19 Feb 2009, at 10:26, Michael Hausenblas wrote:

> Dear TAG members, Tim, Richard,
> Short version: an attempt to defined non-information resources  
> without using
> non-information resource ;)
> Long version:
> In order to contribute in a constructive way to the discussion around
> information resources, non-information resources, CN and 200/303  
> issues, I'd
> like to propose the following, which may be taken as an input to my
> previously raised issue around the boundaries of CN in the Web of  
> Data use
> case. To additionally address emerging usage of RDFa, this area has  
> been
> taken into account as well.
> Please note that this is *not* a philosophical approach and I try to  
> ground
> all my terms and only use *existing* protocols, definitions, etc.:
> ===
> Axiom 1) A URI containing a fragment identifier as of RFC3986 [1]  
> identifies
> a 'thing', that is, a resource which essential characteristics are not
> conveyed in a message as opposed to an information resource as of  
> AWWW1 [2],
> unless otherwise stated.
> Axiom 2) Iff the media type of a representation obtained by  
> dereferencing a
> URI (that is, performing an HTTP GET on the URI) defines the  
> semantics of
> the fragment identifier, the resource is an information resource as  
> of [2];
> this is the case 3 in [3].
> Axiom 3) Iff the authoritative party as defined in section '  
> ownership' of [4], that is the one who can claim URI ownership,  
> explicitly
> states that fragment identifier semantics throughout different
> representations are sufficiently consistent, the resource is an  
> information
> resource as of [2]; this is the case 1 in [3].
> Axiom 4) An authoritative party can explicitly state fragment  
> identifier
> semantics consistency by using POWDER's describedby property as of  
> [5] along
> with HTTP Link: header as of [6] or by embedding RDF as of [7].
> ===
> Please note the following: The intention is to keep the current  
> definition
> of IR as of the AWWW1 and clearly define what else is possible,  
> hence to
> make related (sometimes questioned and IMHO underspecified  
> definitions such
> as for example found in httpRange-14) more usable in a practical  
> context.
> The axiom 1 actually asks people to use frag-ID-URIs as the (one and  
> only)
> default to identify 'things', however, axiom 2 - 4 allow to create
> exceptions based on an explicit set of actions. Further, axiom  allows
> (currently) two ways to 'announce' exceptions, on the HTTP layer or  
> on the
> representation layer - this is open to discussion and should/will be
> extended.
> @Tim: Do you think this helps in getting closer to a written  
> explanation of
> your often articulated thoughts re 'sameness' of information  
> obtained from
> resources, as e.g. in [8]? And also: does this solve our issue wit  
> RDFa as
> discussed in [9]?
> @Richard: Are the axioms consistent with the outcome of your  
> analysis in
> [10]?
> Cheers,
>      Michael
> [1] http://tools.ietf.org/html/rfc3986#section-3.5
> [2] http://www.w3.org/TR/webarch/#def-information-resource
> [3] http://www.w3.org/TR/webarch/#frag-coneg
> [4] http://www.w3.org/TR/webarch/#URI-collision
> [5] http://www.w3.org/2007/05/powder-s#describedby
> [6] http://tools.ietf.org/html/draft-nottingham-http-link-header-03
> [7] http://www.w3.org/2001/tag/doc/selfDescribingDocuments#UsingRDFa
> [8] http://chatlogs.planetrdf.com/swig/2009-02-09#T15-09-20
> [9] http://esw.w3.org/topic/RDFa_vs_RDFXML
> [10] http://lists.w3.org/Archives/Public/semantic-web/2007Dec/ 
> 0157.html
> -- 
> Dr. Michael Hausenblas
> DERI - Digital Enterprise Research Institute
> National University of Ireland, Lower Dangan,
> Galway, Ireland, Europe
> Tel. +353 91 495730
> http://sw-app.org/about.html
> http://webofdata.wordpress.com/
Received on Thursday, 19 February 2009 15:54:01 UTC

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