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RE: Question on the boundaries of content negotiation in the context of the Web of Data

From: Williams, Stuart (HP Labs, Bristol) <skw@hp.com>
Date: Fri, 13 Feb 2009 10:50:12 +0000
To: "wangxiao@musc.edu" <wangxiao@musc.edu>
CC: Michael Hausenblas <michael.hausenblas@deri.org>, "www-tag@w3.org" <www-tag@w3.org>
Message-ID: <233101CD2D78D64E8C6691E90030E5C8293C5892EF@GVW1120EXC.americas.hpqcorp.net>

Hello Xiaoshu

> -----Original Message-----
> From: Xiaoshu Wang [mailto:wangxiao@musc.edu] 
> Sent: 12 February 2009 20:15
> To: Williams, Stuart (HP Labs, Bristol)
> Cc: Michael Hausenblas; www-tag@w3.org
> Subject: Re: Question on the boundaries of content 
> negotiation in the context of the Web of Data
> 
> Williams, Stuart (HP Labs, Bristol) wrote:
> > Hello Xiashou,
> >
> >
> > I'll venture just one response and note that we repeatedly 
> > go round this loop, making little if any progress other than 
> > to increase the size of the permthead...

Hmmm... guess I'm allowing myself another.... but...

<snip/>

> >> The answer, IMHO, can only be "yes". The Web is simply a medium for 
> >> communication.   If you are the owner of the resource, and if you think 
> >> it helps you to communicate with your users by grouping different 
> >> representations under the same URI, just do it as long as it works.
> >> On the other hand, if you don't think such kind of technique don't help 
> >> your cause, just don't do it. I don't think it is about *right* or 
> >> *wrong* here. It is about making the Web pragmatic. 
> >>     
> >
> > Personnally, I think that its is quite a stretch to cast an 
> > image/picture and a graph as being the same thing. I'm sure 
> > that you could carefully construct a corner-case scenario 
> > where it was harder to argue the distinction - eg. I offer 
> > one where the ttl in some suitably grounded vocabulary 
> > describe the pixel colours of each dot in the image or 
> > provided a vector representation with sufficient information 
> > to allow reasonable facsimile to be drawn - then you'd have 
> > representations that were near equivalent.
> >   
> Of course. I have never meant that image/picture and a graph are the 
> same thing.  (That is trivially false). What I intends to say is that 
> they can be the representation of the same thing.  I think this is what 
> Michael really wants to know: if it is O.K. to use content negotiation 
> to sever both a png and a ttl as the representations of the same 
> resource, assuming these two things do not have canonical URI.

Ok... but understand that you are using the word representation in a different sense to the way I have been using it. I have tended (though maybe not always) to use a Pat Hayes induced idiom of prefixing the word representation with awww:representation - to try to be clear that what I am talking about is an ephemeral set of bits and some metadata returned by the web and not the Philosophy of Language, "pol:representation" sense of the word as you are using it in the preceding para.

You are speaking of an image or a description as a pol:representation of some pol:Thing (a house in this case) and FWIW I agree both the picture *awww:resource* and the graph *awww:resource* (or if you prefer awww:resource == pol:Thing) can act as pol:representation's of the house. But of themselves, the image and the graph are different awww:resources and each have their own distinct (sets of) awww:representations. ie the graph and the image are different things and both are different things from the house.

wrt: "Michael really wants to know: if it is O.K. to use content negotiation to sever both a png and a ttl as the representations of the same resource, assuming these two things do not have canonical URI."

...only if they are awww:representations of the same thing - which in this case they are not - one is an awww:representation of picture and the other is an awww:representation of a graph - and as you say "I have never meant that image/picture and a graph are the same thing.  (That is trivially false)."  So... I think the answer to Michael's question is clear.

> The so-called identity crisis is not because there is something called 
> Information Resource but because the intention to think that a 
> representation is the same as what a URI denote.

We (you and I have) been here many times... and I believe that we (you and I) have consistently agreed that URI refer to resources and not to the emphmeral awww:representations returned in response to questions asked of the web. I don't think that there is anything I have said that can cause you to think that I am using the URI from Michael's scenario to refer to awww:representations. I have tried to be careful to speak of house and images and graphs which are the resources in this story. I have referred to representations (PNGs and Turtles), but I have tried to do so with out naming them.

So... you cast thus as an Identity Crisis - and by the way, Michael's scenario doesn't delve into information resource or not - it's about conneg. But I see no crisis here.

Michael's choice of URI (by accident or design) for the house means that we really don't have to visit the httpRange-14 realm - no-one has yet asked what (kind-of) thing http://sw-app.org/home refers to (and I don't ask that now) we have:

	http://sw-app.org/home#my refering to a house
	http://sw-app.org/home.ttl referring to an RDF graph
	http://sw-app.org/home.png referring to an image/picture

> In Michael's case, the 
> URI "http://sw-app.org/sandbox/house#my" denotes a house.  
> The png and ttl doc is a representation of the house, regardless if the 
> latter has canonical URI, such as "http://sw-app.org/sandbox/house.png" or 
> "http://sw-app.org/sandbox/house.ttl".  It is the same if the latter 
> (the ttl) file is served under the "...png" URI.

There is an image and a graph... both, distinct web resources, which are pol:representations of a house.

That does not make either of them awww:representation of said house, they are awww:representations respectively of resources that happen to be a graph and an image (that describe or depict a house).

> If "...png" URI denotes an image, it is an image, not a byte-stream 
> unless there is an explicit assertion.

Yes... have I said anything to the contrary. I don't believe so.

> The representation of 
> an image is a byte-stream.

"The awww:representation of ..." 

> A lot of often exampled so-called URI ambiguity are 
> incorrectly argued.  For instance, to say that, without IR, "a person or 
> a molecule" would have a byte-length, etc. is wrong.  It mistakes a 
> resource from its representation.

Have I made such a mistake in this dicussion? - I don't believe so.
Or are you now introducing rhethorical points making claims that no-one has made?

> It is the receiver's fault.  Another 
> example, saying w/IO, "a molecule would have a creator" is also not 
> ill-founded.  Most time, it is simply its content creator's fault.  If 
> they know what their URI should denote, they would not have made such a 
> mistake.  On the other hand, how do we know that a molecule is not 
> created by someone? 

I really can't parse this example - specifically the premise ...

> I did argued the same point over and over again.  But I really cannot 
> understand why TAG refuses to accept such a simple fact -  What a URI 
> references/denote is not the same thing as what a URI is dereferenced.  
> This is what causes the so-called identity issue.

If by the above paragraph you mean to say that you believe that the TAG (and/or individual members of the TAG) do not accept that URI refer to resources (awww:resources, pol:Things) as opposed to awww:representations (ephemeral bits/byte streams and metadata) there of... then I believe that substantially that is NOT the case.

ie. AFAICT you state that the TAG refuses to accept something that I believe a significant proportion of, if not the whole, TAG do infact believe and *is* the model presented in http://www.w3.org/TR/webarch and is specifically illustrated in the first diagram therein.

> The practical solution, I think, would not be trying to define what is 
> IR (Honestly, I don't think there can ever be).  Rather, it is to find a 
> standard way to denote "representation".   Once we know when we are 
> working with representation, and when we are working with resource 
> (i.e., by way of URI), then all things will be very clear.

I'm sorry... but by way of URI we are working with (referring to) resources (awww:resources, pol:Things). Agreed, there is no 'standard' way of referring to an awww:representation. Certainly we can mint URI (if we want to) to refer to such transcient phenomena and we can write about them in RDF or english or whatever - but they are not named by the URI that gave rise to them as awww:representation in response to an interaction on the Web - for that name is already used up to name the thing that they are an awww:representation of.

Strangely, and mayber preversely, I think that things are already pretty clear.

> Xiaoshu

Stuart
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Received on Friday, 13 February 2009 10:54:50 GMT

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