W3C home > Mailing lists > Public > www-tag@w3.org > February 2009

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

From: Xiaoshu Wang <wangxiao@musc.edu>
Date: Fri, 13 Feb 2009 12:55:08 +0000
Message-ID: <49956DAC.9010804@musc.edu>
To: "Williams, Stuart (HP Labs, Bristol)" <skw@hp.com>
CC: Michael Hausenblas <michael.hausenblas@deri.org>, "www-tag@w3.org" <www-tag@w3.org>

Williams, Stuart (HP Labs, Bristol) wrote:
> 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.
Yes, we are clear about those points.
> 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.
Here, the subtle difference arise.  To make things clear, let's use 
"png", "ttl" to refer the awww:representation and "the:png" and 
"the:ttl" to their URIs.  Then, my point is: with regard to "the:png" 
and "the:ttl", "pgn" and "ttl" are representations of different things. 
That is obviously true.  But let's assume that "the:png" and "the:ttl" 
don't exist (for instance, we don't have to provide it in the 
Content-Location when they are served under "the:house" URI).  Then, 
"png" and "ttl" are awww:representations of "the:house" -- as you said - 
a set of ephemeral set of bits and some metadata.  Please note, now, I 
am not referring to the awww:representation returned from "the:png" and 
the "the:ttl" but from "the:house". Can we say "png" and "ttl" are the 
representations of the same thing - "the:house"?

If the answer is "yes", then we don't differ in anything. If the answer 
is "no", then that is where the difference resides. 

>> 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
Of course. But in Michael's example, the conneg is conducted on the 
"http://sw-app.org/sandbox/house".  The other two URIs does not exist.  
I think Michael minted the other two URIs for the sake of easy 
discussion and implementation.  And it is here that the focus of 
discussion is diverted because (I think) he intended to use the other 
two URI to refer to the awww:representations.  But the URIs was used for 

Perhaps, I have assumed too much (about the identity crisis).  Here is 
my inference:  Given a URI, say http://www.w3.org, we can not really say 
it is a (HTML) web page.  One of its awww:representation is. I think, 
again perhaps over-assummed, the definition of IR is intended to allow 
people to say "http://www.w3.org" is a web page and httpRange-14 tries 
to enforce it be saying under which condition that you can say it.  It 
is the same that we cannot say that "the:png" (the awww:resource) is a 
PNG image unless there is an assertion to say that one of its 
representation is a bit-copy of the resource.

>> 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?
No. But they are often used example argument for the necessity of IR.
For "byte-stuff"
see: http://lists.w3.org/Archives/Public/www-tag/2009Jan/0135.html
For dc:creator
see: http://www.w3.org/2001/tag/2009/01/29-minutes.html
>> 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.
I do believe that TAG accept URI refer to resource.  But I think TAG 
would also like a portion of the resource (i.e., IR) to be the "same" as 
awww:representation.  Otherwise, I don't see the need for IR, hence my 
inability to understand  the reluctance to drop the concept.

> 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
> --
> Hewlett-Packard Limited registered Office: Cain Road, Bracknell, Berks RG12 1HN
> Registered No: 690597 England
Received on Friday, 13 February 2009 12:56:00 UTC

This archive was generated by hypermail 2.3.1 : Wednesday, 7 January 2015 15:33:00 UTC