Resources and representations (was RE: Subgroup to handle semantics of HTTP etc?)

Hello Xiaoshu,

> -----Original Message-----
> From: Xiaoshu Wang [mailto:wangxiao@musc.edu] 
> Sent: 23 October 2007 16:06
> To: Williams, Stuart (HP Labs, Bristol)
> Subject: Re: Subgroup to handle semantics of HTTP etc?

<snip/>

> > Well... I observe that you persist in wanting to speak of 
> > representations as resources.
> >   
> Well, I am a bit confused.  In the eye of semantic web, isn't 
> everything in the world an rdf:Resource?  Isn't 
> representation part of this world too? 

Well, you don't get them as resources for free. You can certainly speak
of them, maybe using b-nodes.

I know that you get that (information) resource access is different from
the representation returned.

That the representation can be referred to:

_:b a webarch:Representation;
    webarch:validFrom	"20071023Z17:39:00"^^xsd:dateTime ;
    webarch:hasMimeType ... ;  
    webarch:representationOf http://example.com/aResource .

would seem to qualify it as a resource.

However, I guess what I'm objecting to is that you seem to me to slip
into speaking of the representation that you obtained from a given
resource as being that resource.

> > Well is suppose that you can conceive of an inverse... and depending

> > on whether you regard a representation as a particular message (in 
> > which case it arises because of an access attempt using a URI) or as
a 
> > type for all messages that carry the specific sequence of 
> > bits/bytes... may or may not be functional, respectively.
> >   
> We don't differ here.
> >> httpRange-14 trying to force the issue.  
> >>     
> >
> > No... at the most, the TAG's httpRange-14 resolution:
> >
> > a) seeks to avoid ambiguity of reference between a thing and a 
> > description/depiction of a thing.
> >   
> That is where the problem is.  A URI denotes a thing but 
> *never* the description of that thing.  The *representation* 
> returned by the server is a description of that thing.

The way I have come to square this is that there resources which are
descriptive in nature eg. some RDF description of a thing (or things),
or some HTML description of a thing (or things). That description (or
depiction) is a separate resource from the thing described and deserving
of its own URI. You can GET representations of a descriptions (or a
depicition) but those are *not* representations of the
described/depicted thing.

Now a question arises (and we have been there, along this path - search
for Partick Sticklers dog or Dan's car) as to whether a representation
of description can also serve as a representation of the described
thing. The TAG's httpRange-14 resolution does advise *not* to do that in
the case of things such as people, places, celestial bodies, physical
artifacts... use either '#' based client side indirection or 303
protocol indirection to a resource that describes (possibly amongst
other things) the thing of interest.

 
> When people think that 
> "http://www.ihmc.us/users/phayes/PatHayes" should NOT denote 
> Pat Hayes because they have confused the returned
> *representation* as the *resource* denoted by that URI.

No... its because they have confused the descriptive document(resource)
that the representation is a representation of with the stated referent
- or at least find the state of affairs inconsistent.

> > b) leave the range of what can be referenced by an http: URI sans 
> > fragment, unconstrained.
> >   
> This becomes non-issue if we straighten out (a), yes?

Well... it becomes a non-issue for those that regard a representation of
a depiction of a thing as an adequate representation of that thing (for
whom a representation of a depiction of a dog is an adequate
representation of that dog). But there are several around for whom that
is *not* the case.

Careful assignment of URIs can be arranged so that on can speak
separately about say, a dog, and its depiction (for example they likely
have different dc:creators).

There remains the question/issue for some as to whether web based
interaction with the resource actually involve interaction with the
resource (search for Pat's missives on distant galaxies as resources -
clearly one would be waiting lengthy periods of time for reponses to
GETs and what on earth would one expect to become of a PUT or a POST).
Both the 303 and the '#' approach acknowledge the absurdity that would
otherwise arise (except in the case of carefully crafted scenarios aimed
at scoring points) if people and places and distant galaxies were 'on
the web' in the sense being concieved as networked informatio objects.
They are not 'on the web' in a networked communicative sense - but they
can be referred to from the web.


> > c) provide a sufficient mechanism for those that care about the 
> > difference to determine that an information resource has been 
> > referenced.
> >
> > [btw: b) comes at the cost of c)]
> >   
> (c) is 303? If (a) is clarified, is 303 becomes unnecessary.

c) is 303 or '#'.

> > None of this tells you what any given resource is - what a give 
> > reference actually denotes.
> >   
> Doesn't httpRange-14 implies this.  If 200, it is an 
> information resource?  This is what starts off this thread.  
> Tim, requested by Jonahan and Alan, asked if any triple can 
> be derived from HTTP response. 

Well, no... 303 doesn't tell you what particular individual the resource
is. It doesn't tell you that it's the current weather forecast in
Bristol, or a house in a surrounding village, or a person, or a dog
or... well anything at all really. At best its says you *may* find
something usefully descriptive over there (an then again you might not -
after all this is the web... and inconsistency is a fact of life).

<snip/>

> >> Why cannot. Assuming your past few email is posted to my website, 
> >> wouldn't it possibly change the state of my mind?
> >>     
> >
> > But... can you convey your "state of mind" before and after our 
> > exchange in a message? :-)
> >   
> But isn't this just a matter of implementation detail?

I don't know... is the current state of you mind expressible in a
message?

>  I can 
> convey a portion of it depending on your request.

	GET http://example.com/xiaoshou/stateOfMind 

/me awaits a response...


> When you 
> send me an email, you have, in fact, manipulated my state of 
> mind - through the mailto URI, yes? 

Yes... its had an impact - somewhat non-deterministic, and probably
without representation.

<snip/>

> 
> Regards,
> 
> Xiaoshu

Stuart
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Registered No: 690597 England

Received on Tuesday, 23 October 2007 17:28:52 UTC