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Re: homework assignment as interpreted by JAR

From: Jonathan Rees <jar@creativecommons.org>
Date: Thu, 17 Apr 2008 08:07:31 -0400
Message-Id: <07794C69-840D-4A57-91F7-71514FBCCAC0@creativecommons.org>
Cc: "public-awwsw@w3.org" <public-awwsw@w3.org>
To: "Williams, Stuart (HP Labs, Bristol)" <skw@hp.com>

On Apr 17, 2008, at 7:35 AM, Williams, Stuart (HP Labs, Bristol) wrote:
>
> Broadly yes, though I would also try to avoid attributing  
> particular action to a server.

Sorry, please strike everything I said about a server, as servers  
don't appear on the diagram at all yet. The point is that regardless  
of the choice of term ("dereferences to") that part of the diagram is  
supposed to commute: that is, the "dereferences to" arc = composition  
of denotes / examine IR at time t / is representable as.  I'd rather  
not use names that evoke the wrong things, so I'll try to come up  
with something else.
> The IR knows things that a faithful value cloud might not; for  
> example, you could have an IR that's a 2000 dpi image (or perhaps  
> even a continuous-space image), while all the values in the value  
> cloud are only 400 dpi. Conversely, the IR doesn't specify the  
> value cloud; there could be many value clouds faithful to the same  
> IR (as we agreed on the call). So neither makes a good model of the  
> other.
>  Ok.... that's interesting (multiple value clouds)... I guess that  
> I could also conceptualise in a way that allowed the value cloud to  
> contain ALL possible representations over all time, and view the  
> selective behaviour of the access protocol as eliminating most of  
> them from view.

That's fair, we can consider that. I'm not sure it's what F&T had in  
mind though. Will investigate.
> The value cloud, even though it's not an IR, is still a thing (as  
> is the network endpoint) and we might want to write about it in  
> RDF, e.g. if we were trying to reason about how well a system  
> implements the REST model. All of these things, including IR,  
> representation, information-resource, and network endpoint, are  
> abstract models of various real world phenomena. To get clarity we  
> have to figure out the relationships among these models and between  
> the models and reality.
> FWIW: I think that the value cloud is a useful why to capture  
> variant representations (meaning roughly conneg) at a moment and  
> change over time - sort of a continuous function of time whose  
> range is sets of values (values being byte-sequences and media-types).
>
> I think that there are some interesting conneg questions to do with  
> the relationship between the value cloud presented by a variant  
> resource and the value cloud of the generic resource which it is a  
> variant of. Loosely it would seem to me that:
>
>     g(t) = union(v_a(t), v_b(t),...,v_k(t),...v_n(t))
>
> where g(t) is the value cloud for the generic and v_k(t) is the  
> value cloud of variant k, allowing k to range over all variants. I  
> also suspect that v_k(t) typically has only one value available at  
> 't' - but thinking about this did cause me to think a bit about  
> nested or multidimensional conneg - and surpisingly perhaps given  
> your mention of dpi's above, I was thinking about say an image  
> available in different dpi's and different formats. Not sure that I  
> can encode dpi or #x,#y pixel constraints into an Accept header.  
> Natural language and format would be more tractable as a 2  
> dimensional conneg... though the details may be that it may be able  
> to all happens in one step.

This is consistent, it just gives a bit more structure to value  
clouds. If some constraints go along with the additional structure  
then we'll be articulating new best practices for 'web architecture'.

> The value-cloud is not an essential part of the picture. I only put  
> it there because I wanted to understand Fielding & Taylor, to make  
> sure my understanding of F&T matched that of others, and to  
> articulate an example of something that IRs are *not*.
>
> I think you're saying that a value cloud is *not* an IR in that it  
> is an artifact of the F&T formulation of resource - I think I'd be  
> inclined to regard there as being a 1:1 correspondence between VC  
> and IR (at least when viewed through a web like lens) - nope that  
> doesn't do it I suppose - i could have two identical 'document  
> controlled' robot arms whose say joint articulations are encoded on  
> a document - and they could happen to go through identical  
> sequences of motion (over all time) - hence the VCs would in fact  
> be identical, but they would each be traces of the movement of  
> distinct robots that happen (coincidentally? or just pervesely for  
> the sake of discussion) to have identical motion over time. Ok...  
> so the VC could in sense type an IR, could it also be one as a type?

I find it much easier to think of VC and IR as completely different  
things and as I said there is a constraint relating them (what I  
called "faithful") but it is not deterministic in either direction,  
as I said above. If what you say about the VC representation set  
being all *possible* representations (not what I had in mind) then  
the VC contains no more information than the IR (e.g. you can't model  
different service levels for the same IR as different VCs).
Received on Thursday, 17 April 2008 12:08:24 GMT

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