Re: homework assignment as interpreted by JAR

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).