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Re: JAR's exploration of TimBL's notion of information resource

From: Harry Halpin <hhalpin@ibiblio.org>
Date: Tue, 26 May 2009 13:14:16 +0300
Message-ID: <b3be92a00905260314t1de91f3fh4cbc1b531c5d4409@mail.gmail.com>
To: Jonathan Rees <jar@creativecommons.org>
Cc: AWWSW TF <public-awwsw@w3.org>
I'm also going to make a quick note:

I think a good deal of confusion could be avoided by just ditching the
idea of conceptualizing a resource as a set of representations,
perhaps indexed by time/HTTP requests. This is the "FTTR" hypothesis
(I'm not sure if Roy would actually agree this is his hypothesis
also). It's great what work JAR has done thinking through generic
resources and their parameters of possible wa:representations.
However, trying to define a resource as just a set of
wa:representations is not a really good idea without some more steps,
steps that may undermine "defining" the URI.

Here's a simple example - "relative URIs in wa:representations".

You can have two *identical* sets of representations given in response
to a single URI. However, the URIs they link to, if they are using
relative URIs, are not identical and can change the meaning of the
document, as well as what you can access mechanically from it.

Thus, to conceptualize representations properly, you would at least
have to absolutize all relative URIs.

On Wed, May 13, 2009 at 5:34 PM, Jonathan Rees <jar@creativecommons.org> wrote:
> JAR's exploration of TimBL's notion of information resource.
>
> [The below does not constitute an endorsement of any particular theory
> of generic resources or information resources - especially not the one
> described herein.]
>
> Terminology / notation:
>  generic-resource = GR = 'information resource' in Tim's sense [1] as
>    JAR currently understands it
>  wa-representation = 'representation' in the REST or web architecture
>    (AWWW) sense (NOT in the Xiaoshu or plain-English sense that
>    permits, say, a rock, or a citizen, to be represented)
>
>  G = some generic resource
>  Z = a multidimensional parameter space
>    (e.g. time * language * content-type * user-agent)
>  P = a point in Z
>
> So far all we know formally is that there is a 3-way relation
>
>  G has wa-representation R at point P
>
> That is, for each P = tuple of parameter values, there is a set
> (possibly empty, or quite large) of wa-representations with the
> property that they are wa-representations of G for the parameters P.
>
> We can derive other relations from this one, e.g.
>
>  G has wa-representation R at time t
>
> meaning G has wa-representation R for some P with P.time = t.
>
> I define the *trace* of G to be the function mapping each point P in Z
> to the set of wa-representations (possibly empty) that G has at P.
>
> If the parameter space Z is one-dimensional consisting only of time,
> we get the formal model of Roy's thesis: a REST resource G is formally
> [modeled as] a function from time to the set of its wa-representations
> at that time.
>
> If the parameter space is Z has two dimensions (time, HTTP GET
> request), AND every set in the image of the trace has at most one
> element, then the resulting class of traces coincides with David's
> FTRR definition.  (So I would say the FTRR is the *trace* of some GR,
> not that it *models* the GR, because it might or might not depending
> on what one wants to use the model for.)  (I assume David means for an
> FTRR to be partial - you don't *have* to have a wa-representation for
> every request and time.)
>
> In Tim's theory we know that Z has at least three axes (time,
> language, content-type), maybe more (user-agent, authorization,
> Russell 2000 index).  We know that a wa-representation can belong to
> the trace of more than one GR, and that a GR can have, at one point P,
> more than one wa-representation (as would e.g. Moby Dick).
>
> As determined on the call, there is nothing that formally rules out a
> "bottom" GR that has no wa-representations (trace is everywhere
> empty), or a "top" GR that has *all* wa-representations (i.e. GR
> has wa-representation R at P for all R and P, or trace is everywhere universal).
> The latter may be useless, but not nonsensical.  In fact it may be the
> case that given an *arbitrary* trace, there is (or could be?) a GR
> with that trace.  This is not essential in what follows, but it would
> be nice to know, if it is not true (ontologically), why it isn't -
> what kinds of traces *do* not have corresponding GRs?
>
> Suppose that two parties, Alice and Bob, get together.  Between the
> two of them they somehow agree to talk about a particular
> generic-resource (such as Moby Dick generically, or perhaps the
> Penguin 2001 edition of Moby Dick, or a GR having as its sole
> wa-representation one whose content-type and content are those from
> [2] with content checksum 137aace70c30eb076407cf28bd78b884), which
> they between themselves call G1.  Suppose that they agree on what G1
> is to the extent that they can each separately distinguish
> wa-representations that are wa-representations of G1 from those that
> aren't, for any parameters P, and do so with perfect agreement - that
> is, they both have full knowledge of the trace of G1.
>
> We determined on the call that the trace isn't adequate to determine
> G1 - there may be some other generic-resource G2 with exactly the same
> trace as G1 that is still somehow different from G1.  So if Alice and
> Bob are to know that they're really talking about the same GR
> (assuming it's possible for them to know that), they will also have to
> exchange additional information.  We don't yet know what additional
> characteristics would be sufficient (essential) for determining
> sameness, and since these characteristics must be message-conveyable
> (according to AWWW), it will be very interesting to learn what they
> are....
>
> OK, now suppose that S is an HTTP server, and G is a generic-resource,
> and U is a URI.  Define "S is consistent with G at U" as follows:
>
>  if whenever S receives an HTTP GET request with request-URI U and
>    responds with a 200 response is received,
>  the RFC2616-entity in the 200 response is a wa-representation of G,
>  then S is consistent with G at U.
>
> Suppose that Alice's server A is consistent with G1 at URI UA, and
> Bob's server B is consistent with the same generic resource G1 at URI
> UB.  Then A and B are each obligated only to respond with
> wa-representations of G1; except as constrained by the HTTP protocol,
> they are *not* required to deliver any *particular* wa-representation
> of G1, or to respond in the same way to the same request.  That is, it
> is the servers A and B that choose the wa-representation, individually
> (subject to the rules of CN of course), which among the many
> wa-representations of G1 they would like to return.
>
> It is entirely possible that certain generic-resources have so few
> wa-representations that the choice is entirely determined by CN
> parameters, in which case A and B *will* deliver the same response for
> the same request.  But this would be a special case.
>
> As an interesting mathematical construction, one could for a given URI
> U the trace T_U corresponding to the GET-request/200-response events
> *actually* occurring over the network for GETs of U from the server
> responsible for U.  Obviously T_U does not determine a single GR, as
> many GRs might have T_U as its trace or as a "subtrace" of its trace
> (i.e. T_U(P) a subset of T2(P) for all P).  Any series of GETs is only
> a sample of the trace of the particular GR that is served at U.  But a
> client might
> still ask: What are some interesting generic resources that *might* be
> the GR that is served at U, according to evidence seen so far?  and
> form plausible hypotheses about the GR on which it might gamble, if
> it were the gambling type.
>
> Jumping ahead, let me take a crack at the web / semantic web
> unification, which I'm sure will be wrong: Let I be an
> (RDFS-)interpretation in the RDF semantics sense [3].  Then I is a
> "web compatible interpretation" if for every URI U, the server S
> responsible for U is consistent with I(U) at U.
>
> (You might want to require that I(U) actually *be* the resource known
> to S as U, as opposed to another one that's merely consistent with
> observation, but it's almost never possible for an observer to
> determine what that GR is.  The nice thing
> about requiring only consistency is that it is equivalent to merely
> adding triples asserting that the wa-representations observed from S
> are representations of whatever U is interpreted to be, which seems
> almost tractable.)
>
>
> [1] http://www.w3.org/DesignIssues/Generic.html
> [2] http://www.gutenberg.org/dirs/etext91/moby.zip
> [3] http://www.w3.org/TR/rdf-mt/
>
>
Received on Tuesday, 26 May 2009 10:14:56 GMT

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