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

From: Alan Ruttenberg <alanruttenberg@gmail.com>
Date: Tue, 26 May 2009 02:39:53 -0400
Message-ID: <29af5e2d0905252339t5e1c60a2n1ae8ef227f744c7@mail.gmail.com>
To: Jonathan Rees <jar@creativecommons.org>
Cc: AWWSW TF <public-awwsw@w3.org>
On Wed, May 13, 2009 at 10:34 AM, 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.
>

Every wa-representation is of some G at some point P, yah?
Just trying the get the all-somes on the classes clear.

> 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.)

I'll just put in a note here that these are going to turn out to not
be orthogonal dimensions as requests includes time, and the below
language and (requested) content type are also parts of requests.

> 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).

Another check in: that a wa-representation can belong to the trace of
more than one GR is a consequence of that fact that a
wa-representation of a more "specific" resource is also a
wa-representation of a more "generic" resource? I mention this because
if this is the reason it implies more structure than you've elaborated
in the above. If there's another case then it's worth noting.

Note that there isn't clarity as to whether you are talking at the
class or individual level. This is important because if we are to
think of "generic resources" as classes of resources, then there is
only one individual with many classes, and my speculation about why a
wa-representation can be shared isn't supported..

Also, are we to take it significant that Tim's trace seems to be
defined in terms of the response, whereas Dave's in terms of the
request?

> 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?

Assuming my speculation about the reasoning behind wa-representations
is wrong, and the term "generic resource" is instead to be taken to
name classes, then rdf:Resource is your "top" GR.

> 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.

Careful. You are giving two scenarios. One in which they know what G
is (e.g. because they meet and point at it) and another in which they
only know the trace. The latter doesn't imply the former unless you
assert that a trace is unique to a resource. Which you don't/

> 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.

Assumes that the only way of knowing is by exchange of information.
However is it possible that they know they are talking about the same
GR because they can each observe it and know the other is observing
the same thing, but can not convey "information" that makes the
distinction.

> 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....

What you say above is asserted to be true only of Information
Resources, if I follow the argument, since it is the n&s condition for
being an information resource. So at this point I am assuming that we
are only talking about IRs. Is it obvious that generic resources are
information resources? (might be but I don't believe it's stated in
the docs)

> 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,
*and* ?
>  the RFC2616-entity in the 200 response is a wa-representation of G,
>  then S is consistent with G at U.

S is a ___ ?

Server? Whatever it is, it can "receive" and it can "respond".
I'm not sure that "consistent" is adding anything here. By the
architecture and what I've heard if the server responds 200 then the
entity *is* the representation, so the antecedent is a tautology.

Obviously, from this point on, given that I think you've just written
true -> x I'm probably not understanding things so anything I say
after this is even more suspect than what I've written before.

> 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.

Well, this suggests an actual link between the representation and the
resource, (sort of a kr-representing kind of thing) that hasn't been
part of your account as of yet. So I don't know how you form any
hypothesis. On what basis?

> 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.

Can't understand "consistent", so I'm lost.

>
> (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.)

What kind of representations of U? What is a criterion for such a
representation. Seems you've moved to kr-representation at this point.

Hope this helps. At least it's a response ;-)

-Alan

>
>
> [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 06:41:04 GMT

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