W3C home > Mailing lists > Public > w3c-rdfcore-wg@w3.org > July 2001

Re: rdfms-graph: Food for thought

From: Graham Klyne <Graham.Klyne@Baltimore.com>
Date: Tue, 17 Jul 2001 19:04:02 +0100
Message-Id: <5.0.2.1.2.20010717185852.03bb5b40@joy.songbird.com>
To: "Stephen Petschulat/CanWest/IBM" <spetschu@ca.ibm.com>
Cc: w3c-rdfcore-wg@w3.org
Steve,

I think I broadly agree with what you say.  My term "awkward" isn't meant 
to imply problematic, or even difficult.  My purpose of engagement here is 
based on:
(a) my perception that representing isolated nodes adds some complexity 
(though maybe not very much), and
(b) questioning whether there is any real purpose in adding this small 
extra complexity to RDF.

That said, Aaaron's proposal to represent isolated nodes as ( <foo> 
rdf:type rdfs:Resource ) overcomes those objections (but introduces another 
because it would make the RDF core dependent on a schema definition, viz 
rdfs:Resource).

You also say:
>"An RDF Subject that does not have any associated Properties corresponds to
>a disconnected node in a graph. The value of the about/ID attribute of this
>element is the label of the disconnected node."

With which I'd pick a nit:

My take on the current M&S is that the concept of "an RDF Subject" is 
meaningful only in the context of a property -- a "Subject" doesn't exist 
in isolation.  A resource can be any or all of Subject, Object or Property 
depending on how it is used.

(This isn't affected by your rewording in a different message.)

#g
--

At 08:44 AM 7/17/01 -0700, Stephen Petschulat/CanWest/IBM wrote:

>I don't really see this as being about the abstract syntax as much as the
>graph theoretical model. Right now RDF pays lip service to being a "graph",
>but doesn't formalize this in the model. If we do intend to lay down a
>graph theoretical foundation for RDF then this issue is fundamental. Graph
>theory makes use of disconnected nodes in graphs (ie. a graph is defined
>such that it may contain disconnected nodes) so it would seem we should
>either explicitly define what it means or have a good reason to disallow
>out it (an possibly lose out on the body of graph theory that requires a
>graph be able to have edgeless/arcless nodes). As far as being awkward to
>define, I don't think this is the case for the graph theoretical model,
>although I don't know how the logic people would deal with it. The
>definition can be as simple as:
>
>"An RDF Subject that does not have any associated Properties corresponds to
>a disconnected node in a graph. The value of the about/ID attribute of this
>element is the label of the disconnected node."
>
>Or a more formally, it might look something like:
>
>"For a given RDF graph G defined by the vertex set V and edge set E of
>ordered pairs, an RDF Subject S is considered a disconnected node of G iff
>S is an element of V and forall ordered pairs e* contained in E,  S is not
>an element of e*."
>
>We'd first need to decide what constitutes an RDF graph and how you include
>RDF statements within a single identifiable graph. We would also have to
>deal with oddball cases like:
>
><rdf:Description></rdf:Description>   or    <rdf:Description/>
>
>where they are top level elements.
>
>The example I gave is a fairly trivial one since there are other (better
>maybe, but not necessarily simpler) ways to solve the problem. However,
>there are other use cases to consider. For example, what about using RDF to
>map out the Gnutella network over time? A node (subject or object) is a
>Gnutella peer and the only predicate you're interested in is
>foo:isConnectedTo.
>
>gnutella://myserver.com:6234 --foo:isConnectedTo-->
>gnutella://yourserver.com:6234
>
>Nodes wink in and out of existence, however they are still (for a while) a
>part of the gnutella "network" and hence a part of your graph. A node that
>isn't currently connected to anyone can still be downloaded from and often
>will reconnect in a short period of time.
>
>These kinds of dynamic RDF graphs seem to need the idea of a disconnected
>node, otherwise the implementer is forced to 'invent' a property just to
>keep a Subject connected to the rest of the graph.
>
>I'm not sure how this would effect the logical model... I suppose an
>edgeless/arcless node might correspond to a simple existential statement
>("URI exists in my domain of discourse")... now is that the existence of
>the Resource, the URI as a label, the URI as a reference to the resource,
>or the string? ;-)
>
>cheers,
>
>- steve
>
>Stephen Petschulat
>
>
>
> 
>
>                     Graham 
> Klyne 
>
>                     <Graham.Klyne@Balti       To:     Stephen 
> Petschulat/CanWest/IBM@IBMCA
>                     more.com>                 cc: 
> w3c-rdfcore-wg@w3.org
>                     Sent by:                  Subject:     Re: 
> rdfms-graph: Food for thought
>                     w3c-rdfcore-wg-requ 
>
>                     est@w3.org 
>
> 
>
> 
>
>                     16/07/2001 12:48 
> PM 
>
>                     Please respond 
> to 
>
>                     Graham 
> Klyne 
>
> 
>
> 
>
>
>
>
>Steve,
>
>Your approach seems entirely reasonable to me as an application design, but
>
>I'm not sure that it's something we want to be forced to standardize, which
>
>I fear may be the result of allowing it in the abstract RDF syntax.  E.g.
>when exchanging RDF between systems (the reason for standardization), do we
>
>really want to specify that the existence of a node, without properties, is
>
>significant?  If so, we must define the significance, and that looks
>awkward to me.
>
>I suppose I'm not arguing to prohibit property-less nodes so much as saying
>
>I don't think we should try to define them as part of a standard.  IMO, the
>
>abstract syntax and associated semantics where such things appear is
>intended to unambiguously specify the intent of what we do define, not to
>prohibit (or say anything about) what we don't define.
>
>#g
>--
>
>At 09:41 AM 7/16/01 -0700, Stephen Petschulat/CanWest/IBM wrote:
>
> >The use case I have in mind is a metadata repository where new resources
> >are constantly being added, but they may be 'tagged' at a later time. In
> >this case, an empty
> >
> ><rdf:description about="urn:myscheme:some_object_id">
> ></rdf:description>
> >
> >would indicate that the resource urn:myscheme:some_object_id is a part of
> >the current domain of discourse, but nothing has been said about it. So
>the
> >resource is flagged as one that needs someone to meta-tag it. Of course,
> >there are many other ways to solve this such as meta-meta info to indicate
> >the tagging information, but this isn't reason enough to explicitly
> >disallow it...
> >
> >cheers,
> >
> >- steve
> >
> >Stephen Petschulat
> >
> >
> >
> >
> >
> >                     Graham
> > Klyne
> >
> >                     <Graham.Klyne@balt       To:     Stephen
> > Petschulat/CanWest/IBM@IBMCA
> >                     imore.com>               cc:
> > w3c-rdfcore-wg@w3.org
> >                                              Subject:     Re:
> > rdfms-graph: Food for thought
> >                     16/07/2001
> > 05:36
> >
> >                     AM
> >
> >                     Please respond
> > to
> >
> >                     Graham
> > Klyne
> >
> >
> >
> >
> >
> >
> >
> >
> >At 10:55 AM 7/13/01 -0400, Stephen Petschulat/CanWest/IBM wrote:
> > >Agreed. Note that it brings the Subject into the RDF graph... the lack
>of
> > >arcs itself can be meaningful.
> >
> >Really?
> >
> >That (i.e. "the lack of arcs itself can be meaningful") goes against my
> >understanding of RDF.
> >
> >#g
> >
> >
> >------------------------------------------------------------
> >Graham Klyne                    Baltimore Technologies
> >Strategic Research              Content Security Group
> ><Graham.Klyne@Baltimore.com>    <http://www.mimesweeper.com>
> >                                  <http://www.baltimore.com>
> >------------------------------------------------------------
> >
> >
> >
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>Graham Klyne                    Baltimore Technologies
>Strategic Research              Content Security Group
><Graham.Klyne@Baltimore.com>    <http://www.mimesweeper.com>
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------------------------------------------------------------
Graham Klyne                    Baltimore Technologies
Strategic Research              Content Security Group
<Graham.Klyne@Baltimore.com>    <http://www.mimesweeper.com>
                                 <http://www.baltimore.com>
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Received on Tuesday, 17 July 2001 14:42:21 EDT

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