- From: Stephen Petschulat/CanWest/IBM <spetschu@ca.ibm.com>
- Date: Tue, 17 Jul 2001 08:54:43 -0700
- To: "Stephen Petschulat/CanWest/IBM" <spetschu@ca.ibm.com>
- Cc: w3c-rdfcore-wg@w3.org
> "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." Correction... to avoid mixing syntax & model, the second sentence would be better written as: "The URI of the Subject is the label of the disconnected node." - steve Stephen Petschulat Stephen Petschulat/CanWest/ To: Graham Klyne <Graham.Klyne@Baltimore.com> IBM@IBMCA cc: w3c-rdfcore-wg@w3.org, w3c-rdfcore-wg-request@w3.org Sent by: Subject: Re: rdfms-graph: Food for thought w3c-rdfcore-wg-requ est@w3.org 17/07/2001 08:44 AM Please respond to Stephen Petschulat 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> >------------------------------------------------------------ > > > > ----------------------------------------------------------------------------------------------------------------- > >The information contained in this message is confidential and is intended >for the addressee(s) only. If you have received this message in error or >there are any problems please notify the originator immediately. The >unauthorized use, disclosure, copying or alteration of this message is >strictly forbidden. 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Received on Tuesday, 17 July 2001 12:00:05 UTC