# Re: rdfms-graph: Food for thought

From: Stephen Petschulat/CanWest/IBM <spetschu@ca.ibm.com>
Date: Tue, 17 Jul 2001 08:44:40 -0700
To: Graham Klyne <Graham.Klyne@Baltimore.com>

Message-ID: <OF8E749C29.B4CC5AE6-ON88256A8C.005155F6@mkm.can.ibm.com>
```
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
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>
>
>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
>
> 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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>
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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 11:50:07 UTC

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