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

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>
Cc: w3c-rdfcore-wg@w3.org, w3c-rdfcore-wg-request@w3.org
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

gnutella://myserver.com:6234 --foo:isConnectedTo-->

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? ;-)


- 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                            
                    16/07/2001 12:48 PM                                                                                 
                    Please respond to                                                                                   
                    Graham Klyne                                                                                        


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.


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">
>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
>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...
>- 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
> >arcs itself can be meaningful.
>That (i.e. "the lack of arcs itself can be meaningful") goes against my
>understanding of RDF.
>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. Baltimore Technologies plc will not be liable for
>special, indirect or consequential damages arising from alteration of the
>contents of this message by a third party or as a result of any virus
>passed on.
>This footnote confirms that this email message has been swept by
>Baltimore MIMEsweeper for Content Security threats, including
>computer viruses.
>This footnote confirms that this email message has been swept by
>MIMEsweeper for the presence of computer viruses.

Graham Klyne                    Baltimore Technologies
Strategic Research              Content Security Group
<Graham.Klyne@Baltimore.com>    <http://www.mimesweeper.com>
Received on Tuesday, 17 July 2001 11:50:07 UTC

This archive was generated by hypermail 2.4.0 : Friday, 17 January 2020 20:24:02 UTC