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Subject: More on $has predicate (was RE: [xdi] Agenda: XDI TC Telecon Thursday 1-2PM PT 2009-01-15)
I had a very productive 1-on-1 call with Nick
Nicholas after the XRI TC telecon today, where we went into the metagraph model
diagrams even more deeply. Several key conclusions came out of it. Nick is
doing his own writeup, which may have even more detail, but in this email I
wanted to highlight two key conclusions that directly address the questions
Bill raised this morning in his message below. Also, I posted a few minor edits
in a V2 of the metagraph/graph diagrams at: http://www.oasis-open.org/committees/download.php/30719/xdi-rdf-graph-model-diagrams-v2.pdf
Note: I’m noticing how much we need
to refer to the NON-metagraph, but talking about a non-metagraph hurts my head,
I propose the term we use to talk about “the XDI RDF graph that does not
include any metagraph statements” is “base graph”, simply
because something can only be meta if it has a base to work against. Please post
if you prefer another term. ************* The first conclusion is that the V1
diagrams are, in fact, correct. In other words, the metagraph statement
+x/$has/+y does in fact translate into two base graph statements: +x/+y/+x+y +x+y Both of them are reflected in the diagram: This is a different conclusion than I gave
in my first response to Bill earlier today when I said that +x/$has/+y was a
way of asserting that the target of the outgoing arc +y originating from +x was
a blank node. Saying the object is in fact +x+y is a better solution for two
reasons: 1)
There is no need to invoke the concept of a blank node (and all its baggage). 2)
There is no need to specify that a $has statement means there can be only one
arc +y originating from +x. Instead what +x/$has/+y describes is that: There
is an exactly one arc +y originating from +x terminating in the object
identified as +x+y. This is consistent with: a)
RDF, because there is exactly one RDF statement with the same subject,
predicate, and object (multiples of that same statement are ignored). b)
Our use of $has in the metagraph to define XDI addresses in the base graph: $has
identifies the singleton arcs in the base graph that serve this addressing role.
No more, no less. Well, actually, a little more. There is one other statement that can be derived
from a $has statement – but it really is just a logical conclusion that fundamentally
falls out of the logical structure of the metagraph itself. It is the following
inference: Every
metagraph statement +x/$has/+y infers the statement +x+y/$is$a/+y. Kudos to Nick for recognizing this inference.
The funny thing is that it’s plain as the nose on your face if you study
the metagraph/base graph diagrams. Diagram from section 4A/4B of the PDF, which
is the diagram of the metagraph statements +x/$a/+y and +y/$is$a/+x, shows: Because both $a and $is$a statements are assertions
about an object having an incoming arc, it follows that any metagraph statement
that specifies an outgoing arc terminating in an object is also a statement
that the object has an incoming arc. Thus all
four of the following statements infer the other three: +x/$has/+y +y/$is$has/+x +x+y/$is$a/+y +y/$a/+x+y This jibes with the English semantics if
we substitute real words for x and y: +dog/$has/+collar +collar/$is$has/+dog +dog+collar/$is$a/+collar +collar/$a/+dog+collar So the English phrase “dog collar”
only means that the identified object is-a collar, not that it is-a dog. However
it is not just any collar, it
is-a collar in the context of being something a dog has. This finally addresses the question Bill
raised in his message below In the RDF he provided, his assumption was that for
the metagraph statement +x+y, the RDF statements were: 1)
+x+y is a +x 2)
+x has the property +y. What Nick and I concluded is that the RDF
statements are: 1)
+x+y is a +y 2)
+x has the property +y whose object is +x+y Bill, we also agreed that since you have
more RDF experience than we, it may be easier for you to express the RDF then
we can. Hope this helps. =Drummond From: Barnhill,
William [USA] [mailto:barnhill_william@bah.com] Hi Drummond, I'm unclear on the relationship between metagraph and graph. I had
thought the relationship was comparable to the relationship between the
T-Box and the A-Box in RDFS, OWL, etc. but your diagrams seem to be
saying otherwise. For an example the metagraph diagram for 2A (+x+y) leads to a
graph statement of +x/+y/+x+y, which I am having trouble relating to RDF.
In my mind the XDI metagraph statement +x+y is comparable to the metagraph
statements stated in the following RDF/XML notation: <owl:Class rdf:about="http://plus.xri.net/x"> <rdfs:subClassOf
rdf:resource="&owl;Thing" /> <rdfs:subClassOf> <owl:Restriction>
<owl:onProperty rdf:resource=http://plus.xri.net/y
/>
<owl:minCardinality
rdf:datatype="&xsd;nonNegativeInteger">1</owl:minCardinality> </rdfs:subClassOf> </owl:Class> Which then might lead an XDI reasoner to entail the implicit graph
statements expressed in <rdf:Description
rdf:about="http://equals.xri.net/Bill.Barnhill" > <rdf:type
rdf:resource="http://plus.xri.net/x" /> </rdfDescription> from the explicit graph statements expressed in: <rdf:Description
rdf:about="http://equals.xri.net/Bill.Barnhill" > <rdf:type rdf:resource="&owl;Thing"
/> <plus:y>!1234!234</plus:y> </rdfDescription> In XRI terms the following implicit XRI in the XDI graph
would be determined by an XDI reasoner: =Bill.Barnhill/$a/+x from the following explicit XRIs in the graph (playing fast and loose
with name spaces here): =Bill.Barnhill/$a/+Thing =Bill.Barnhill/+y/!1234!234 and a query on the graph of $$X/$a/+x will answer [{"$$X",
"=Bill.Barnhill"}] Could you explain how the above would be done with the metagraph
predicate framework described by your diagram and recent wiki update? Regards, =Bill.Barnhill From: Drummond
Reed Following is the agenda for the unofficial telecon of the XDI TC at:
Date: Thursday, 15 January 2009 USA Time: 1:00PM - 2:00PM Pacific Time (21:00-22:00 UTC)
TO ACCESS THE AUDIO CONFERENCE: Dial In Number: 571-434-5750 Conference ID: 5474
AGENDA
1) XDI ADDRESSER
We will start with a tour from Markus of his latest XDI RDF utility:
http://graceland.parityinc.net/xdi-addresser/XDIAddresser
2) SOLVING THE RUBIK'S CUBE OF THE XDI 1.0 RDF METAGRAPH MODEL
Based on insights from Monday's aborted XRI/XDI editor's telecon (which turned into a long informal XDI telecon) and Tuesday's special XRI Syntax 3.0 telecon (which also ended out as a long XDI discussion), Drummond had a key revelation about the metagraph model.
He has uploaded a PDF with a new set of diagrams illustrating the metagraph model and how it describes the graph model.
http://www.oasis-open.org/committees/download.php/30700/xdi-rdf-graph-model- diagrams-v1.pdf
He also updated the XDI RDF Graph Model wiki page with new text descriptions of each metagraph predicate and references to the diagrams.
http://wiki.oasis-open.org/xdi/XdiOne/RdfGraphModel
The main topic of the call will be to review this and discuss how it returns the metagraph back to a pure description of the graph with no other semantics and thus enables the metagraph predicates to be used with any description logic:
http://en.wikipedia.org/wiki/Description_logic
3) MULTI-PART SPEC DESIGN AND NEXT STEPS WITH THE FIRST WORKING DRAFT
Drummond had his questions answered by Mary McRae about how a multi-part specification should work. See the DITA example:
http://docs.oasis-open.org/dita/v1.1/OS/overview/overview.html
Given this model, Drummond has updated the specification names on:
http://wiki.oasis-open.org/xdi/XdiOneSpecs
If there is consensus on this, our next step is to choose a template and begin the first Working Draft.
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