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Subject: RE: XDI RDF addressing alternatives? (was RE: Minutes: XDI TCTelecon Thursday 1-2PM PT 2009-04-02)


Giovanni,

First, thank you very much for this explanation. I now understand how you
would create a new subject based on an existing subject. In fact, let me
test my understanding by restating all this as an algorithm. What you are
proposing is:

	Every XDI RDF statement +x/+y/+z infers two additional XDI subjects:

	1) +x+y
	2) +x+y+z

The first one, +x+y, is the address of a single node in the XDI RDF graph
that is the target of exactly one outgoing arc +y whose source is the XDI
RDF subject +x. However I believe what you are saying is that this new
subject, _as a node in the graph_, represents the set of all possible
targets of an outgoing arc +x from the source node +x. 

Is that right?

The second one, +x+y+z, is also the address of a single node in the XDI RDF
graph. This one is the target of exactly one outgoing arc +z whose source is
the XDI RDF subject +x+y above. However I believe what you are saying is
that this new subject, _as a node in the graph_, represents the member +z of
the set represented by +x+y as defined above.

Is that right?

Lastly, what you are saying is that the same way it has always been true in
XDI RDF, this pattern is recursive to any depth. In other words, the
following algorithm would also be true:

	Every XDI RDF statement +x+y+z/+d/+e infers two additional XDI
subjects:

	1) +x+y+z+d
	2) +x+y+z+d+e

Once again, like all XDI RDF subjects, both of these represent a single node
in the XDI RDF graph. However the first one, +x+y+z+d, specifically
represents the set of all possible targets of an outgoing arc +d from the
source node +x+y+z.

And the second one, +x+y+z+d+e, represents the member +e of the set
represented by +x+y+z+d as defined above.

Is that right?

Lastly, although there is a clear set/member recursive pattern here (just
like the subject/predicate//subject/predicate recursive pattern to XDI
addressing as it crosses subcontexts), it actually still works even in at
"odd" intervals. In other words, the following algorithm would also be true:

	Every XDI RDF statement +x+y/+z/+d infers two additional XDI
subjects:

	1) +x+y+z
	2) +x+y+z+d

Once again, like all XDI RDF subjects, both of these represent a single node
in the XDI RDF graph. However the first one, +x+y+z, specifically represents
the member +z of the set represented by +x+y as defined above, while the
second one, +x+y+z+d, specifically represents the set of all possible
targets of an outgoing arc +d from the source node +x+y+z.

Is that right?

I want to confirm all this with you because this is a truly fascinating new
way of looking at XDI RDF addressing. Whatsmore, I believe it's completely
consistent with exactly what Bill has been saying about XDI RDF simply being
a way of expressing sets and set membership (which I find very attractive).

Bill, is that right?

=Drummond 

> -----Original Message-----
> From: Giovanni Bartolomeo [mailto:giovanni.bartolomeo@uniroma2.it]
> Sent: Thursday, April 09, 2009 2:17 AM
> To: Drummond Reed
> Cc: xdi@lists.oasis-open.org
> Subject: Re: XDI RDF addressing alternatives? (was RE: Minutes: XDI
> TCTelecon Thursday 1-2PM PT 2009-04-02)
> 
> Hello Drummond,
> 
> Thanks for your mail. A clarification to your first concern: note that
> X3 is only the serialization format it is used to define new subjects,
> and it does not replace the graph. As I wrote:
> 
> ...Using these compound identifiers, it is possible to *introduce in
> the XDI/RDF graph* five new nodes: @example.org+employee
> @example.org+ceo @example.org+employee=j.doe
> @example.org+employee=m.smith @example.org+ceo=r.walker which are not
> part of the original graph, but are useful to make entailments on the
> graph itself (metamodel)...
> 
> This means that the new nodes *are* actually new subjects in the
> graph, and they can appear as roots in the tree, when you make
> assertions about them. For example, you can have
> 
> @example.org+employee=m.smith
> 	+mail
> 		(mailto:m.smith@example.org)
> 
> thus you have two new nodes:
> 
> @example.org+employee=m.smith+mail
> @example.org+employee=m.smith+mail(mailto:m.smith@example.org)
> 
> I've thought a bit about your second concern ("addressing capability
> of the XDI RDF graph is not something that can be expressed in
> conventional RDF"). Well, I'm doing my best to investigate this and to
> try to remain inside RDF as much as possible.
> Of course if we will find that something cannot really be really
> addressed, then it will make sense to shift to another model, but,
> this should be justified and supported by evidences. Otherwise we risk
> to reinvent things that already exist (and work!), and we risk our
> work to be not accepted by the scientific community (my biggest
> concern).
> 
> Kind Regards,
> Giovanni
> 
> At 08.58 09/04/2009, Drummond Reed wrote:
> Giovanni,
> 
> Thanks for making this posting. In reading through "A Different Proposal"
> at
> the end, I am trying but have not yet been able to fully understand the
> proposal. It appears that you are proposing that for the same X3 graph
> rooted on the one XDI RDF subject, you are proposing that +x/+y addresses
> one part of that graph (rooted on the +x subject) and +x+y addresses
> another
> part of the same graph still rooted on the +x subject.
> 
> That seems to destroy the extensiblity model of the XDI RDF graph. You
> can't
> actually create a new subject. As I understand the model you have written
> up, you can't go beyond three levels, i.e., you can talk about +x, +x+y,
> and
> +x+y+z. But what do you do about +x+y+z+j+k?
> 
> The $has verb in the proposed metagraph model in
> http://wiki.oasis-open.org/xdi/XdiOne/RdfGraphModel is not limited to 2 or
> 3
> levels deep. You can extend and create new XDI RDF subjects, predicates,
> or
> objects to any depth.
> 
> The more I think about this topic, the more I believe that the addressing
> capability of the XDI RDF graph is not something that can be expressed in
> conventional RDF. It simply can't be done. It's like trying to express a
> three-dimensional space in two dimensions. We are adding the dimension of
> addressability to RDF graphs. It results in identifiers being combined
> into
> new identifiers. RDF graphs have no such notion. I can't see any way
> around
> it.
> 
> Thoughts? (We'll just have to continue the conversation in email this week
> since I can't attend the XDI TC call tomorrow.)
> 
> =Drummond
> 
> > -----Original Message-----
> > From: Giovanni Bartolomeo [mailto:giovanni.bartolomeo@uniroma2.it]
> > Sent: Wednesday, April 08, 2009 5:23 AM
> > To: xdi@lists.oasis-open.org
> > Subject: RE: [xdi] Minutes: XDI TC Telecon Thursday 1-2PM PT 2009-04-02
> >
> > Hello,
> >
> > After our call of last week, I've updated today the XDIMetamodel page:
> > http://wiki.oasis-open.org/xdi/XdiMetamodel
> >
> > Kind Regards,
> > Giovanni
> >
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> >
> >
> >
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