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Subject: Re: [xdi] Minutes: XDI TC Telecon Thursday 1-2PM PT 2010-02-04
Dear Drummond, all,
During next call I'd like to illustrate a set of slides which summarizes the assumptions I made for a semantic model for XDI and how they address the issues encountered during our work in last months.
As below reported, these slides are mainly intended as a support for writing a technical paper on semantic aspects in XDI, but hopefully parts of its content could then be used to complement the working draft.
If someone of you likes to become a coauthor, please let me know. Estimated time for the completition of the paper is two-three months since now. In any case, I will thank the TC in an acknowledgement at the end of the article.
Best Regards,
Giovanni
Def. Quota "Drummond Reed" <drummond.reed@xdi.org>:
Federal Identity, Credentialing, and Access Management TFPAP, Version 1.0.1,
September 4, 2009article<http://en.wikipedia.org/wiki/Article_%28grammar%29#Definite_article>“the”.
Following are the minutes of the unofficial telecon of the XDI TC at:
Date: Thursday, 04 February 2010 USA
Time: 1:00PM - 2:00PM Pacific Time (21:00-22:00 UTC)
ATTENDING
Giovanni Bartolomeo
Markus Sabadello
Drummond Reed
1) XDI RDF METAGRAPH MODEL & ALTERNATE METAMODELS
On last week’s telecon, Giovanni said that he would like to propose an
alternative XDI metamodel. While Drummond supports the research into this,
he has concerns about the TC pursuing more than one metamodel for the XDI
1.0 specification suite, given that with the current metamodel we are very
close to finally producing the first formal XDI 1.0 specifications, and that
several real world implementations badly need these specifications
formalized.
The discussion began with Giovanni asking if the goal was to publish the 1.0
specifications while semantic issues still remained. Drummond clarified that
he was talking about at least pushing forward to the Working Draft level of
specification, because the TC has taken so long (6 years) just to reach that
point.
There was a short discussion of the timing to move to Working Draft 01.
Drummond said that if he is going to be the editor, he can’t do it until
after the RSA Security conference the first week of March. This would give
us about a month to close issues.
Drummond clarified that he certainly does not want to build any logical
inconsistencies into XDI 1.0. Giovanni explained that his motivation for
proposing a different metamodel is to define formal semantics for the XDI
metamodel that help make sure logical inconsistencies are avoided. The
proposal he is developing are based on the same requirements as the current
metagraph model but makes some different assumptions, thus resulting in
different semantics.
Giovanni suggests that he would like to author a paper about the XDI
metamodel in order to: a) raise the visibility of the work, and b) receive
the feedback of the scientific and academic community on the metamodel and
formal semantics.
Markus observed that he has been working with implementations of the current
metagraph model for over a year now and not experienced any problems, so he
wondered how much value that proof of semantic correctness will add.
Drummond said he pictured Markus’ question as asking about two levels: a
syntactic level (“graph correctness”) and a semantic level (“logical
correctness”). Consistency/correctness at the syntactic/graph level is
necessary but not sufficient for consistency/correctness at the
semantic/logic level.
However Drummond said he believes that the metagraph model will prove to be
logically consistent and correct at both levels. So he is eager to continue
to explore any issues of semantic correctness.
He cited the next topic as an example, so the focus moved to that.
2) $HAS SEMANTICS CONTINUED
Drummond said that part of the power of the metagraph model is that when
used very precisely, it suggests novel solutions to challenges we have faced
at modeling in XDI certain common constructs in natural language. He sent an
example to the list just before the call:
http://lists.oasis-open.org/archives/xdi/201002/msg00003.html
He reviewed the message with Giovanni and Markus to go over how a precise
application of the metagraph predicate $, which identifies the current
context, can be used to express both possession and plurality of XDI RDF
subjects. Due to the coincidence that the $ global context symbol was chosen
to represent this resource, what is truly stunning is how much the resulting
XDI expression ends out reading like the same natural language expressions
in English.
POSSESSIVE
=bill/$has/$ <==> (=bill/$)
<==> =bill$
=bill$/$has/+car <==> (=bill$/+car)
<==> =bill$+car
PLURAL
+car/$has/$ <==> (+car/$)
<==> +car$
POSSESSIVE + PLURAL
=bill$/$has/+car$ <==> (=bill$/+car$)
<==> =bill$+car$
Both Markus and Giovanni laughed as hard as Drummond did when they first saw
this result.
Markus then asked how one would render an English expression like “the car’s
windshield”. Drummond pointed out that there is a significant difference
between “the car’s windshield” and “car’s windshield”. The latter identifies
a specific class of windshield. The former identifies one instance of that
class by adding the definitivearticle<http://en.wikipedia.org/wiki/Article_%28grammar%29#Indefinite_article>such
Giovanni said he has had the same issue of identifying an instance of an
object that is not a person or an organization.
Drummond said this same issue had come up in his consideration of the
English language expressions above. Ironically, the solution appears to once
again to involve the key XDI concept of context. The definitive article
“the” expresses that the subject being referenced is known in the current
context, or more precisely, “is uniquely distinguished in the current
context”. In that case, since $ represents the current context in the
current XDI RDF metagraph model, and since all XDI subjects within that
context are uniquely XRI-addressable, this means the XDI equivalent of the
English phrase “the car” would be:
$+car
The literal translation would be “the XDI RDF subject +car in the current
context”. Note that this identifies a different XDI RDF graph than +car by
itself, because +car by itself identifies the generic class of “car”, and
not any specific car.
$+car is definitive since $ represents the current context, of which there
can be only one. This means the way to express an indefinite
as “a car” would be to make it explicit that the context being
referenced could be any context. That can be done with a cross-reference,
which is always relative to the XRI that it follows. So, the XDI equivalent
of the English phrase “a car” would be:
$(+car)
3) NEXT CALL
The next call is next week at the regular time. Drummond noted that due to
work commitments leading up to RSA Conference the first week of March, he
may have to miss several calls after next week’s call.
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