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Subject: Re: [xdi] Proposal for separation of subject/predicate andparent-subject/child-subject semantics
Yes, that's correct. The "magic" you are referring is called
"reasoner" and should be placed at the XDI endpoint which receives the
incoming query.
As a side note, the difficulty about implementation is not to
implement the reasoner itself (there are already available products,
open source, in java) rather, to provide a formal, *consistent* model
for XDI documents and XDI query which could be described to the
reasoner in terms of Description Logic.
Talk with you all in today's phc,
Kind Regards,
Giovanni
Def. Quota "Markus Sabadello" <markus.sabadello@xdi.org>:
> Okay I understand.. And you call this behavior "logical equivalence", right?
> Whereas "resolution equivalence" is based on $is statements?
>
> So from your point of view, both "logical equivalence" and "resolution
> equivalence" would affect how an XDI endpoint works, i.e. add some "magic"
> to the plain data in a low-level XDI store/document?
>
> Markus
>
> On Thu, Apr 22, 2010 at 11:06 AM, Giovanni Bartolomeo <
> giovanni.bartolomeo@uniroma2.it> wrote:
>
>> Thank you Markus and Drummond for your replies.
>>
>> I think Markus got the point. Here our views are probably different here.
>> My answer to Markus is that this query
>>
>> =markus
>>>> $get
>>>> /
>>>> =markus+email
>>>>
>>>
>> should return
>>
>> =markus+email
>> $1
>> "markus.sabadello@gmail.com"
>>
>> This might be not an asserted statement (there is no need that a human
>> writes this in the document) but an inferred one (a reasoner produces this).
>>
>> In this, my opinion is that XDI should behave more similarly to RDF/OWL
>> rather than to XML (i.e. XDI is more an ontology language rather than a
>> simple markup language).
>>
>> As Bill pointed out that +a <=> (+a), I'm saying that there are also other
>> logical equivalences and that +a/+b <=> +a+b is one of them (and that's why
>> I see no need for $has$a) - sorry, I'm disagreeing here with you Drummond.
>>
>> But you're probably right in saying that I'm not fully understanding what
>> you're proposing ("semantically linked" and the golden triangle). So looking
>> forward to talk with you for a clarification on this.
>>
>>
>> Kind Regards,
>> Giovanni
>>
>> Def. Quota "Drummond Reed" <drummond.reed@xdi.org>:
>>
>> It's late so I will just put short replies inline.
>>>
>>> On Wed, Apr 21, 2010 at 9:11 AM, Markus Sabadello
>>> <markus.sabadello@xdi.org>wrote:
>>>
>>> So if I send the following XDI message to an endpoint:
>>>>
>>>> =markus
>>>> $add
>>>> /
>>>> =markus
>>>> +email
>>>> "markus.sabadello@gmail.com"
>>>>
>>>> and then I send this XDI message:
>>>>
>>>> =markus
>>>> $get
>>>> /
>>>> =markus+email
>>>>
>>>> What will be the XDI response?
>>>>
>>>> Markus
>>>>
>>>
>>>
>>>
>>> Markus, my answer is that the answer to the query (your second message)
>>> would be no such subject exists (in other words, the response would be
>>> null). In order to get a response to the second query, you would have to
>>> first made this operation:
>>>
>>>
>>> =markus
>>> $add
>>> /
>>> =markus+email
>>> ...
>>>
>>> where ... is whatever the contents of the subject =markus+email is.
>>>
>>> Giovanni,
>>>
>>> I understand what you are saying below, but you may not understand what I
>>> am saying. I am saying that +a/+b and +a+b are neither logically
>>> equivalent
>>> or resolution equivalent.
>>>
>>> I am only saying that they are *semantically linked* - in other words,
>>> that
>>> if you have +a/+b in the graph, it implies that +a+b would be a
>>> semantically
>>> valid XRI in the graph, and vice versa. However it does NOT imply that
>>> +a+b
>>> exists, or vice versa.
>>>
>>> The reason this is semantically consistent -- and I agree with you that of
>>> course XDI needs semantic consistency -- is because in the analysis I have
>>> been doing of the implications of what it would mean IF +a/+b and +a+b
>>> were
>>> logically equivalent, I keep finding deeper and deeper problems.
>>>
>>> I don't have time to go into in more deeply now, but I would summarize
>>> that
>>> the golden triangle model I posted yesterday reveals the crucial
>>> difference
>>> between $has, as a predicate that identifies another predicate, and $has$a
>>> as a predicate that identifies an object.
>>>
>>> Because of this, $has and $has$a, as now proposed (vs. some of the
>>> discussions of this topic over the last year that you reference below) are
>>> not logically equivalent, and thus obviously not resolution equivalent
>>> either.
>>>
>>> Let's definitely make this the subject of tomorrow's XDI TC telecon.
>>>
>>> Talk to you then,
>>>
>>> =Drummond
>>>
>>>
>>>
>>>
>>>>
>>>> On Wed, Apr 21, 2010 at 11:10 AM, Giovanni Bartolomeo <
>>>> giovanni.bartolomeo@uniroma2.it> wrote:
>>>>
>>>> Hello Markus and Drummond,
>>>>>
>>>>> thanks for your replies. I think there is however some confusion which I
>>>>> want to clarify once for all. I'm not saying that +a/+b is resolution
>>>>> equivalent to +a+b. I think we are now all agreeing on this (at least I
>>>>> think :-).
>>>>> Logical equivalence is a different concept. It means that if +a/+b is
>>>>> resolvable then also +a+b must be. Note that they DO NOT resolve to the
>>>>> SAME
>>>>> node (this is instead resolution equivalence).
>>>>> The requirement I'm talking about (if +a/+b is resolvable then also +a+b
>>>>> must be and viceversa) is needed to ensure consistency in my semantic
>>>>> model.
>>>>> For instance, having in a graph
>>>>>
>>>>> =example
>>>>> $has
>>>>> +email
>>>>>
>>>>> will allow to deal with =example+email as the set of emails which
>>>>> =example
>>>>> owns (=example+email), and consequently make assertions on them (pls
>>>>> refer
>>>>> to http://lists.oasis-open.org/archives/xdi/201004/msg00004.html for
>>>>> details on this, btw I can clarify them in next phc). This is also
>>>>> highlighted in point 1) of
>>>>> http://lists.oasis-open.org/archives/xdi/201003/msg00014.html.
>>>>>
>>>>> Note that resolution of certain statements is needed, even if not
>>>>> asserted
>>>>> in the graph, but inferred using a reasoner (again pls refer to my mail
>>>>> for
>>>>> details) in order to implement what we have discussed in
>>>>>
>>>>> http://lists.oasis-open.org/archives/xdi/201001/msg00047.html
>>>>>
>>>>>
>>>>> =bill.barnhill: We can get the graph containing statements about any
>>>>>
>>>>>> statement in the graph, and that graph will only exist if and only if
>>>>>>> the
>>>>>>> statement is in the graph. put more simply (+a/+b) is resolvable, or
>>>>>>> R(+a/+b) = True. Generalizing this we see that if R(X) = True then
>>>>>>> R((X)) =
>>>>>>> True, meaning and (X) are logically equivalent
>>>>>>> Drummond: It seems the import of what you are saying is that +a <=>
>>>>>>> (+a)
>>>>>>>
>>>>>>>
>>>>>> giovanni: resolution = your statement is true; fault = your statement
>>>>> is
>>>>>
>>>>>> false
>>>>>>>
>>>>>>>
>>>>>> =bill.barnhill: but not +a = (+a)
>>>>>
>>>>>> Drummond: Yes!
>>>>>>> Drummond: A $is statement in an XDI graph establishes what Bill calls
>>>>>>> "resolution equivalence"
>>>>>>>
>>>>>>>
>>>>>> where we used symbol <=> to address logical equivalence, and = to
>>>>> address
>>>>> resolution equivalence.
>>>>>
>>>>> Now my point is: either the TC has a different semantic model to propose
>>>>> for XDI (in this case, I would be glad to know more and discuss about
>>>>> it),
>>>>> or the TC wants to discard semantic consistency for XDI. If the latter
>>>>> is
>>>>> the case, then, frankly, I think the TC is underestimate a tremendously
>>>>> powerful feature which could instead change the whole concept of web
>>>>> applications in next few years. Please let me know your opinion about
>>>>> it.
>>>>>
>>>>> Thank you,
>>>>> Giovanni
>>>>>
>>>>> Def. Quota "Markus Sabadello" <markus.sabadello@xdi.org>:
>>>>>
>>>>>
>>>>> What I'm afraid of is that people will never understand resolution
>>>>>
>>>>>> equivalence, and that it would be difficult to implement at the storage
>>>>>> and
>>>>>> messaging levels of an XDI stack.
>>>>>>
>>>>>> Can't we just have logical equivalence without resolution equivalence?
>>>>>> On the logical level a high level application/reasoner/etc can infer
>>>>>> lots
>>>>>> of
>>>>>> stuff based on some $is, $has, $is$a, etc rules we make up, but I'm
>>>>>> really a
>>>>>> bit scared of having to implement such extra XDI "magic" just for plain
>>>>>> XDI
>>>>>> storage and messaging..
>>>>>>
>>>>>> Markus
>>>>>>
>>>>>> On Tue, Apr 20, 2010 at 5:21 PM, Giovanni Bartolomeo <
>>>>>> giovanni.bartolomeo@uniroma2.it> wrote:
>>>>>>
>>>>>> Dear Drummond, All,
>>>>>>
>>>>>>>
>>>>>>> thank you for this proposal - two main thoughts from my side:
>>>>>>>
>>>>>>> 1) it is FUNDAMENTAL that if +a/+b is resolvable then also +a+b MUST
>>>>>>> be,
>>>>>>> and viceversa. We used the symbol <==> to say this, +a/+b <==> +a+b.
>>>>>>> Obviously the stored document can only contain one of the two, e.g.
>>>>>>> the
>>>>>>> document only contains +a/+b, but +a+b MUST be resolvable as well.
>>>>>>> This is about the relationship between asserted and inferred
>>>>>>> statements
>>>>>>> which I illustrated in the second part of my email
>>>>>>> http://lists.oasis-open.org/archives/xdi/201004/msg00004.html - we
>>>>>>> didn't
>>>>>>> discuss this yet because we ran out of time last week, but it would be
>>>>>>> good
>>>>>>> to address this in next phc.
>>>>>>>
>>>>>>> 2) About the following statements (in which the symbol <==> is used in
>>>>>>> a
>>>>>>> somehow different way than in point 1)
>>>>>>>
>>>>>>>
>>>>>>> +a/$has/+b <==> +a/+b
>>>>>>>
>>>>>>> +a/+/+b <==> +a+b
>>>>>>>> +a/$/$b <==> +a$b
>>>>>>>> +a/=/=b <==> +a=b
>>>>>>>> +a/@/@b <==> +a@b
>>>>>>>> +a/*/*b <==> +a*b
>>>>>>>> +a/!/!b <==> +a!b
>>>>>>>> +a/()/(b) <==> +a(b)
>>>>>>>>
>>>>>>>>
>>>>>>>> note that the statements on the left side in their own give origin
>>>>>>> to
>>>>>>> metastatements like +a/$has/+, +a/$has/$, +a/$has/() which in turns
>>>>>>> are
>>>>>>> logically equivalent to +a+, +a$, +a() whose semantics is totally
>>>>>>> unknown to
>>>>>>> me.
>>>>>>>
>>>>>>> Kind Regards,
>>>>>>> Giovanni
>>>>>>>
>>>>>>> Def. Quota "Drummond Reed" <drummond.reed@xdi.org>:
>>>>>>>
>>>>>>>
>>>>>>> Per my action item from Thursday's telecon, following is more about
>>>>>>> my
>>>>>>>
>>>>>>> analysis and proposal for solving the "$has semantics problem" once
>>>>>>>> and for all.
>>>>>>>>
>>>>>>>> *** ANALYSIS ***
>>>>>>>> My first conclusion, on which I think we all agree, is that while
>>>>>>>> +a+b
>>>>>>>> might imply +a/+b and vice versa, that does not mean that: a) they
>>>>>>>> have the same semantics, or b) that they identify the same XDI graph.
>>>>>>>> Indeed, since one is an XDI subject and the other adds an XDI
>>>>>>>> predicate, by definition they cannot identify the same XDI graph.
>>>>>>>>
>>>>>>>> To illustrate in English, most English verbs can be made into nouns
>>>>>>>> by
>>>>>>>> putting them in their infinitive form (“to be”, “to run”, “to
>>>>>>>> shout”).
>>>>>>>> But “run” and “to run”, while clearly linked semantically, do not
>>>>>>>> have
>>>>>>>> the same meaning. They cannot be interchanged with each other.
>>>>>>>>
>>>>>>>> Thus my second conclusion is that none of the three XDI graph
>>>>>>>> examples
>>>>>>>> we were discussing on our telecon two weeks ago were in fact
>>>>>>>> equivalent. They were each different XDI graphs that had similar but
>>>>>>>> not identical semantics.
>>>>>>>>
>>>>>>>> My third conclusion is that this means that we do in fact need
>>>>>>>> clearly
>>>>>>>> distinguished ways of semantically and syntactically of expressing
>>>>>>>> that an XDI subject (e.g., +a) : 1) has a child subject (e.g., +a+b),
>>>>>>>> and 2) has a predicate (e.g., +a/+b).
>>>>>>>>
>>>>>>>> My fourth conclusion was that this is the problem we have been
>>>>>>>> struggling with for the last year: by using $has (with or without
>>>>>>>> variants like $has$a) to express both parent subject/child subject
>>>>>>>> relationships (+a+b) and subject/predicate relationships (+a/+b), we
>>>>>>>> have wrapped ourself in a classic Gordian knot.
>>>>>>>>
>>>>>>>> *** PROPOSAL ***
>>>>>>>>
>>>>>>>> Once the problem was framed this way, it became much easier (for me
>>>>>>>> anyway) to see that a clean solution would be to dedicate the $has
>>>>>>>> predicate to only one semantic – either the parent subject/child
>>>>>>>> subject relationship or the subject/predicate relationship. Of those
>>>>>>>> two options, I strongly prefer dedicating the $has verb to expressing
>>>>>>>> the subject/predicate relationship (+a/+b), since this is the simpler
>>>>>>>> and more intuitive to understand, and mirrors the semantics of the $a
>>>>>>>> predicate, which expresses predicate/object relationships.
>>>>>>>>
>>>>>>>> If $has is dedicated to that expression, then we simply need a
>>>>>>>> different predicate (or set of predicates) to express parent
>>>>>>>> subject/child subject relationships.
>>>>>>>>
>>>>>>>> For that, an obvious answer is to use XRI delimiters as predicates.
>>>>>>>> This is intuitive since it is syntactically required to have a
>>>>>>>> delimiter between XRI subsegments, and each of the six XRI delimiters
>>>>>>>> (not counting parentheses for cross-references) has its own
>>>>>>>> XRI-defined "semantics", e.g., * means reassignable identifier, !
>>>>>>>> means persistent identifiers, etc.
>>>>>>>>
>>>>>>>> So to summarize using +a and +b notation:
>>>>>>>>
>>>>>>>> +a/$has/+b <==> +a/+b
>>>>>>>> +a/+/+b <==> +a+b
>>>>>>>> +a/$/$b <==> +a$b
>>>>>>>> +a/=/=b <==> +a=b
>>>>>>>> +a/@/@b <==> +a@b
>>>>>>>> +a/*/*b <==> +a*b
>>>>>>>> +a/!/!b <==> +a!b
>>>>>>>> +a/()/(b) <==> +a(b)
>>>>>>>>
>>>>>>>> Semantically, it would still be true that +a+b/$is$a/+b (and the
>>>>>>>> inverse that +b/$a/+a+b). And while +a+b would imply that +a/+b is
>>>>>>>> valid, and vice versa, the existence of one in an XDI graph would not
>>>>>>>> imply the existence of the other.
>>>>>>>>
>>>>>>>> So, if =example wanted to do a query to determine if +a+b and +a/+b
>>>>>>>> both existed in an XDI graph, it would be:
>>>>>>>> =example
>>>>>>>> $get
>>>>>>>> /
>>>>>>>> +a
>>>>>>>> $has
>>>>>>>> +b
>>>>>>>> +
>>>>>>>> +b
>>>>>>>>
>>>>>>>> I have been trying out this proposal in sample XDI documents for the
>>>>>>>> past week and it seems to work very well. Please consider and
>>>>>>>> experiment with it and post your thoughts. We'll put it on the agenda
>>>>>>>> for next week's telecon.
>>>>>>>>
>>>>>>>> =Drummond
>>>>>>>>
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>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
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