Re: lack of support for claims regarding Concise Bounded Descriptions (see MSGs)

From: Giovanni Tummarello <giovanni@wup.it>
Subject: Re: lack of support for claims regarding Concise Bounded Descriptions (see MSGs)
Date: Mon, 06 Jun 2005 01:14:53 +0200

> 
> >I'm looking at [1], and I am puzzled by the definition of an MSG.
> >
> >The MSG of a statement that has a blank node as a subject or object is
> >defined in terms of the MSGs of all statements that use that blank node as
> >a subject or object.  This seems to me to be a self-referential definition.
> >
> >  
> >
> isnt it a standard declarative definition?  

Nope.  

> by using the word "set" of 
> triple duplicates are removed so you dont get in a loop really, if 
> that's what you meant. 

This doesn't work.

> We might add "statements not included so far" 
> somewhere if that makes it more clear or more correct. We'll look into it.

I don't know if this would solve the issue.    It depends on your wording.


Consider the graph
	 ex:a ex:b _:a .
	 _:a ex:c ex:d .
The MSG of the first triple includes the MSG of the second
triple, which includes the MSG of the first triple.  So if something is in
the MSG then it is in, but just what is in?

> It might be easier to understand in other terms however: an MSG of a 
> statement is the "blank node closure graph" including that statement.
> 
> >>CBDs are the union of all the Minimum Self contained Graphs, "involving" 
> >>a starting URI. 
> >>    
> >>
> >
> >I don't believe that this is the case.  Consider the RDF graph
> >
> >	ex:a ex:r ex:c .
> >
> >The CBD of ex:c for this graph is the empty graph, but the MSG of ex:c is
> >the entire graph.

> Patric has several definitions of CBD.. some are simmetric wrt subject 
> and object some are not.

Yes, but the CBD is asymmetric.  The "symmetric" one is Symmetric CBD.  In
any case I do not believe that the Symmetric CBD of a node is the union of
the MSGs of statements involving the node.

> Our is simmetric as there is, clearly, no reason for using an asymmetric 
> one given that there is no reason why ontologies should use active 
> rather than passive terms.
> So the union of the msgs involving a node should be identical to patric 
> symmetric cbd .. it might be different ifhe also adds reificatiions of 
> ground triples which are not the original one should check that but it 
> could be reconcilied (if he changes his definition ;-)  )


Yes, but the CBD is asymmetric.  The "symmetric" one is Symmetric CBD.  In
any case I do not believe that the Symmetric CBD of a node is the union of
the MSGs of statements involving the node.

> >>rebuild it correctly, you cant do it in any fine granularity or in any 
> >>coarser without duplicating transfers of information (or loosing some). 
> >
> >You need to have a particular model of information flow for this to work
> >out right, though.  
> >
> well given that a statement belongs to 1 and only 1 msg and that you acn 
> extract i the model is straightforward, you extract it, you send it (its 
> a perfectly valid self contained RDF) and ot ther other end it can be 
> marged right away. Iterate and you have transferred the whole graph 
> without ever sending statements twice and with minimum bits.
> In our P2P system we actually transfer "RDFN" which is similar to CBD 
> basically a peer asks another "what do you know about URI x" and this 
> will provide all the MSGs involving X. This causes some duplicate 
> triples sometimes. Say you first ask a peer about the song "like a 
> virgin" , well you'll get a msg which says that madonna sang it.. then 
> you might ask about madonna and you get the same MSG back.  But that's 
> really not a bug, its a normal behaviour, its clear that it makes ense 
> to have that MSG since it does in fact involve the 2 uris. This makeit 
> that peers dont all have to know the same thing, one might be interested 
> justin madonna another just in that song .. the'll have some overlapping 
> information but not all of course.

Suppose that you could just transfer the statements one at a time, with
bnode identities retained.  This would allow transmittal of the entire
graph.

> >>MSgs are 
> >>guaranteed not to interfere with other MSGs so they can be safely 
> >>inserted and removed, MSGs can be uniquely named leading to a few 
> >>intersting properties (i.e. we're working on efficent syncinc of 
> >>RDFGraphs, results due soon).
> >
> >I don't understand what "interfere" means here.
> >
> Inserting an MSG into a triplestore will not alter any other existing 
> MSG. this is fairly obvious from the definition, but really nice if you 
> think of it. The key point is that bnodes are not addressable from the 
> outside and we're considering just the case where an "update" is a piece 
> of rdf coming from the outside ..in that case you will not alter the 
> content or structure of exisitng MSGs in your DB. which in our case is 
> what guarantees that digital signatures will be shipped along with the 
> data they refer to and will not break no matter wher ethey are inserted 
> into (which triplestore) and which other information is added at a later 
> time.

OK, sure.

> On the other hand, interesting behavours where people change the 
> structure of MSGs remotely are possible .. for example in DBin a local 
> client might retract or modify an msg from its DB if it receives another 
> MSG that says so by pointing at the one to retract in the terms of the 
> hash of its the canonical form. This way a "group administrator" or the 
> original author of the msg can state "I say please remove that one and 
> put this one instead" remotly , and , since the hash is really a digital 
> signature, in a way that can be trusted.
> 
> Sorry if this gets a bit involved in what seems to be out application 
> specific behaviours, it is however the case that these are properties 
> spawning from RDF (basically from the definitions of bnodes) which  
> gives them more applicability that just DBin.. for example  see the 
> "revokation without quotation" issue.
> 
> Giovanni


peter

Received on Monday, 6 June 2005 03:13:06 UTC