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Re: Final text for Basic Graph Patterns

From: Enrico Franconi <franconi@inf.unibz.it>
Date: Wed, 18 Jan 2006 11:54:52 +0100
Message-Id: <CCF9968C-FA45-4CCF-9566-C6E74BEB14F0@inf.unibz.it>
Cc: RDF Data Access Working Group <public-rdf-dawg@w3.org>, bparsia@isr.umd.edu
To: Pat Hayes <phayes@ihmc.us>

On 18 Jan 2006, at 03:44, Pat Hayes wrote:

> Given an entailment regime E, a basic graph pattern, BGP, E-matches  
> with pattern solution S on graph G with respect to a scoping graph  
> G' and a scoping set B, just when the following three conditions  
> all hold:
>
> (1)  S(G' OrderedMerge BGP) is an appropriately well-formed RDF  
> graph for E-entailment
> (2)  G E-entails S(G' OrderedMerge BGP)
> (3)  the identifiers introduced by S all occur in B.
>
> Several conditions must be met by the scoping graph and scoping  
> set. The scoping graph and scoping set must be identical for all  
> answers to a query; the scoping graph G' must be graph-equivalent  
> to G; and B must contain every term in G'.

OK, *now* you are starting to converge to us :-)

Still, as I said, I would leave G outside the above semantic  
definition, since the above is *completely* equivalent if we replace  
(2) with:
(2)  G' E-entails S(G' OrderedMerge BGP)

As I already said in <http://lists.w3.org/Archives/Public/public-rdf- 
dawg/2006JanMar/0137>, and you also said:

> Moreover, we require that any scoping graph G' must be graph- 
> equivalent to G, and that a single scoping graph must be used for  
> all answers to a single query.
>
> <<Note, this last point will need to be firmed up by text elsewhere  
> in the document.>>

the definition of G' wrt G should be stated at the beginning of any  
processing of the SPARQL server, just to be sure that G' is the same  
for any kind of processing within the server.

And also *very* important for us, B should be left free to *not*  
include any bnode name. This would cover the only current formal  
understanding of OWL-DL SPARQL, where answers do not include bnodes,  
and the syntax of the BGPs is restricted in an analogous way OWL-DL  
expressions are (i.e., what we have defined as non-high order RDF  
graphs in [1]). Note that our current text also says something about  
the necessary restrictions on BGPs:

"where the syntactic restrictions in OWL-DL or OWL-Lite should be  
reflected in suitable syntactic restrictions on the form of basic  
graph patterns"

> << The only point we are leaving open, really, is exactly how to  
> define the scoping vocabulary B for OWL. I remain concerned that  
> this may have to be allowed to contain enough vocabulary to  
> construct OWL syntax using RDF collections.>>

As you can understand, this is exactly what we don't want to leave  
open. And your current proposal still wouldn't allow for it, but I  
like the idea of having the scoping set B. So, we could have several  
meaningful kinds of Bs, which may include an arbitrary combination of  
the following sets (as I see it now):
- all URIs excluding RDF/RDFS/OWL vocabularies
- the RDF/RDFS/OWL vocabularies
- the bnodes names
- the terms in G'

For simple entailment, you have that B is just the terms in G'.
If you want told bnode simple entailment, you have in addition that G  
is equal to G'.
For standard OWL-DL entailment, B contains only all URIs excluding  
RDF/RDFS/OWL vocabularies (and a restriction on the syntax of BGPs).

> << This is Enrico's definition modified with the scoping graph,  
> which I suggest is necessary to avoid requiring that engines  
> deliver the actual bnodeIDs from the dataset in all answer  
> bindings. Without this, we are in effect defining things so that  
> told-bnodes are automatic. But in case, I think it is intuitive, as  
> the actual role of G' isn't anything to do with entailment: it is  
> semantically transparent. It is only to keep the bnodes properly in  
> line in answer sets. >>

I understand this, with the proviso that we move away the  
introduction of G' from the semantic definition.

And I *insist* that in the normative standard we have the equivalence  
to subgraph matching by deafult: this happens if G=G'. So, if you  
insist to have separate G and G', I insist that the default should be  
having G=G' (which guarantees the equivalence of the semantic  
definition with the implementations using subgraph matching). SPARQL  
may be extended (by enhancing the protocol) to allow servers to  
declare that they do not guarantee G=G'.

So, the new proposed central part of the text would be now:

"""
Definition: Scoping Set.
A scoping set B is a set of terms formed by a combination of:
     - all URIs excluding RDF/RDFS/OWL vocabularies;
     - the RDF/RDFS/OWL vocabularies;
     - the bnodes names.

Definition: Basic Graph Pattern matching.
A Basic Graph Pattern is a set of triple patterns.
Given an entailment regime E, a basic graph pattern, BGP, E-matches  
with pattern solution S on graph G with respect to a scoping set B, if:
     - S( G OrderedMerge BG P) is an appropriately well-formed RDF  
graph for E-entailment;
     - G  E-entails  S(G OrderedMerge BGP);
     - the identifiers introduced by S all occur in B.

The default normative choices in SPARQL are:
(a) E-entailment is simple entailment (as defined in [RDF-MT]);
(b) B is restricted to the terms in G.

These default choices allow for the basic operation of querying the  
"syntax" of RDF graphs, completely neglecting its semantics. In this  
way, the basic option for SPARQL is to manipulate graphs, rather than  
involving reasoning on knowledge bases; the latter may be possible by  
choosing another form of E-entailment. In fact SPARQL may be extended  
to provide a way to override the default "simple entailment" with  
"RDF entailment", "RDFS entailment" (as defined in [RDF-MT]) by  
releasing the restriction on B, or with "OWL entailment" (as defined  
in [OWL-Semantics]) where B should be restricted to include only URIs  
and the syntactic restrictions in OWL-DL or OWL-Lite should be  
reflected in suitable syntactic restrictions on the form of basic  
graph patterns.
"""

Is this of your satisfaction? :-)

cheers
--e.

[1] Jos de Bruijn, Enrico Franconi, Sergio Tessaris (2005). Logical  
Reconstruction of normative RDF. Proc. of the Workshosp on OWL  
Experiences and Directions (OWLED 2005), Galway, Ireland, November  
2005. <http://www.inf.unibz.it/~franconi/papers/owled-05.pdf>
Received on Wednesday, 18 January 2006 10:55:14 GMT

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