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Re: [RDF-OWL] what if there are no graphs at import locations?

From: Axel Polleres <axel.polleres@deri.org>
Date: Fri, 13 Feb 2009 00:41:00 +0000
Message-ID: <4994C19C.1010106@deri.org>
To: Jos de Bruijn <debruijn@inf.unibz.it>
CC: RIF WG <public-rif-wg@w3.org>

I am a bit surprised to see that we refer to a "RDF graphs accessible 
from the locations u1,...,un" in RIF imports, which suggests that we 
talk about URLs here, rather than URIs.

Neither OWL, nor SPARQL, nor OWL2 do this:

OWL [2, Section 3,4]:
"The imported ontology is the one, if any, that has as name the argument 
of the imports construct. (This treatment of imports is divorced from 
Web issues. The intended use of names for OWL ontologies is to make the 
name be the location of the ontology on the Web, but this is outside of 
this formal treatment.)"

SPARQL [3, section 8]:
"A SPARQL query is executed against an RDF Dataset which represents a 
collection of graphs. An RDF Dataset comprises one graph, the default 
graph, which does not have a name, and zero or more named graphs, where 
each named graph is identified by an IRI."

OWL2 [4, Section 9]:
"Definition 3.1 (Import Closure): Let K be a collection of RDF graphs. K 
is imports closed iff for every triple in any element of K of the form x 
owl:imports u then K contains a graph that is referred to by u. The 
imports closure of a collection of RDF graphs is the smallest imports 
closed collection of RDF graphs containing the graphs."

Neither of these specs require the URI/IRI of an ontology (or for a 
named graph in the case of SPARQL) to be dereferenceable on the Web, but 
this is - IMO intentionally - left open in the specs, just mentioning 
that the URI/IRI at identifies a graph/ontology. How this identification 
is specified is not part of the specs. While accessing the URI as a URL 
from the Web might be the default behavior, there are use cases where 
this may not be desirable (e.g. in a Triple store which has several 
named graphs stored, these graphs may not be (web) accessible, but only 
be called by these "names" within the triple store.)

Likewise, I would be reluctant if we made any stronger assumptions here, 
which might be restrictive. I rather suggest to adopt something similar 
to the formulation in OWL above.

Long written, briefly summarized:
I suggest to replace
"accessible from the locations u1,...,un"
by
"referred to by u1,...,un"


This is though not directly related to your question, I see. But we 
could state e.g. something like "If there is no RDF graph (or, resp. 
ontology) referred to by uri u_i in an imports statement, the respective 
   graph SHOULD be treated as empty." (in case this is the behavior we 
want to advocate)

Axel

2. http://www.w3.org/TR/owl-semantics/direct.html
3. http://www.w3.org/TR/rdf-sparql-query/
4. http://www.w3.org/TR/2008/WD-owl2-rdf-based-semantics-20081202/
Jos de Bruijn wrote:
> In section 5.2 of the RDF-OWL document [1] we specify how RIF documents
> with two-ary import statements must be interpreted. however, we do not
> say anything about the case that an import statement refers to a
> location does not have an RDF graph.
> 
> So, if
> 
> Import(<u1> <p1>)
>   ...
> Import(<un> <pn>)
> 
> are the 2-ary import statements and one of u1,...,un does not point to
> an RDF graph, what should happen?  Do we say that the document could be
> rejected, or do we leave this unspecified?
> 
> related: what if pi denotes the OWL DL profile, but ui does not point to
> an OWL DL ontology?  Should the document be rejected?  I think so.
> 
> Best, Jos
> 
> 
> [1] http://www.w3.org/2005/rules/wiki/SWC#Interpretation_of_Profiles


-- 
Dr. Axel Polleres
Digital Enterprise Research Institute, National University of Ireland, 
Galway
email: axel.polleres@deri.org  url: http://www.polleres.net/
Received on Friday, 13 February 2009 00:41:51 GMT

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