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

From: Dave Reynolds <der@hplb.hpl.hp.com>
Date: Fri, 13 Feb 2009 09:22:27 +0000
Message-ID: <49953BD3.8010300@hplb.hpl.hp.com>
To: Jos de Bruijn <debruijn@inf.unibz.it>
CC: Axel Polleres <axel.polleres@deri.org>, RIF WG <public-rif-wg@w3.org>

The OWL (1) specs are not entirely internally consistent in this regard 
as you've jointly pointed out.

In current practice many applications and tools deal with ontologies 
(and datasets in the case of SPARQL) not published at their URI, while 
recommended good practice is to publish there.

My preference would be:

- use Axel's suggested phrasing of "referred to by u1,...,un"

- say that recommended good practice is to resolve this reference by 
treating the URI as a web location but that implementations MAY support 
other mechanisms (e.g. local caches)

- leave behaviour in the case where the reference cannot be resolved 
undefined (in practice any tool would report such a failure but there 
are use cases where proceeding with the subset of data you have 
available at present is preferable to a complete abort)

> 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.

Seems reasonable so long as the requirement is SHOULD and not MUST. A 
species check is moderately expensive and allowing implementers the 
possibly to omit this in some circumstances would also seem reasonable.


Jos de Bruijn wrote:
> As a clarification:
> the OWL2 document you are referring to is merely a "normal" working
> draft; it is not in last call, and I know for a fact that there have
> already been substantial (and substantive) changes in the document since
> the working draft of December.  So, this is not something we should base
> our decisions on.
> The section from the OWL semantics you are referring to is concerned
> with the abstract syntax, not with RDF documents on the web.  In fact,
> when going to the RDF world in section 5.4 [1] I read:
> "[A collection of OWL DL ontologies] O is said to be imports closed iff
> for any URI, u, in an imports directive in any ontology in O the RDF
> parsing of the document accessible on the Web at u results in T(K),
> where K is the ontology in O with name u."
> This is very much in line with what we wrote in the RDF and OWL
> compatibility document.
> all that said, I don't care too much about this issue.  But we need to
> get our facts straight when referring to other specifications.
> Best, Jos
> [1] http://www.w3.org/TR/owl-semantics/rdfs.html#5.4
> Axel Polleres wrote:
>> 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
Received on Friday, 13 February 2009 09:23:30 UTC

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