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

From: Jos de Bruijn <debruijn@inf.unibz.it>
Date: Tue, 17 Feb 2009 12:21:31 +0100
Message-ID: <499A9DBB.5080705@inf.unibz.it>
To: Axel Polleres <axel.polleres@deri.org>
CC: Dave Reynolds <der@hplb.hpl.hp.com>, RIF WG <public-rif-wg@w3.org>
I tried to implement your wishes.  Please check whether the current text
is okay for you:


Axel Polleres wrote:
> Dave Reynolds wrote:
>> 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)
> +1
>> - 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)
> we are actually developing tools which try to be tolerant in this
> respect, i.e. issue a warning only, but still proceed in case of e.g. a
> 404 for some RDF import. So, indeed I would be either in favor of
> leaving this to the implementation or suggesting to treat such graph as
> emptyt and getting back a warning rather than prescribe to terminate
> with an error.
>>> 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.
> interesting aspect indeed.
>> 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.
>> Dave
>> 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

Jos de Bruijn            debruijn@inf.unibz.it
+390471016224         http://www.debruijn.net/
No one who cannot rejoice in the discovery of
his own mistakes deserves to be called a
  - Donald Foster

Received on Tuesday, 17 February 2009 11:21:30 UTC

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