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Re: LC Comment on Entailment Regimes: Illegal Handling, specifically OWL 2 Direct Regime

From: Birte Glimm <birte.glimm@comlab.ox.ac.uk>
Date: Thu, 10 Nov 2011 10:15:27 +0100
Message-ID: <CABt65OcxAMb2haU=TEQKDPmG6admg3ny06DTe32GwweTObQBiw@mail.gmail.com>
To: Michael Schneider <schneid@fzi.de>
Cc: public-rdf-dawg-comments@w3.org

Thank you for your comment about the SPARQL Entailment Regimes document.

The error handling for SPARQL with basic graph pattern matching
(simple entailment) is defined in the protocol document. It was
decided that instead of extending the protocol document for the case
of systems that use entailment that the entailment regimes document
itself specifies and extends the error handling where necessary. Since
the protocol specification (already in version 1.0 of SPARQL) uses
MUST for the case of malformed queries and graphs, the WG has decided
not to change the MUST into a SHOULD in the entailment regimes
specification in order to be consistent with SPARQL with basic graph
pattern matching.

Regarding your comment about the Direct Semantics extra condition,
which allows for dropping triples that violate OWL 2 DL restrictions,
the WG has decided to remove the part in question. The relevant text
now says: If the queried ontology is not an OWL 2 DL ontology or the
query is not legal for the ontology, the system MAY refuse the query
and raise a QueryRequestRefused error.

We would be grateful if you would acknowledge that your comment has
been answered by sending a reply to this mailing list.

Birte, on behalf of the SPARQL-WG

On 26 July 2011 14:34, Michael Schneider <schneid@fzi.de> wrote:
> Dear all!
> Document: SPARQL 1.1 Entailment Regimes
> State: LCWD
> The definition tables for the entailment regimes have an entry "Illegal
> Handling". It is generally defined that a system must signal an error, if
> either the queried graph or the query is not a syntactically valid RDF graph
> or BGP. In the case of the "OWL 2 Direct Semantics Entailment Regime" (Chap.
> 6), there is additional treatment for the special case that the graph or the
> query is not an OWL DL ontology:
>    If the queried ontology is not an OWL 2 DL
>    ontology or the query is not legal for the
>    ontology, the system MAY refuse the query
>    and raise a QueryRequestRefused error or the
>    system MAY use only a subset of the triples
>    in the ontology or query.
> In general, I would be very cautious about specifying required ("MUST")
> behavior of a system in the case of syntactically invalid input. This may be
> too much of a requirement for an implementer to provide a compliant system,
> and might significantly affect the performance of a system due to additional
> checks. So, I suggest to reconsider your decision of how to handle
> syntactically invalid input: maybe a "SHOULD" is sufficient, or maybe
> nothing at all should be said. The latter would have the advantage that
> future extensions, such as support for generalized RDF (e.g. bnodes in
> predicate position) or named graphs beyond what SPARQL allows today would be
> possible without breaking compliance to SPARQL 1.1.
> In any case, I strongly propose to /not/ make any concrete suggestions of
> the form: "on invalid OWL DL input, a system may only use a subset of the
> triples in the query". Not even with a "MAY". This makes a wrong behavior
> with unexpected results on wrong input into a kind of "best practice
> recommendation" to implementors and leads to certain expectations of users.
> If a system works outside its specification, there should be either a strong
> requirement to not process the input (as currently for illegal RDF data), or
> nothing at all should be said, so the system behavior is strictly
> implementation dependent. It is then up to the implementor to do what he
> considers best (and potentially to describe the behavior in the system's
> manual). Again, system implementers may decide to support more than plain
> OWL 2 DL (e.g. by relaxing on the global syntactic constraints to retain
> decidability), and there should not be a general expectation that the system
> will use only parts of the input query in such cases.
> Best regards,
> Michael
> --
> Dipl.-Inform. Michael Schneider
> Research Scientist, Information Process Engineering (IPE)
> Tel  : +49-721-9654-726
> Fax  : +49-721-9654-727
> Email: michael.schneider@fzi.de
> WWW  : http://www.fzi.de/michael.schneider
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> Haid-und-Neu-Str. 10-14, D-76131 Karlsruhe
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> Studer
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> ==============================================================================

Dr. Birte Glimm, Room 309
Department of Computer Science
University of Oxford
Parks Road
United Kingdom
+44 (0)1865 283520
Received on Thursday, 10 November 2011 09:16:21 UTC

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