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Re: OWL S&AS comment - owl:imports

From: Jeff Heflin <heflin@cse.lehigh.edu>
Date: Fri, 20 Jun 2003 10:05:35 -0400
Message-ID: <3EF314AF.7B5A74F1@cse.lehigh.edu>
To: Dave Beckett <dave.beckett@bristol.ac.uk>
Cc: public-webont-comments@w3.org

Dear Dave,

Thank you for you comment. As the original issue owner for imports, I
have been asked to respond to you.

You say:

> Firstly it seems that owl:imports potentially will import the entire
> semantic web of OWL Ontologies.  Have you considered the security and
> denial-of-service implications of this mechanism?

It may be true that the imports closure of some documents will be very
large. However, it is not likely to be much larger than the set of
documents found by following all the URIs in each document, and in many
cases will
be smaller than that. Still the result may be so large that applications
have to give up after a certain point and like search engines, which
don't truly cover the whole Web, admit that they only provide partial
results. To make it clear that not every OWL tool need to load every
ontology, we will add the following text to "OWL Web Ontology Language
1.0 Reference":

"Note that whether or not an OWL tool must load an imported ontology
depends on the purpose of the tool. If the tool is a complete reasoner
(including complete consistency checkers) then it must load all of the
imported ontologies. Other tools, such as simple editors and incomplete
reasoners, may choose to load only some or even none of the imported
ontologies."

I assume by denial-of-service implications you are imagining a scenario
where someone creates an ontology or set of ontologies that imports a
target some large number of times, causing any application that loads
the ontology to "attack" the target. Note that an application gains no
benefit from reloading an ontology, so it is expected that efficient
applications will cache their ontologies and only load the target a
single time, severely hampering any attempt at denial-of-service via the
imports mechanism.

You also comment:

> Secondly, is not clear at what stage that this (Imports Closure
> http://www.w3.org/TR/2003/WD-owl-semantics-20030331/rdfs.html#5.3 )
> should be done.  In an example where you have some RDF/XML that will
> map to RDF triples describing an OWL ontology, what are the steps
> that you expect to happen?

There was a minor omission in 5.3 that explains how imports relates to
entailment. The last definition should say that K and Q are
imports-closed. I propose to change it to:

"Definitions: Let K and Q be imports-closed collections of RDF graphs.
Then K OWL Full entails Q whenever every OWL Full interpretation (of any
vocabulary V that includes the RDF and RDFS vocabularies and the OWL
vocabulary that satisfies all the RDF graphs in K also satisfies all the
RDF graphs in Q. K is OWL Full consistent if there is some OWL Full
interpretation that satisfies all the RDF graphs in K."

There would also be a similar change in section 5.4 for OWL DL.

Note that these changes mean that in order to compute entailment, you
must first compute the import closure (as defined earlier in Section
5.3). As I mentioned before not every tool needs to process imports. For
those that do, a straight-forward approach is to recursively load the
documents imported by each document you load and then execute any
reasoning. However, it is not the purpose of the Semantics
document to mandate a particular processing strategy.

Finally, thank you for pointing out that the issues list did not include
a link to the formal objection. It has now been updated.

Thank you again for you comments. Please respond to this mailing list to
let me know if I have adequately addressed your concerns.

Jeff Heflin

Dave Beckett wrote:
> 
>   OWL Web Ontology Language Semantics and Abstract Syntax
>   W3C Working Draft 31 March 2003
> 
>   owl:imports, as defined in 2.1 Ontologies
>   http://www.w3.org/TR/2003/WD-owl-semantics-20030331/syntax.html#2.1
> 
> A semantic import mechanism for the RDF-based languages looks
> potentially useful and in considering if and how to implement it,
> there are some problems.
> 
> Firstly it seems that owl:imports potentially will import the entire
> semantic web of OWL Ontologies.  Have you considered the security and
> denial-of-service implications of this mechanism?
> 
> Secondly, is not clear at what stage that this (Imports Closure
> http://www.w3.org/TR/2003/WD-owl-semantics-20030331/rdfs.html#5.3 )
> should be done.  In an example where you have some RDF/XML that will
> map to RDF triples describing an OWL ontology, what are the steps
> that you expect to happen?
> 
> A process point - the WebOnt issue list as of 6 May 2003
> does not record the details of the objections made when this
> syntax based daml:imports was decided:
>   http://www.w3.org/2001/sw/WebOnt/webont-issues.html#I5.6-daml:imports-as-magic-syntax
> 
> Thanks
> 
> Dave
Received on Friday, 20 June 2003 10:07:21 GMT

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