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RE: Ontology locations: OntologyURI vs. xml:base and namespaces (ISSUE-21)

From: Boris Motik <boris.motik@comlab.ox.ac.uk>
Date: Wed, 21 May 2008 10:50:03 +0100
To: "'Rinke Hoekstra'" <hoekstra@uva.nl>, "'OWL Working Group WG'" <public-owl-wg@w3.org>
Message-ID: <002801c8bb28$0b33cfd0$4012a8c0@wolf>

Hello Rinke,

If I understood your message correctly, you are worried about possible mismatches between xml:base, ontologyURI and versionURI,

If this is the case, I would just like to point out that xml:base lives at a completely different level in the "food chain" than the
ontologyURI and the versionURI. That is, xml:base is a low-level XML mechanism for resolution of relative URIs and, is reflected
neither in the RDF model nor in the OWL 2 structural specification. The resolution of URIs relative to xml:base happens at the level
of the parsers processing various XML-based syntaxes. In the case of OWL/RDF-XML, this happens before you even see any triples; in
case of OWL/XML, this happens before you see the axioms.

Thus, the presence and/or the content of xml:base is completely orthogonal to location and version concerns. xml:base can be equal
to ontologyURI and/or versionURI, but it does not need to be. Please note that xml:base is not a property of an XML document: ANY
element in an XML document can have an xml:base specification, which is then used while parsing this element's children. This just
reinforces my belief that xml:base is something similar to expansion of named entities: it happens at the XML level.

Thus, if xml:base is there, a parser should process it to resolve the relative URIs in the document, and if it is not, a parser
should resolve the relative URIs against the location that the ontology was loaded from. It should be clear that the usage of
relative URIs without xml:base is dangerous; however, this is a low-level syntax issue that has nothing to do with the structural
specification. Finally, it should also be clear that ontologyURI and versionURI play no part whatsoever in the resolution of the
relative URIs.

If you feel this is necessary, we can make these issues clear in the documents rescribing the respective syntaxes (but not in the
structural specification document, which doesn't know anything about xml:base at all).



> -----Original Message-----
> From: public-owl-wg-request@w3.org [mailto:public-owl-wg-request@w3.org] On Behalf Of Rinke Hoekstra
> Sent: 19 May 2008 14:20
> To: OWL Working Group WG
> Subject: Ontology locations: OntologyURI vs. xml:base and namespaces (ISSUE-21)
> Hi All,
> (long email, sorry)
> Although the current publishing guidelines and imports sections are
> masterpieces of clarity (thanks Boris), I feel that the consequences
> of imports-by-location resolution to ISSUE-21 are still not entirely
> covered.
> If I understand correctly, the use of the OntologyURI in Boris' and
> Peter's proposal allows us to keep track of where our axioms and
> objects come from, or rather, where they 'belong'. It is, in fact,
> interpreted as a URL. The VersionURI is an additional construct that
> can be used to add an integrity check when the ontology specified by
> the OntologyURI is not physically located at that URI. Nonetheless,
> when an ontology is imported from the VersionURI, it is still to be
> interpreted *as if* it came from the OntologyURI.
> However, XML has a built-in way of managing the 'what came from
> where?' question: through the xml:base attribute. This is not
> precisely what RFC2396 [1] says, but it is a very common
> interpretation of the value of the xml:base defined on the root
> element of an XML file, cf. [2] which states that "the base URI is the
> URI used to retrieve the document entity or external entity". This is
> what OntologyURI does in import statements.
> The way in which current tools (TopBraid/Jena and Protege4/OWL API)
> deal with imports and the like is primarily through this xml:base
> attribute. In fact, the RDF/XML serialisation of both leaves the
> rdf:about attribute on the owl:Ontology element empty. Also, both give
> a warning and do some repair when the OntologyURI is not the same as
> the xml:base (or empty). Additionally, both tools specify the default
> namespace as equal to the xml:base. In conclusion: the current state
> is that ontologyuri=xml:base=namespace. I think this is quite
> sensible, as it allows all relative URIs of classes etc. to be
> relative to the Ontology URI (see my last point on imports at the end
> of this message)
> This use of xml:base interferes with the proposed 'rules' in several
> ways. The rules are publishing guidelines, i.e. they are prescriptions
> of where an ontology creator SHOULD publish his/her ontology, rather
> than a specification for retrieving the proper ontology given some
> OntologyURI, VersionURI or both. But I don't think the two
> perspectives can easily be disentangled.
> Rule 1:
> If O does not contain an ontology URI (and, consequently, without a
> version URI as well), then O can be physically located anywhere.
> There are two scenario's (in the RDF/XML case):
> 1) An empty rdf:about on the owl:Ontology element: the id/URI of the
> owl:Ontology element will be interpreted as being the same as the
> value of the xml:base attribute of the RDF/XML file. If the xml:base
> is not specified or empty (?), the OntologyURI will be the same as the
> physical location of the file (either online or on a local harddisk),
> because the xml:base is considered to be the same as that physical
> location.
> 2) No rdf:about on the owl:Ontology element: the owl:Ontology is
> nameless. Currently TopBraid will add a new owl:Ontology element with
> an empty rdf:about and then complains that the file contains two
> owl:Ontology elements. Protege4 currently does the proper thing, and
> interprets the owl:Ontology to be anonymous, i.e. its URI will be
> inferred to be something like 'xml:base'#'generatedID'.
> This touches on ISSUE-15, which was resolved: ontologies can in fact
> be without name.
> Rule 2:
> If O contains an ontology URI ou but no version URI, then O should be
> physically located at the location ou.
> This serves exactly the same function as the common use of the
> xml:base attribute.
> * If O contains an ontology URI ou and a version URI vu, then O should
> be physically located at the location ou or vu.
> In this case, the combination of ou and vu says 'yes this is the
> ontology ou, but I may also be found at vu'. This evidently adds
> functionality over the use of just xml:base and allows an ontology
> publisher to provide extra information on the whereabouts of the
> ontology on the web for versioning purposes. Nonetheless, when
> retrieving the ontology from vu through an imports statement, it is
> interpreted as coming from ou. This complies with the principle
> ou=xml:base=namespace.
> RDF/XML, Turtle and XML, each support (some form of specifying) an
> xml:base, the only syntax that does not support it is the functional
> style syntax.
> I might be mistaken, but if imports is by location, then OntologyURI
> has no real value over just using xml:base, apart from the fact that
> the fss does not support it.
> As I see it, there are three ways out of it, given that we do have
> imports by location:
> 1) Drop OntologyURI alltogether, and just use xml:base + VersionURI
> 2) Do not drop OntologyURI, but enforce it to be equal to xml:base (or
> empty). Make the appropriate adjustments to the mapping to ensure that
> a missing OntologyURI (i.e. rdf:about) is not translated to a bnode.
> 3) Treat OntologyURI similar to VersionURI, and say that the file
> owl:imports points to should have the same base URI as the import URI,
> or should have the same ontology URI as the import URI, or should have
> the same version URI as the import URI.
> A combination of 2 and 3 is possible as well.
> A last issue, that I'm still chewing on is what happens when an
> ontology imports another ontology from the versionURI location. The
> currrent practice in P4 is imports by name: imported classes are in
> the namespace of the imported ontology, and their names are relative
> to its ontology uri. In other words:
> importsuri=ontologyuri=xml:base=namespace. The imports by location
> principle breaks this chain, as the imports URI is does longer have to
> be the same as the ontology URI. But in the case where some specific
> version URI is imported, all local names of imported classes are still
> relative to the ontology URI. This means that someone who imports an
> ontology has no longer any way of knowing the namespace of the
> imported classes. Don't really know whether that's bad... but it's
> good to realise this consequence.
> -Rinke
> [1] http://www.ietf.org/rfc/rfc2396.txt
> [2] http://www.w3.org/TR/xmlbase/
> -----------------------------------------------
> Drs. Rinke Hoekstra
> Email: hoekstra@uva.nl    Skype:  rinkehoekstra
> Phone: +31-20-5253499     Fax:   +31-20-5253495
> Web:   http://www.leibnizcenter.org/users/rinke
> Leibniz Center for Law,          Faculty of Law
> University of Amsterdam,            PO Box 1030
> 1000 BA  Amsterdam,             The Netherlands
> -----------------------------------------------
Received on Wednesday, 21 May 2008 09:51:47 UTC

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