- From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
- Date: Fri, 30 Nov 2007 12:32:52 -0500 (EST)
- To: conrad.bock@nist.gov
- Cc: public-owl-wg@w3.org
From: "Conrad Bock" <conrad.bock@nist.gov> Subject: RE: UFDTF Metamodeling Document Date: Fri, 30 Nov 2007 11:59:46 -0500 > Peter, [...] > > I also think that using the OMG metamodel for RDF would be > > detrimental to the use of RDF. > > Again, you'd need to justify that with comments on the metamodels. > > Conrad OK, how about the following divergences between the metamodel (which I take to be the diagrams) and the RDF recomendation: 1/ The metamodel on page 35 allows triples (RDFStatment) in RDF graphs (RDFGraph) that are missing subject, predicate, and or object, contrary to the RDF recommendation. Controlling text at http://www.w3.org/TR/rdf-concepts/#section-data-model Each triple represents a statement of a relationship between the things denoted by the nodes that it links. Each triple has three parts: 1. a subject, 2. an object, and 3. a predicate (also called a property) that denotes a relationship. 2/ The metamodel on page 35 make an RDF graph (RDFGraph) be a ordered collection of triples (RDFStatment), contrary to the RDF recommendation. (Yes the text says that an RDF graph is a set, but this contradicts the metamodel.) Controlling text at http://www.w3.org/TR/rdf-mt/#graphdefs An RDF graph, or simply a graph, is a set of RDF triples. 3/ The metamodel on page 35 has names for graphs, which is not supported in the RDF recommendation. 4/ The metamodel on page 35 has reification status as a part of a triple (RDFStatement), which is not supported in the RDF recommendation. 5/ The metamodel on page 35 allows subjects to be literals, and predicates to be literals or blank nodes. (Yes this is later removed in the text, the metamodel allows this.) Controlling text is http://www.w3.org/TR/rdf-concepts/#section-triples 6.1 RDF Triples An RDF triple contains three components: * the subject, which is an RDF URI reference or a blank node * the predicate, which is an RDF URI reference * the object, which is an RDF URI reference, a literal or a blank * node An RDF triple is conventionally written in the order subject, predicate, object. The predicate is also known as the property of the triple. 6/ The metamodel on page 35 does not have URI references (URIReferenceNode), blank nodes (BlankNode), and literals (RDFSLiteral) pairwise disjoint. (Yes, the disjointness is clearly stated in the text, but not in the metamodel.) Controlling text is http://www.w3.org/TR/rdf-concepts/#section-blank-nodes 6.6 Blank Nodes The blank nodes in an RDF graph are drawn from an infinite set. This set of blank nodes, the set of all RDF URI references and the set of all literals are pairwise disjoint. 7/ The metamodel on page 43 has the comment (rdfs:comment) and label (rdfs:label) relationships 0/1 to many. The RDF recommendation does not restrict these properties and thus allows them to be many to many. 8/ The metamodel on page 43 has the "range" of comments and labels be plain literals (PlainLiteral). However the RDF recommendation has the range of these two relationships be rdfs: Controlling text is in http://www.w3.org/TR/rdf-mt/#RDFSINTERP rdfs:label rdfs:range rdfs:Literal . rdfs:label rdfs:range rdfs:Literal . 9/ The metamodel on page 50 has the both relationship between rdf lists (RDFList) and their first elements and the relationship between rdf lists (RDFList) and their tailss be 0/1 to many. However, the RDF recommendation does not so limit lists. A list can have multiple first elements as well as multiple tails. The explanatory text for this is in http://www.w3.org/TR/rdf-mt/#collections Also, RDF imposes no 'well-formedness' conditions on the use of this vocabulary, so that it is possible to write RDF graphs which assert the existence of highly peculiar objects such as lists with forked or non-list tails, or multiple heads: _:666 rdf:first <ex:aaa> . _:666 rdf:first <ex:bbb> . _:666 rdf:rest <ex:ccc> . _:666 rdf:rest rdf:nil . It is also possible to write a set of triples which underspecify a collection by failing to specify its rdf:rest property value. I don't think that this is an exhaustive list. I just put it together in about 30 minutes. I have also not included anything to do with the mixture of RDF syntax and semantics that occurs throughout the metamodel. In sum, the OMG metamodel for RDF has significant and pervasive differences from the W3C RDF recommendation. An application written to use the OMG metamodel is likely to not conform to the W3C recommendation. Therefore, the OMG metamodel for RDF is unsuitable for use. Peter F. Patel-Schneider Bell Labs Research
Received on Friday, 30 November 2007 17:49:45 UTC