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Jeff Z. Pan 1 and Ian Horrocks 1,2 {pan | horrocks}@cs.man.ac.uk 1 Information Management Group

RDFS(FA) and RDF MT: Two Semantics for RDFS. Jeff Z. Pan 1 and Ian Horrocks 1,2 {pan | horrocks}@cs.man.ac.uk 1 Information Management Group Computer Science Department University of Manchester 2 Network Inference Ltd London, UK. ???. ???. ???. ?. ?. Semantic Web Vision.

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Jeff Z. Pan 1 and Ian Horrocks 1,2 {pan | horrocks}@cs.man.ac.uk 1 Information Management Group

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  1. RDFS(FA) and RDF MT:Two Semantics for RDFS Jeff Z. Pan1 and Ian Horrocks1,2 {pan | horrocks}@cs.man.ac.uk 1 Information Management Group Computer Science Department University of Manchester 2 Network Inference Ltd London, UK IMG, University of Manchester

  2. ??? ??? ??? ? ? Semantic Web Vision • Semantic Web (SW) aims at machine understandability • SW languagesdescribe content/function of Web resources • RDF(S) is proposed as the base for SW languages • (In)famous “layer cake”:  Semantics+reasoning  Relational Data  Data Exchange IMG, University of Manchester

  3. worksIn worksIn worksWith name email horrocks@cs.man.ac.uk Ian Horrocks Dual Roles of RDF(S) - I • RDF(S) is used to add metadata annotations to Web resources • Subject-predicate-object triples used to link resources • i.e., triples represent knowledge about domain (such as Ian Horrocks worksWith Jeff Pan) IMG, University of Manchester

  4. Parent Restriction hasChild subClassOf onProperty minCardinality 1 equivalentClass subClassOf subPropertyOf subClassOf Class Resource type Dual Roles of RDF(S) - II • RDF(S) also used to define syntax and semantics of subsequent language layers (and even of itself), e.g.: IMG, University of Manchester

  5. RDF(S) Features/Limitations • Not clear that RDF(S) is appropriate for bothfunctions (at once) • Limited set of syntax constructs (triples) • Not possible to extend syntax (as it is, e.g., when using XML) • Uniform semantic treatment of triple syntax • i.e., “syntax” and “knowledge” triples have same semantics • Confusing (for some) cyclicalmeta-model • Semantics given by “non-standard” Model Theory IMG, University of Manchester

  6. RDF(S) Model Theory (RDF MT) • Let V be a set of vocabulary, IR the universe of discourse • I is a mapping from V to IR • IP is the set of property objects • IEXT(x), the extension of a property object x, is a set of pairs <R,C> IEXT(I(rdf:type)) <P,C> IEXT(I(rdf:type)) <J,P> IEXT(I(rdf:type)) IEXT(S) IEXT IEXT(T) IMG, University of Manchester

  7. Language Layering • More expressive ontology languages layered on top of RDF(S) • E.g., OIL, DAML+OIL, and now OWL • Include logical connectives, quantifiers, transitive properties, etc. • Need to extend RDF MT to “RDF+ MT” to give semantics to them • However… • Several known problems with the “RDF+ MT” approach • Difficult to ensure that RDF+MT gives all and only desired entailments • Classes whose extension is not well defined • Size of the MT universe Should I use owl:Class or rdfs:Class? IMG, University of Manchester

  8. RDF(S) Features/Limitations (reprise) • Problems stemfrom features/limitations of RDF(S) • Triples, all triples and nothing but triples! • Classes and properties are treated as objects in the domain • Including RDF/OWL/… built-in classes and properties • No restrictions on the use of built-in vocabularies • E.g. the users can write triples as follows: • Can lead to unwanted/unexpected consequences, particularly with more expressive langauges (like OWL) ex:my-type rdfs:subPropertyOf rdf:type rdf:type rdfs:domain rdfs:Property IMG, University of Manchester

  9. Proposed Solution: RDFS(FA) • RDFS(FA)is a sub-language of RDF(S) • It stands for “RDFS with Fixed layer metamodeling Architecture” • Hasa First Order/Description Logic style semantics • The universe of discourse is divided up into a series of strata • User defined facts/vocabulary and RDF/OWL built-in vocabulary are (typically) in different strata • Each modelling primitive belongs to a certain stratum (layer) • Labelled with different prefixto indicate the stratum IMG, University of Manchester

  10. Metamodeling Architecture (Four Strata) fa:MResource, fa:MClass fa:MProperty … Stratum 3 (Meta-Language Layer) fa:LResource, fa:LClass fa:LProperty … Stratum 2 (Language Layer) fa:OResource Person, Researcher workWith … Stratum 1 (Ontology Layer) Stratum 0 (Instance Layer) Ian, Jeff … IMG, University of Manchester

  11. Syntax and Semantics • RDFS(FA) introduces some new syntax to RDF(S) • Disallows arbitrary use of built-in vocabulary • Supports meta-classes and meta-properties (in specified strata) • RDFS(FA) doesn’t invalidate existing RDF(S) syntax • Users don’t need to change their RDF(S) data sets • Classes and Properties are not objects in RDFS(FA) • Classes interpreted as sets of resources in the adjacent lower stratum • Properties interpreted as sets of pairs of resources in the adjacent lower stratum • The only exception is “type” property IMG, University of Manchester

  12. Example: Stratification fa:MClass fa:m-type fa:l-subClassOf fa:l-subClassOf fa:LResource fa:LClass fa:LProperty fa:l-type fa:l-type eg:Person fa:l-type fa:o-subClassOf fa:OResource eg:workWith fa:o-subClassOf fa:o-domain fa:o-subClassOf eg:Researcher fa:o-range fa:o-type fa:o-type eg:workWith Jeff Ian IMG, University of Manchester

  13. Interpretation of RDFS(FA) … IMG, University of Manchester

  14. Advantages of RDF MT • RDF(S) (RDF MT) is more expressive than RDFS(FA) • No stratification restrictions • Anyone can say anything about anything • Properties can be defined between any two resources • Any resource can be defined as an instance of any resource (including itself) • Be careful: an object can become a class or a property some time later What are the motivations of the extra expressive power? IMG, University of Manchester

  15. Advantages of RDFS(FA) • No problems layering FO languages on top of RDFS(FA) • Bottom two layers form standard FO models • RDFS(FA) supports use of meta-classes and meta-properties • In stratum above classes and properties • RDFS(FA) metamodel very similar to that of UML • Possible to define a new sub-language of OWL: OWL FA • Extends OWL DL with meta-classes/properties and support for annotation properties • Fully compatible with OWL DL semantics • Amenable to reasoning (even for meta-classes/properties) IMG, University of Manchester

  16. Conclusion • RDF(S) is proposed as base for SW languages • Language architecture may be too complex for base layer • Known problems layering FO languages on top of RDF(S) • We propose RDFS(FA) as a sub-language of RDF(S) • Users can choose between • Layeredstyle: RDFS(FA) • Non-layeredstyle: full RDF(S) Should I use fa:Class or rdfs:Class? IMG, University of Manchester

  17. Acknowledgement • Thanks to: • Peter Patel-Schneider • Peter Aczel IMG, University of Manchester

  18. Thank you for your attention! Jeff Z. Pan1 and Ian Horrocks1,2 {pan | horrocks}@cs.man.ac.uk 1 Information Management Group Computer Science Department University of Manchester 2 Network Inference Ltd London, UK IMG, University of Manchester

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