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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. - PowerPoint PPT Presentation
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IMG, University of Manchester 1ISWC-2003 Sanibel Island, FL
Jeff Z. Pan1 and Ian Horrocks1,2 {pan | horrocks}@cs.man.ac.uk
1 Information Management Group Computer Science DepartmentUniversity of Manchester
2 Network Inference LtdLondon, UK
RDFS(FA) and RDF MT:Two Semantics for RDFS
IMG, University of Manchester 2ISWC-2003 Sanibel Island, FL
Semantic Web Vision
• Semantic Web (SW) aims at machine understandability– SW languages describe content/function of Web resources
• RDF(S) is proposed as the base for SW languages– (In)famous “layer cake”:
Data Exchange
Semantics+reasoning
Relational Data
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IMG, University of Manchester 3ISWC-2003 Sanibel Island, FL
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)
worksWith
worksIn
worksIn
Ian Horrocks [email protected] email
IMG, University of Manchester 4ISWC-2003 Sanibel Island, FL
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.:
ParentsubClassOf
RestrictiononProperty
minCardinality
hasChild
1
equivalentClasssubPropertyOf
subClassOf
Class
subClassOf
Resource
type
IMG, University of Manchester 5ISWC-2003 Sanibel Island, FL
RDF(S) Features/Limitations
• Not clear that RDF(S) is appropriate for both functions (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) cyclical meta-model
– Semantics given by “non-standard” Model Theory
IMG, University of Manchester 6ISWC-2003 Sanibel Island, FL
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
IEXTIEXT(T)
IEXT(S)
<R,C> IEXT(I(rdf:type))<P,C> IEXT(I(rdf:type))<J,P> IEXT(I(rdf:type))
IMG, University of Manchester 7ISWC-2003 Sanibel Island, FL
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 8ISWC-2003 Sanibel Island, FL
RDF(S) Features/Limitations (reprise)
• Problems stem from 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:typerdf:type rdfs:domain rdfs:Property
IMG, University of Manchester 9ISWC-2003 Sanibel Island, FL
Proposed Solution: RDFS(FA)
• RDFS(FA) is a sub-language of RDF(S)– It stands for “RDFS with Fixed layer metamodeling Architecture”
– Has a 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 prefix to indicate the stratum
IMG, University of Manchester 10ISWC-2003 Sanibel Island, FL
Metamodeling Architecture (Four Strata)
Stratum 0 (Instance Layer) Ian, Jeff …
Stratum 1 (Ontology Layer)
Stratum 2 (Language Layer)
Stratum 3 (Meta-Language Layer)
fa:OResourcePerson, ResearcherworkWith …
fa:LResource, fa:LClassfa:LProperty …
fa:MResource, fa:MClassfa:MProperty …
IMG, University of Manchester 11ISWC-2003 Sanibel Island, FL
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 12ISWC-2003 Sanibel Island, FL
Example: Stratification
fa:LResourcefa:LClass
eg:Person
eg:Researcher
fa:LProperty
eg:workWith
Jeff Ian
fa:l-subClassOf
fa:o-subClassOf
fa:l-subClassOf
fa:o-subClassOf
fa:l-typefa:l-type fa:l-type
fa:o-type fa:o-typeeg:workWith
fa:OResourcefa:o-subClassOf
fa:MClass
fa:m-type
fa:o-domain
fa:o-range
IMG, University of Manchester 13ISWC-2003 Sanibel Island, FL
Interpretation of RDFS(FA)
…
IMG, University of Manchester 14ISWC-2003 Sanibel Island, FL
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 15ISWC-2003 Sanibel Island, FL
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 16ISWC-2003 Sanibel Island, FL
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
–Layered style: RDFS(FA)
–Non-layered style: full RDF(S)
Should I use fa:Class or rdfs:Class?
IMG, University of Manchester 17ISWC-2003 Sanibel Island, FL
Acknowledgement
• Thanks to:–Peter Patel-Schneider
–Peter Aczel
IMG, University of Manchester 18ISWC-2003 Sanibel Island, FL
Thank you for your attention!
Jeff Z. Pan1 and Ian Horrocks1,2 {pan | horrocks}@cs.man.ac.uk
1 Information Management Group Computer Science DepartmentUniversity of Manchester
2 Network Inference LtdLondon, UK