The Semantic Web – WEEK 5: RDF Schema + Ontologies The “Layer Cake” Model – [From Rector & Horrocks Semantic Web cuurse]

The Semantic Web Recap ‘Resources’ on the web are things that have a URI, like a web page (a URL), a fragment of a web page (ie a URL followed by a fragment identifier), a book, a person, a web service etc Last week we met RDF – a way of writing descriptions of resources by using attributes (or property/predicate names) and values. Hence we defined an RDF document to consist of triples of the form resource, attribute, value Or in logic subject, predicate, object An RDF document can be drawn as a directed, labeled graph

The Semantic Web Classes and Properties n RDF is a kind of simple, ‘flat’ relational database language. For web applications we need the kind of structuring and representational capability of an OODB. n It is desirable that we can represent common object classes (eg Person, Book, Company, Product) and common properties of these classes. We also want to make our class definitions explicit and SHARE them. n RDF does not give any ‘meaning’ to attributes or resources

The Semantic Web RDF -> RDF Schema n RDF(S) allows users to define vocabularies of terms, using Class, Property, type, subClassOf, subPropertyOf, range, domain Resources can be members of classes, and classes can be put into a class hierarchy So we expect triples of this form: type Class type subClassOf

The Semantic Web Properties can have Sub-Properties We expect triples of this form: type Property subPropertyOf Property> x subPropertyOf y Means.. “all pairs of resources which are related by x are also related by y” So motherof is a sub-property of what ??

The Semantic Web Properties can have defined range and domains Domain and range give properties of properties ie the class of thing that the property can be applied to (domain) and the class of things that could be the value (ranges) So we expect triples of this form: domain range Eg fatherof range Person

The Semantic Web RDF Schema example Cat type Class hasParent type Property Kitten subClassOf Cat Dyllan type Kitten sameLitter range Cat sameLitter domain Cat

The Semantic Web RDFS – another example - longhand <rdf:RDF xmlns:rdf= " xmlns:rdfs=" xml:base= "

The Semantic Web RDFS – example – shorter (type implicit)

The Semantic Web example – even shorter Animal type class Horse type class Horse subClassOf Animal Dog subClassOf Animal.. Bit of an ontology!

The Semantic Web Ontologies – many defns n An ‘Ontology’ is --- an agreed on, shared, common understanding of a domain written as an explicit, formal specification. -- a specification of a conceptualization, where a conceptualization is “an abstract, simplified view of the world” -- etc Because of the taxonomic nature of things, the specification is often written in terms of classes and properties etc n RDFS can be used as an ontology language as it has Classes and properties, Sub/super-classes, range and domain n Ontologies are often used to share the common meanings of terms used in semantic web pages.

The Semantic Web Ontologies – from philosopy ONTOLOGY - “The systematic study of existence” “the science or study of being” EPISTEMOLOGY - “The study of knowledge”

The Semantic Web Ontologies - history n First used for Knowledge-Sharing and Knowledge Re-Use in KBS. They can be as simple as a ‘concept hierarchy’ or as complex as an axiomatic theory of sets. n There are various “kinds” of ontology: eg – -- representation ontology (axiomatization of basic operations used in many applications) or application ontology (capturing a taxonomy of the animal kingdom).

The Semantic Web Databases vs Ontologies Databases are - Large bodies of persistent data - Interpreted as a list of facts - Generally stored relationally in tables (OO databases may never catch on…) - Complete with highly optimised access procedures and interface mechanisms

The Semantic Web Databases vs Ontologies In comparison, ontologies - Can store more information than factual eg quantified (Fred has at least one pet) or disjunctive (Bill is a boy or a girl) - Come with implied reasoning mechanisms - Are more akin to OO DBs - Classes and properties may have more than one description / definition

The Semantic Web Conclusions - RDF schema + languages above it are considered suitable languages for Semantic Web encoding - Ontologies are used to encode and share knowledge in the Semantic Web