Chapter 8: Web Ontology Language (OWL) Service-Oriented Computing: Semantics, Processes, Agents – Munindar P. Singh and Michael N. Huhns, Wiley, 2005

Chapter 82Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns Highlights of this Chapter “Species” or Dialects Constructors Axioms Inference Dialects Compared Expressiveness

Chapter 83Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns Web Ontology Language (OWL) OWL stands for Web Ontology Language OWL is built on top of RDF OWL is for processing information on the web OWL was designed to be interpreted by computers OWL was not designed for being read by people OWL is written in XML OWL has three sublanguages OWL is a web standard

Chapter 84Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns Web Ontology Language (OWL) Ontology is about the exact description of things and their relationships. Knowledge Structure RDF captures the basics, i.e., an object-oriented type system Additional subtleties of meaning are needed for effective knowledge Representation (KR) OWL standardizes additional constructs to show how to capture such subtleties of meaning Builds on RDF Gives particular semantics to new terms Example: Life OntologyLife Ontology

Chapter 85Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns Why OWL? OWL is a part of the "Semantic Web Vision" - a future where: Web information has exact meaning Web information can be processed by computers Computers can integrate information from the web OWL provides the ability to specify classes and properties in a form of description logic with the terms in its expressions related using Boolean operators analogous to AND, NOT, and OR. OWL was designed to provide a common way to process the content of web information (instead of displaying it).

Chapter 86Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns OWL in Brief OWL has three species /sublanguages: OWL Lite OWL DL (includes OWL Lite) OWL Full (includes OWL DL) Specifies classes and properties in a form of description logic (DL) Class operators analogous to Boolean operators and, not, and or Constraints on properties: transitive, … Restrictions: constructs unique to DL

Chapter 87Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns OWL Constructors Classes correspond to sets of objects. Properties relate pairs of individuals. Class expressions are used to express

Chapter 88Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns OWL Constructors OWL distinguishes between constructors and axioms. The OWL constructors are the primitives that help us specify new classes. The OWL axioms are the primitives that help us make additional assertions about classes and properties. The OWL dialects provide class constructors that are based on description logic. These constructors build on the data types defined in XML schema. Classes correspond to sets of objects. Properties relate pairs of individuals.

Chapter 89Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns OWL Constructors OWL comes into its own through a sophisticated set of class expression constructors. A class name A Boolean combination of class expressions. owl:intersectionOf owl:unionOf owl:complementOf An enumeration enclosed in the owl:Class elemnts. owl:oneOf A property restriction. OWL enables classes to be constructed out of properties through the mechanism of the property restriction.

Chapter 810Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns Custom Metadata Vocabularies Creating metadata for services and their information resources they rely upon presupposes custom vocabularies for such metadata The metadata must be given a standard semantics so that different parties interpret it the same way, and so that tools can function appropriately.

Chapter 811Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns Ontologies to Define Vocabulary Semantics A trivial ontology defining our vocabulary Uses simple subclasses and properties Disjointness goes beyond RDF Object properties refine RDF properties; relate two objects

Chapter 812Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns Simple Inference Inference can be specified as part of the semantics of the language. Given the definition for the property hasParent and the snippet we can infer that Fido is an Animal Figure 8.1 presents an RDF schema that includes the main entities and relationships of OWL.

Chapter 813Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns OWL Entities and Relationships

Chapter 814Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns Constructing OWL Classes Explicitly (as in the examples above) or Anonymously, using Restrictions (next page) Set operators: intersectionOf, unionOf, complementOf, e.g.,

Chapter 815Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns Restrictions: 1 A unique feature of description logics Kind of like division in arithmetic: define classes in terms of a restriction that they satisfy with respect to a given property Anonymous: typically included in a class def to enable referring them Key primitives are someValuesFrom a specified class allValuesFrom a specified class hasValue equal to a specified individual or data type minCardinality maxCardinality Cardinality (when maxCardinality equals minCardinality)

Chapter 816Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns Restrictions: 2 Examples of restriction fragments 1

Chapter 817Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns Restrictions: 3... The maker of a Wine must be a Winery The allValuesFrom restriction is on the hasMaker property of this Wine class (Makers of other products such as cheese are not constrained by this local restriction)

Chapter 818Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns OWL Axioms Instances of both classes (objects) and properties (pairs of objects) are written in the XML syntax for RDF. Data values are written in RDF syntax with a string representation of the desired value. A pair of classes or properties is identical or equivalent. They behave as synonyms. Properties can be introduced as inverse of other properties. It is helpful to relate ontologies to binary relations as elementary algebra.

Chapter 819Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns Axioms: 1 Assertions that are given to be true Can be especially powerful in combination with other axioms, which may come from different documents Some primitives rdfs:subClassOf owl:equivalentClass

Chapter 820Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns Axioms: 2 <!– in essence, pair-wise inequalities 

Chapter 821Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns Restrictions versus Axioms Axioms are global assertions that can be used as the basis for further inference Restrictions are constructors When we state that hasFather has a maxCardinality of 1, we are Defining the class of animals who have zero or one fathers: this class may or may not have any instances Not stating that all animals have zero or one fathers Often, to achieve the desired effect, we would have to combine restrictions with axioms (such as based on equivalentClass)

Chapter 822Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns Inference OWL is about content, not the syntax Statements from different documents about the same URI are automatically conjoined OWL can appear unintuitive to the uninitiated Declare that no one can have more than one mother Declare Mary is John’s mother Declare Jane is John’s mother A DBMS would declare an integrity violation An OWL reasoner would say Mary = Jane

Chapter 823Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns Dialects Compared OWL DL: the core dialect, includes DL primitives; not necessarily (but often practically) tractable OWL Lite: adds restrictions to OWL DL make it tractable OWL Full: lifts restrictions to allow other interpretations; extremely general; potentially intractable (undecidable); included just for fancy expressiveness needs

Chapter 824Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns Expressiveness Limitations: 1 OWL DL cannot express some simple requirements Non-tree models: because instance variables are implicit in OWL restrictions, OWL cannot express conditions that require that two variables be identified Think of siblings – two people who have the same parents – but in terms of classes Do the same thing with class definitions

Chapter 825Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns Expressiveness Limitations: 2 Specialized properties Cannot state that the child of a mammal must be a mammal and so on, without Defining new child properties for each class Adding an axiom for each class stating that it is a subClassOf the restriction of hasChild to itself Analogous to the problem in a strongly typed object- oriented language without generics You have to typecast the contents of a hash table or linked list

Chapter 826Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns Expressiveness Limitations: 3 Constraints among individuals Cannot define tall person: class of persons whose height is above a certain threshold Can define ETHusband: class of persons who have been married to Elizabeth Taylor Cannot capture defeasibility (also known as nonmonotonicity) Birds fly Penguins are birds Penguins don’t fly

Chapter 827Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns Chapter 8 Summary OWL builds on RDF to provide a rich vocabulary for capturing knowledge Synthesizes a lot of excellent work on discrete, taxonomic knowledge representation Fits well with describing information resources – a basis for describing metadata vocabularies Critical for unambiguously describing services so they can be selected and suitably engaged