1. Sycara and Paolucci 1 OWL Katia Sycara and Massimo Paolucci OWL section partially based on OWL Tutorial by Roger L. Costello and David B. Jacobs

2. Sycara and Paolucci 2 Overview OWL What can be said in OWL-S OWL vs XML RDF/RDFS/OWL OWL language Description Logic What do they are good for What inference do they support

3. Sycara and Paolucci 3 What can be said in OWL Classes: ComputerManifacturer Subclasses: ComputerManifacturer < Manifacturer Instances : IBM Properties : products Restriction on properties : all products are computers Cardinality restrictions : At lest two production facilities Set Theoretic statements : At lest two production facilities

4. Sycara and Paolucci 4 The basis of OWL From RDF/RDFS XML Syntax Concepts of rdfs:subclassOf, rdfs:Property etc… From Description Logics Axiomatic description Proof theory: inference mechanism Constraints on the language to maintain inference computable OWL Description LogicsRDF/RDFS XML

5. Sycara and Paolucci 5 Why isn‘t XML enough? … What is an SLR? What is this XML snippet talking about?

6. Sycara and Paolucci 6 SLR - which one? SLR Single Lens Reflex (camera) SLR Satellite Laser Ranging SLR Self Loading Rifle SLR Sending Loudness Rating (telecommunications) SLR Service Level Report SLR Service Location Register SLR Side Looking Radar SLR Single Line Restoral SLR Single Linear Recording SLR Slide Raft (aircraft door) SLR Slush on Runway(s) SLR Solectron SLR Spacelift Range SLR Sri Lanka Rupee (national currency) SLR Statutory Liquidity Ratio SLR Stock Level Report SLR Stock Level Requirement SLR Straight Leg Raise SLR Straight Leg Raising SLR System Level Requirement(s)

7. Sycara and Paolucci 7 Meaning (semantics) applied on a per-application basis XML app#1 Semantics: Code to interpret the data Action: Code to process the data app#2 Semantics: Code to interpret the data Action: Code to process the data

8. Sycara and Paolucci 8 Problem with attaching semantics on a per- application basis application Semantics: Code to interpret the data Action: Code to process the data Problems with burying semantic definitions within each application: - Duplicate effort - Each application must express the semantics - Variability of interpretation - Each application can take its own interpretation - Example: Mars probe disaster - one application interpreted the data in inches, another application interpreted the data in centimeters. - No ad-hoc discovery and exploitation - Applications have the semantics pre-wired. Thus, when new data is encountered an application may not be able to effectively process it. This makes for brittle applications.

9. Sycara and Paolucci 9 Better approach: XML app#1 Action: Code to process the data app#2 Action: Code to process the data OWL Document Semantic Definitions Separate concept definition (semantics) from application Express concept definitions using a standard vocabulary

10. Sycara and Paolucci 10 What is added by RDF Provide basic syntax for OWL Use of URI for unique identification of concepts, instances and relations Expression of relations between objects and concepts RDF triples Problem: no structure

11. Sycara and Paolucci 11 RDF Schema Add basic structure to RDF Class/Subclass declaration Instances Properties (relations) Multiple inheritance OWL greatly expands the vocabulary of possible constructs

12. Sycara and Paolucci 12 OWL as Description Logics Language Subset of First Order Logics used to describe objects in a domain Allows three types of objects Concepts: describe general concepts of things in the domain They are usually represented as sets Ex: Bird: the generic description of bird (or whatever is common to all the things that are birds) Individuals: an object in the domain Ex: Tweedy the bird Properties: relations between concepts One special relation is ISA (or subclassOf) Ex: Bird isa Animal or Bird has Wing Superficially OO looking

13. Sycara and Paolucci 13 OWL and logics OWL relies on Description Logics Logics provide automatic Check of consistency of concept definitions Completion of concept definitions Classification of new instances and concepts Extraction of implicit knowledge in the documents None of this is available in XML XML Schema provides some of those properties to some extent

14. Sycara and Paolucci 14 Types and quantifiers on Properties Different types of properties Transitivity, Symmetry, Function, Inverse etc… Transitivity: Joe siblingOf Sally and Sally siblingOf Bob then Joe siblingOf Bob Inverse: Joe siblingOf Sally then Sally siblingOf Joe Cardinality restrictions Specify how many elements are in relation with each other at-most, at-least, exactly, optionality (0 or more) Bird has (exactly) 2 Wing Type restrictions Identifies subclasses that have some restriction on a property P Bottle madeOf Material Glass subclassOf Material GlassBottle subclassOf Bottle madeOf Glass

15. Sycara and Paolucci 15 Equivalence between concepts Equivalence of concepts Allows concepts that have been defined in different ontologies to be equated Ont1:LiquidContainer equvalentClass ont2:Bottle As a consequence any instance of LiquidContainer iis also an instance of Bottle and any instance of Bottle is also an instance of LiquidContainer Equivalence of individuals Allows instances defined in different ontologies to describe the same objects EveningStar sameAs MorningStar Difference of individuals Assert values that are mutually distinct.

16. Sycara and Paolucci 16 Using Set Theory Complex types to support set theory union, intersection and complement Human= unionOf Woman and Man Enumerated Classes means to specify a class via a direct enumeration of its members, this is done through the oneOf construct Gender oneOf male or female Disjoint Classes It guarantees that an individual that is a member of one class cannot simultaneously be an instance of a specified other class. Cat disjoint Dog

17. Sycara and Paolucci 17 What are Description Logics good for Inheritance (Subsumption) The properties of the super class are inherited to the subclass and eventually to the instances Automatic classification You can define a new object of type Thing (the top level node) and depending on its property that object is automatically classified (and completed) Bird subclassOf Animal with exactly 2 Wing Tweedy is an instance of Thing Tweedy has wing LeftWing Tweedy has wing RightWing Tweedy has not more wings Tweedy is automatically classified as Bird

18. Sycara and Paolucci 18 Limitations Description Logics Description Logics provides a snapshot of the World Describe objects in a given time frame They are intrinsically very static Difficult to represent processes (sequences of situations) There is no explicit notion of variables and rules It is impossible to say that X father Y and Y fatherZ  X grandfather Z

19. Sycara and Paolucci 19 OWL Full, OWL DL, and OWL Lite Description Logics provides a careful balance between expressivity and computational complexity OWL provides sublanguages with reduced expressivity and computational complexity OWL Full OWL DL OWL Lite

20. Sycara and Paolucci 20 Comparison OWL FullOWL DL OWL Lite Everything that has been shown in this tutorial is available. Further, you can mix RDF Schema definitions with OWL definitions. You cannot use owl:cardinality with TransitiveProperty. You cannot use a class as a member of another class, i.e., you cannot have metaclasses. FunctionalProperty and InverseFunctionalProperty cannot be used with datatypes (they can only be used with ObjectProperty). All the DL restrictions plus: You cannot use owl:minCardinality or owl:maxCardinality. The only allowed values for owl:cardinality is 0 and 1. Cannot use owl:hasValue. Cannot use owl:disjointWith. Cannot use owl:oneOf. Cannot use owl:complementOf. Cannot use owl:unionOf.

21. Sycara and Paolucci 21 Advantages/Disadvantages Full: The advantage of the Full version of OWL is that you get the full power of the OWL language. The disadvantage of the Full version of OWL is that it is difficult to build a Full tool. Also, the user of a Full-compliant tool may not get a quick and complete answer. DL/Lite: The advantage of the DL or Lite version of OWL is that tools can be built more quickly and easily, and users can expect responses from such tools to come quicker and be more complete. The disadvantage of the DL or Lite version of OWL is that you don't have access to the full power of the language.

22. Sycara and Paolucci 22 Resources OWL specifications W3C page on OWL http://www.w3c.org/2001/sw/WebOnt/ OWL Guide and language reference http://www.w3.org/TR/owl-guide/ http://www.w3.org/TR/owl-ref/ Tutorials Costello and Jacobs’ OWL tutorial: http://www.xfront.com/owl/

23. Sycara and Paolucci 23 OWL Tools Racer: OWL-Lite inference engine www.sts.tu-harburg.de/~r.f.moeller/racer/ Tools at Network Inference http://www.networkinference.com/ OilEd: http://oiled.man.ac.uk/ Editor for ontologies Mostly for DAML+OIL, exports OWL but not a current representation OWL Validator: http://owl.bbn.com/validator/ Web-based or command-line utility Performs basic validation of OWL file OWL Ontology Validator: http://phoebus.cs.man.ac.uk:9999/OWL/Validator a "species validator" that checks use of OWL Lite, OWL DL, and OWL Full constructs Euler: http://www.agfa.com/w3c/euler/ an inference engine which has been used for a lot of the OWL Test Cases Chimaera: http://www.ksl.stanford.edu/software/chimaera/ Ontology evolution environment (diagnostics, merging, light editing) Mostly for DAML+OIL, being updated to export and inport current OWL Extensive list of tools, http://www.w3.org/2001/sw/WebOnt/impls