Introduction to Ontologies

Transcription

1 Introduction to Ontologies Jon Atle Gulla Ontology (from the Greek nominative ὤν: being, genitive ὄντος: of being (participle of εἶναι: to be) and -λογία: science, study, theory) is a study of conceptions of reality and the nature of being. In philosophy, ontology is the study of being or existence and forms the basic subject matter of metaphysics. It seeks to describe or posit the basic categories and relationships of being or existence to define entities and types of entities within its framework. [ Jon Atle Gulla Ontology Introduction 1 Outline of Presentation Ontology ABC Terminology Ontology Languages Semantic Web building blocks URIs and namespaces XML RDF/RDFS Impact of ontology on information systems (N. Guarino) Classification of ontology applications (R. Jasper & M. Uschold) Jon Atle Gulla Ontology Introduction 2 1

2 Terminology Jon Atle Gulla Ontology Introduction 3 Communication between People Jon Atle Gulla Ontology Introduction 4 2

3 Ontology ABC Ontology attracts attentions across many fields in computer science The term ontology originates from philosophy and its current use in computer science (first introduced in AI) is far from its philosophical origin. There is no consensus definition about ontology. One most cited is Ontology is an explicit representation of a conceptualization, the conceptualization includes a set of concepts, their definition and interrelationships. In many cases the term ontology is another name for familiar activities like conceptual analysis and domain modeling. The roles of ontology vary from knowledge management to semantic interoperability. Ontologies are considered the key enablers of the Semantic Web. Jon Atle Gulla Ontology Introduction 5 What are ontologies? Studer(98): Formal, explicit specification of a shared conceptualization Machine readable Consensual knowledge Concepts, properties, functions, axioms are explicitly defined Abstract model of some phenomena in the world Jon Atle Gulla Ontology Introduction 6 3

4 More Terminology Ontology: engineering artifact Constituted by a vocabulary (concepts, relations) Assumptions about intended meaning Formalization: Logical theory accounting for the intended meaning of a formal vocabulary Committed to a particular conceptualization of the world Ontology vs. conceptualization Conceptualization is language-independent Ontology is language-dependent Jon Atle Gulla Ontology Introduction 7 Example 1. Ontology of American Universities SHOE ontology of university concepts <?xml <?xml version version = = encoding= ISO encoding= ISO standalone= no standalone= no?>?> <!DOCTYPE <!DOCTYPE ontology ontology SYSTEM SYSTEM /onto.dtd > /onto.dtd > <ontology <ontology id= university-ont id= university-ont version= 2.1 version= 2.1 description= > description= > <def-category <def-category name= Department name= Department isa= EducationalOrganization isa= EducationalOrganization short= university short= university department department /> /> <def-category <def-category name= Activity name= Activity isa= SHOEEntity isa= SHOEEntity short= activity short= activity /> /> <def-category <def-category name= Work name= Work isa= Activity isa= Activity short= work short= work /> /> <def-category <def-category name= Course name= Course isa= Work isa= Work short= teaching short= teaching course course /> />.. </ontology> </ontology> Jon Atle Gulla Ontology Introduction 8 4

5 Example 2. Business Process Ontology MIT process handbook Sell loan Sell reserve credit Sell credit card Sell instalment loan Sell letter of credit Sell mortgage Sell credit line Sell financial service Sell savings & investment service Sell account Sell certificate of deposit Sell mutual funds Sell retirement plan Sell management service Sell account access services Sell ATM access Sell telephone access Jon Atle Gulla Ontology Introduction 9 Example 3. Hierarchical Categories? Can hierarchical categories be ontologies? Conceptualization of medical domain? Jon Atle Gulla Ontology Introduction 10 5

6 More Confusion Differences and similarities Ontology Ontology Thesaurus Thesaurus Dictionary Dictionary?Categories Categories Jon Atle Gulla Ontology Introduction 11 Ontology Spectrum Strong semantics Taxonomy Thesaurus ER Has narrower meaning than Schema Weak semantics Local Domain Theory Description logic Is disjoint subclass OWL of with transitive UML property Conceptual Model RDF/S Is subclass of Extended ER Is subclassification of First order logic Jon Atle Gulla Ontology Introduction 12 6

7 Ontology and Interoperability Web Ontology Language (OWL) Recommended by W3C Logic language (based on Description Logic) with XML syntax 3 versions Jon Atle Gulla Ontology Introduction 13 Ontology Languages Jon Atle Gulla Ontology Introduction 14 7

8 Ontology languages OWL OWL Jon Atle Gulla Ontology Introduction 15 Ontology Languages Wide variety of languages for Explicit Specification Graphical notations Semantic networks Jon Atle Gulla Ontology Introduction 16 8

9 Ontology Languages Wide variety of languages for Explicit Specification Graphical notations Topic Maps Jon Atle Gulla Ontology Introduction 17 Ontology Languages Wide variety of languages for Explicit Specification Graphical notations UML Jon Atle Gulla Ontology Introduction 18 9

10 Ontology Languages Wide variety of languages for Explicit Specification Graphical notations RDF Jon Atle Gulla Ontology Introduction 19 Ontology Languages Wide variety of languages for Explicit Specification Logic based Description Logics (e.g., OIL, DAML+OIL, OWL) Rule (e.g. SWRL, RuleML, Prolog) First Oder Logic (e.g., KIF) Jon Atle Gulla Ontology Introduction 20 10

11 Ontology Languages Wide variety of languages for Explicit Specification Logics based Conceptual graphs John F. Sowa Jon Atle Gulla Ontology Introduction 21 Ontology Languages Degree of formality varies widely Increased formality makes languages more amenable to machine processing (e.g., automated reasoning) Jon Atle Gulla Ontology Introduction 22 11

12 Semantic Web Building Blocks Jon Atle Gulla Ontology Introduction 23 The Semantic Web Goal: Evolve the Web From sites designed for human consumption To sites also understandable and usable by computer programs. What would that do for us? Query answering rather than document retrieval Services findable, usable, and composable by automated agents Information exchange among independently designed programs How do we get there from here? For services Service description Ontologies to provide intended meaning of service item. For documents Structure, ala XML Ontologies to provide intended meaning of terms The Semantic Web is an extension of the current web in which information is given welldefined meaning, better enabling computers and people to work in cooperation. Jon Atle Gulla Ontology Introduction 24 12

13 W3C Semantic Web Activity Semantic Web Activity ( Established to serve a leadership role, in both the design of enabling specifications and the open, collaborative development of technologies that support the automation, integration and reuse of data across various applications. Successor to the W3C Metadata Activity RDF Core Working Group ( Responsible for the Resource Description Framework (RDF) Web Ontology Working Group ( Charter: Build upon the RDF Core work a language for defining structured web based ontologies which will provide richer integration and interoperability of data among descriptive communities Developing Ontology Web Language (OWL) Based on DAML+OIL, developed in DARPA s Agent Markup Language program Jon Atle Gulla Ontology Introduction 25 Uniform Resource Identifiers (URIs) Need unique references to information representation constructs A URI is a string of characters identifying a particular resource URIschemeName Scheme-specific part Fragment URI Terminology: URIref: Fragment included in reference Absolute URI: full specification of URI Relative URIref: food-ont#pizza Jon Atle Gulla Ontology Introduction 26 13

14 Namespaces Associates namespaces with URIref to disambiguate duplicate names in XML documents Simplifies writing URIrefs <rdf <rdf xmlns:rest= > xmlns:rest= > The prefix rest is associated with Qualified names (QNames) Namespace Namespace declaration: declaration: xmlns:rest= xmlns:rest= Qname Qname rest:pie rest:pie expands expands to to Default Default namespace namespace declaration: declaration: xmlns= xmlns= Qname Qname #pie #pie expands expands to to Jon Atle Gulla Ontology Introduction 27 XML Describes Document Structure HTML Language for describing how to display document content E.g., tag a word to be displayed in bold or italic XML Language for describing the structure of document content E.g., declare data to be a retail price, a sales tax, a book title,... Uniform method for describing and exchanging data using HTTP Provides a syntactic schema XML allows authors to create their own markup (e.g. <AUTHOR>), which seems to carry some semantics. However, from a computational perspective tags like <AUTHOR> carries as much semantics as a tag like <H1>. A computer simply does not know, what an author is and how the concept author is related to e.g. a concept person. Jon Atle Gulla Ontology Introduction 28 14

15 Bibliographical Entry in XML <Publication URL = "ftp://db.stanford xml.ps > <Title> From Semistructured Data... Language </Title> <Author> R. Goldman </Author> <Published> Proceedings of... Databases </Published> <Location> <City> Philadelphia </City> <State> Pennsylvania </State> </Location> <Date> <Month> June </Month> <Year> 1999 </Year> </Date> </Publication> Location of what? When in June? Jon Atle Gulla Ontology Introduction 29 XML is a Syntactic Basis Language for describing the structure of document content Uniform method for describing and exchanging data using HTTP Ontologies enable independently developed programs to exchange datad Provides a syntactic schema Provides no means of specifying intended meaning of tags Ontologies specify intended meaning in a computer interpretable form XML is like HTML, where you make up your own tags. But in XML, you can t say what your tags mean. Jon Atle Gulla Ontology Introduction 30 15

16 Resource Description Framework A simple representation language for describing Web resources All sentences are triples of the form (Property Subject Object) Property is a binary relation Subject is a URI reference Object is either a URI reference or a literal (plain literal or typed literal) E.g.: creatorof Ora Lassila xsd:integer^^6 Model theoretic semantics Includes a resource Class and properties type, subclassof, etc. Supports classes of resources and literals E.g., (type Elephant Clyde) Supports subclass hierarchies E.g., (subclassof Elephant Mammal) Jon Atle Gulla Ontology Introduction 31 RDF Statements XML used to serialize RDF(S) statements Start Start tag: tag: <rdf:rdf <rdf:rdf xmlns:rdf= xmlns:rdf= Create Create new new subject subject resource resource (rdf:id) (rdf:id) <rdf:description <rdf:description rdf:id= Person345 /> rdf:id= Person345 /> Describe Describe existing existing subject subject resource resource (rdf:about) (rdf:about) <rdf:description <rdf:description rdf:about= #Person345 > rdf:about= #Person345 > <profile:favoritepie>key <profile:favoritepie>key Lime Lime Pie Pie </profile:favoritepie> </profile:favoritepie> </rdf:description> </rdf:description> Typed Typed literal literal in in statement statement (rdf:datatype) (rdf:datatype) <rdf:description <rdf:description rdf:about= rdf:about= <closingtime <closingtime rdf:datatype= &xsd;time >22:00:00</closingTime> rdf:datatype= &xsd;time >22:00:00</closingTime> </rdf:description> </rdf:description> Short-hand Short-hand notation notation for for types: types: <food:starch <food:starch rdf:id= Rice > rdf:id= Rice > <rdf:type <rdf:type rdf:about= rdf:about= </food:starch> </food:starch> Jon Atle Gulla Ontology Introduction 32 16

17 RDF Schema Classes 4 Class 4 ContainerMembershipProperty Literal Container Bag Seq Alt Resource Class Property Statement ContainerMembershipProperty Properties 4subClassOf 4subPropertyOf 4seeAlso 4isDefinedBy 4comment 4label 4range 4domain 4member Jon Atle Gulla Ontology Introduction 33 RDFS Predefined Constructs CLASS Rdfs:Resource Rdf:Property Rdfs:Class Rdfs:Literal Rdfs:XMLLiteral Rdfs:Datatype DESCRIPTION Root class of all resources Class of all properties Class of all classes Class of all literal values XML strings within RDF statements Identifies datatypes RDFS CATEGORY CONSTRUCT DOMAIN RANGE Core properties Rdf:type &rdfs;resource &rdfs;class Rdfs:subClassOf &rdfs;class &rdfs;class Rdfs:subPropertyOf &rdfs;property &rdfs;property Constraint properties Rdfs:range &rdfs;property &rdfs;class Rdfs:domain &rdfs;property &rdfs;class Clarification properties Rdfs:seeAlso &rdfs;resource &rdfs;resource Rdfs:isDefinedBy &rdfs;resource &rdfs;resource Documentation properties Rdfs:label &rdfs;resource &rdfs;literal Rdfs:comment &rdfs;resource &rdfs;literal Jon Atle Gulla Ontology Introduction 34 17

18 RDF-S Class and Property Definitions <rdf:class ID="MotorVehicle"> <rdfs:subclassof rdf:resource="http.../pr-rdf-schema #resource"/> </rdf:class> <rdf:class ID="PassengerVehicle"> <rdfs:subclassof rdf:resource="#motorvehicle"/> </rdf:class> <rdf:class ID="Van" <rdfs:subclassof rdf:resource="#motorvehicle"/> </rdf:class> <rdf:class ID="MiniVan"> <rdfs:subclassof rdf:resource="#van"/> <rdfs:subclassof rdf:resource="#passengervehicle"/> </rdf:class> <rdf:property ID = "registeredto"> <rdfs:domain rdf:resource = #MotorVehicle /> <rdfs:range rdf:resource = #Person /> </rdf:property> Christine is a passenger vehicle. Is Christine a motor vehicle? Yes. Christine is registered to Arnie. What is Arnie? A person. Jon Atle Gulla Ontology Introduction 35 Comments on RDF and RDF-S Severely lacking in expressive power Domain and range constraints rather than Value-Type E.g., can t define class of people all of whose children are male No cardinality constraints Particularly important for exactly 1 and at most 1 No decompositions Particularly important for disjoint and exhaustive No axioms No negation (!) Not useful for checking consistency E.g., can t prove an object is not an instance of a class Basically a typing system More powerful ontology representation languages are needed. Jon Atle Gulla Ontology Introduction 36 18

19 Nicola Guarino: Formal Ontology and Information Systems Jon Atle Gulla Ontology Introduction 37 Formal Ontology and Information Systems This paper is trying to offer a systematic account of the central role ontologies may play in information systems. Ontology may affect three main components of information systems: information resources, user interfaces and application programs. In AI, an ontology is an engineering artifact. In the simplest case, an ontology describes a hierarchy of concepts related by subsumption relationships; in more sophisticated cases, suitable axioms are added in order to express other relationships between concepts and to constrain their intended interpretation. Jon Atle Gulla Ontology Introduction 38 19

20 Ontologies at Different Levels of Generality Top-level ontologies. Domain ontologies. Task ontologies. Application ontologies. Jon Atle Gulla Ontology Introduction 39 Differences in Describing Reality subtypes of concepts. task dependence of ontologies. Jon Atle Gulla Ontology Introduction 40 20

21 Ontology-Driven Information Systems An IS consists of components of three different types: application programs, information resources, and user interfaces. Ontologies can play a central role here. Two dimensions for analysis: Temporal dimension: using ontologies at development time or run time. Structural dimension: impact of ontologies on different IS components. Jon Atle Gulla Ontology Introduction 41 The Temporal Dimension Using an ontology at development time. If we have a set of reusable ontologies (ontology library) The result of the development phase is an application ontology, which is specialization of both domain ontology and task ontology. The available off-the-shell ontologies today is limited. (But ontology companies are now emerging) If we have generic ontology. More realistic. Not building block, but tool The generic ontology can increase the quality of analysis process, analogous to any other CASE tool. Jon Atle Gulla Ontology Introduction 42 21

22 The Temporal Dimension Using ontologies at run time. Ontology aware IS An IS component is just aware of the existence of a (possibly remote) ontology and can use it (i.e. query it) for whatever specific application purpose. Ontology driven IS Enabling communication between software agents. Software agents are communicating with each other via messages that contain expressions formulated in terms of an ontology. Jon Atle Gulla Ontology Introduction 43 The Structural Dimension Using an ontology as a database component. An ontology can be compared with the schema component of a database. At development time, the resulting conceptual model of requirement analysis can be represented as a computer processable ontology and from there mapped to concrete target platforms. Another main use of ontology in development time is information integration. At run time, explicit ontologies (run-time accessible database schema) are at the core of the mediation based approach to information integration. Jon Atle Gulla Ontology Introduction 44 22

23 The Structural Dimension Using an ontology as a user interface component. Allow the user to query and browse the ontology. The user can browse the ontology to understand the vocabulary used by the IS, being able to formulate queries at the desired level of specificity. Another use is vocabulary detaching: the user can use his own natural language terms which are mapped to the IS vocabulary with the help of the ontology. Jon Atle Gulla Ontology Introduction 45 The Structural Dimension Using an ontology for the application program component. Application programs encode knowledge in the form of type or class declaration and procedures. The ontological commitment of the program can be made explicit using ontologies. Programs turned into knowledge-based systems with expandable growing knowledge bases Jon Atle Gulla Ontology Introduction 46 23

24 Conclusions Ontology driven information system. Different types of ontologies. The role of ontology in IS Time dimension Development time vs. run time Structural dimension. Information resource, user interfaces, and application program. Jon Atle Gulla Ontology Introduction 47 Robert Jasper and Mike Uschold: A Framework for Understanding and Classifying Ontology Applications Jon Atle Gulla Ontology Introduction 48 24

25 A framework for understanding and classifying ontology application The paper presents a framework for understanding ontology applications. We will study The framework Various ontology application scenarios (use cases). Jon Atle Gulla Ontology Introduction 49 Overview of Framework Purpose and benefits Communication Inter-operability System engineering benefits Reusability Search Reliability Specification Maintenance Knowledge acquisition Role of the ontology Three level of information L 0 : operational data (a process description) L 1 : an ontology (PIF) L 2 : ontology representation language (Ontolingua) To share or exchange information at L n requires reference to a model at level L n+1 Jon Atle Gulla Ontology Introduction 50 25

26 Overview of Framework Actors Ontology Author (OA) Data Author (DA) Application Developer (AD) Application User (AU) Knowledge Worker (KW) Supporting technologies Ontology representation languages. Knowledge interchange languages. Translation tools Distributed objects Maturity level Jon Atle Gulla Ontology Introduction 51 Ontology Application Scenarios These scenarios are abstractions of specific ontology applications in industry or research. Four main categories of scenarios Neutral authoring Authoring ontologies Authoring operational data Ontology as specification Common access to information Human communication Data access via shared ontology Data access via mapped ontology Shared services Ontology-based search Jon Atle Gulla Ontology Introduction 52 26

27 Neutral Authoring The main idea is to author an artifact in a single language, and to have that artifact translated into a different format for use in multiple target application. The authored artifact to translate can be an ontology or operational data. Jon Atle Gulla Ontology Introduction 53 Ontology as Specification Author an ontology which models the application domain, and provides a vocabulary for specifying requirements for one or more target application. (building a concept model of a domain in UML) Examples. KADS/CML Protégé Information Modeling Object oriented modeling Software synthesis Maturity Jon Atle Gulla Ontology Introduction 54 27

28 Common Access to Information Use ontologies to enable multiple target applications (or humans) to have access to heterogeneous sources of information (ontology based information integration). Four categories. Human communication Data access via shared ontology Data access via mapped ontology Shared services Jon Atle Gulla Ontology Introduction 55 Human Communication Promote common understanding among knowledge workers. Supporting technologies include ontology editors and browsers. Example: the work flow management coalition reference documents. Maturity: library classification skills have a long history (KWs sharing an ontology in the form of a glossary) Jon Atle Gulla Ontology Introduction 56 28

29 Data Access via Shared Ontology An ontology can be used as an interchange format to enable common access to operational data. Example: Process Interchange Format (PIF) and EcoCyc Maturity: commercial success exists in some context, while in others, the technology is a long way from being mature. Difficult to agree on common ontology Jon Atle Gulla Ontology Introduction 57 Data Access via Mapped Ontology No explicit shared ontology, instead mapping rules are used to define what a term in one ontology means in another ontology. Example: mediator based interoperability Jon Atle Gulla Ontology Introduction 58 29

30 Shared Services Similar to data access via shared ontology, but different in the focus of what is being shared. The ontology defines interfaces in multiple target languages. Example: Using UML to create an ontology for product data management, this ontology is then used to generate interface code for the client and server. Maturity: relatively mature Jon Atle Gulla Ontology Introduction 59 Ontology-Based Search Use an ontology for searching an information repository for desired resources. Example: Yahoo Maturity: Many commercial internet portals are beginning to explore the use of concepts for ontology-based search. Jon Atle Gulla Ontology Introduction 60 30

31 Conclusions The paper presents a framework for understanding ontology applications. We studied The framework Various ontology application scenarios (use cases). Neutral authoring Authoring ontologies Authoring operational data Ontology as specification Common access to information Human communication Data access via shared ontology Data access via mapped ontology Shared services Ontology-based search Jon Atle Gulla Ontology Introduction 61 Summary Ontology ABC Terminology Ontology Languages Semantic Web building blocks URIs and namespaces XML RDF/RDFS Impact of ontology on information systems Classification of ontology applications Jon Atle Gulla Ontology Introduction 62 31