Cornell CS 502 20020307

Semantic WebOntologies & Data Models

CS 502 – 20020307Carl Lagoze – Cornell University

Acknowledgements:Eric MillerDieter Fensel

Cornell CS 502 20020307

Components of the Semantic Web

Cornell CS 502 20020307

What is an Ontology?

• A formal specification of conceptualization shared in a community

• Vocabulary for defining a set of things that exist in a world view

• Formalization allows communication across application systems and extension

• Parallel concepts in other areas:– Domains: database theory– Types: AI– Classes: OO systems– Types/Sorts: Logic

• Global vs. Domain-specific

Cornell CS 502 20020307

XML and RDF are ontologically neutral

• No standard vocabulary just primitives– Resource, Class, Property, Statement, etc.

• Compare to classic first order logic– Conjunction, disjunction, implication, existential,

universal quantifier

Cornell CS 502 20020307

Components of an Ontology

• Vocabulary (concepts)• Structure (attributes of concepts and

hierarchy)• Logical characteristics of concepts & attributes

– Domain and range restrictions– Properties of relations (symmetry, transitivity)

Cornell CS 502 20020307

Wordnet

• On-line lexical reference system, domain-independent

• >100,000 word meanings organized in a taxonomy with semantic relationships– Synonymy, meronymy, hyponymy, hypernymy

• Useful for text retrieval, etc.• http://www.cogsci.princeton.edu/~wn/online/

Cornell CS 502 20020307

CYC

• Effort in AI community to accommodate all of human knowledge!!!

• Formalizes concepts with logical axioms specifying constraints on objects and classes

• Associated reasoning tools• Contents are proprietary but there is OpenCyc

– http://www.opencyc.org/

Cornell CS 502 20020307

Ontologies for the Web

• Lots of Participants and $$$– Web Ontology Working Group– Distributed Agent Markup Language– Ontology Inference Layer– OntoWeb– Schemas Project

• DAML+OIL – develop standard for encoding ontologies on top of RDF Schema

Cornell CS 502 20020307

Extending RDF(S) with DAML+OIL

RDFS DAML+OIL

•Class, sub-class definition

•Property (attribute), sub-property definition

•Domain, range constraints

•Class definition: Conjunction, disjunction, negation

•Property constraints: universality, existence, cardinality

•Properties of properties: transitivity, symmetry

Cornell CS 502 20020307

DAML class building operations

• disjointWith– No vegetarians are carnivores

• sameClassAs (equivalence)• Enumerations (on instances)

– The Ivy League is Cornell, Harvard, Yale, ….

• Boolean set semantics (on classes)– Union (logical disjunction)– Intersection (logical conjunction)– complimentOf (logical negation)

• All non-carnivores are vegetarians

Cornell CS 502 20020307

DAML property building operations & restrictions

• Unique Property: subject identifies object• Unambiguous Property: object identifiers

subject• Inverse of

– hasChild is inverse of hasParent

• Transitivity (e.g., descendent relationship)• Cardinality (exact, max, min)

Cornell CS 502 20020307

DAML+OIL DataTypes

• Full use of XML schema data type definitions• Examples

– Define a type age that must be a non-negative integer

– Define a type clothing size that is an enumeration “small” “medium” “large”

Cornell CS 502 20020307

DAML+OIL Instance Creation

• Create individual objects filling in slot/attribute/property definitions

<Person ref:ID=“William Arms”><rdfs:label>Bill</rdfs:label><age><xsd:integer rdf:value=“55”/></age><shoesize><xsd:decimal rdf:value=“10.5”/></shoesize>

</Person>

Cornell CS 502 20020307

Language ComparisonDTD XSD RDF(S) DAML+OIL

Bounded lists (“X is known to have exactly 5 children”)

X

Cardinality constraints (Kleene operators) X X XClass expressions (unionOf, complementOf) XData types X XEnumerations X X XEquivalence (properties, classes, instances) XFormal semantics (model-theoretic & axiomatic)

X

Inheritance X XInference (transitivity, inverse) XQualified contraints (“all children are of type person”

X

Reification X X

Cornell CS 502 20020307

Some useful RDF tools

• JENA toolkit for manipulating RDF models– http://www.hpl.hp.com/semweb/jena-top.html

• RDFSviz for visualizing ontologies expressed as RDF schema– http://www.dfki.uni-kl.de/frodo/RDFSViz/

• W3C RDF validation service for parsing and view RDF instances– http://www.w3.org/RDF/Validator/

Cornell CS 502 20020307

But modeling the way things “are” is not always enough

ABC – Modeling how things (or their descriptions) change

Cornell CS 502 20020307

ABC Example

Leo Tolstoy, who was born in Moscow in 1828, authored a manuscript called “War and Peace” in 1855. In 1860, that manuscript as the book “Illustrated War and Peace”, was published by Russia Publishers, with Tolstoy supplying the illustrations.

Cornell CS 502 20020307

ABC Model Overview

• (Digital) objects have inherent lifecycle characteristics– Model creation, evolution, and transformation of

objects over time.

• Notions of temporality are given first-class status

• Measuring utility of metadata through query-ability– Ability to answer who, what, when, where comments

that are difficult in simpler models

• Descriptions provide a world context• From ambiguous to exact information – only say

what you know

Cornell CS 502 20020307

Entities and their Properties

• Entities are anchor points for set of properties• Entities "change" by modification of property

sets– Model does not distinguish between "change of

nature" and "change of description"

• Dual facets of Entities– Universal – object and its property set that is "global"

to the description– Existential – object instances and their property sets

that are periodic ("stateful")

Cornell CS 502 20020307

Situations

• Establish a time period – granularity determined by the longevity of entity

state within situation

• Situations and actualities– Situations provide context for associating entities in

their existential facet.– Entities can exist out of situation to express their

universal properties

Cornell CS 502 20020307

Events

• Transition marker point between situations– Always have a time property

• Levels of increasing knowledge– Something happened at a time (that caused change

in situation)– Type thing happened at some time– Anchor point for multiple actions (verbs) within a

happening.

Cornell CS 502 20020307

Events, Actions, Agents

• Events provide a context for associating actions ("verbs"), 1-n

• Actions provide a context for participation of agents, 1-n

• Participation type can be specialized for domain

Cornell CS 502 20020307

Causality – verbs & predicates

• Association of actions to Actualities in preceding situation is weak

• Increasing knowledge of association (esp. important in rights management)– involves– hasTool– hasPatient

Cornell CS 502 20020307

Intellectual Property

• Notion of "ability to copy" is the determining factor

• Promote abstraction to work and actuality to manifestation and item