OWL 2 Web Ontology Language

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Extracted from the W3C Working Draft on OWL 2 Web. Ontology Language ... E.g., if a man and woman are married to each other, they can't also be friends ☺.
OWL 2 Web Ontology Language: A Quick Introduction

Kavi Mahesh Principal Consultant, Knowledge Management Education and Research Group, Infosys Technologies Ltd., Electronics City, Hosur Road, Bangalore, 560100 INDIA © 2009 Infosys Technologies Limited

© 2009 Infosys Technologies Limited

Source

Extracted from the W3C Working Draft on OWL 2 Web Ontology Language released in December 2008 http://www.w3.org/TR/2008/WD-owl2-new-features-20081202/

© 2009 Infosys Technologies Limited

New Features in OWL 2 (To be) Supported by new algorithms and tool vendors:

•extra syntactic sugar, •additional property and qualified cardinality constructors, •extended datatype support, •simple metamodeling, and •extended annotations

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So what?

Please see the concluding slides for an opinion on the impact of OWL 2 for our purposes

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1. Extra Syntactic Sugar • DisjointUnion • Shortcut for UnionOf and DisjointWith (declaring a class as a union of mutually-disjoint subclasses) • We don’t care • DisjointClasses • Shortcut for repeated pairwise DisjointWith statements • We don’t care • NegativeObjectPropertyAssertion • Stating a negative fact about a class • Wasn’t possible in OWL 1 • Nice, but not very useful • NegativeDataPropertyAssertion • Same thing for data properties • Nice, but not very useful

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2. New Property Constructors • ObjectHasSelf • Defining a class using a self restriction, i.e., a constraint on a reflexive property • We don’t care • ObjectMinCardinality, ObjectMaxCardinality and ObjectExactCardinality • Also allows further constraints on the values (called qualified cardinality, e.g., people who have two children, both girls) • In OWL 1, whenever cardinality was exact, we had to state both min and max cardinalities with equal values • This may be a useful capability • ReflexiveObjectProperty, IrreflexiveObjectProperty and AsymmetricObjectProperty • OWL 1 only had SymmetricProperty and TransitiveProperty • The key is to be able to reason using these; hence, we don’t care much

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2. New Property Constructors (contd.) • DisjointObjectProperties • Two objects cannot be related by two (or more of these) properties • E.g., if a man and woman are married to each other, they can’t also be friends  • This may be useful in some situations • PropertyChain • Deals with the inheritance of properties via part-of relations • This again may be useful in some cases • HasKey • An inverse-functional datatype property (as in relational databases) • Very useful!

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3. Extended Datatype Capabilities • (Unary) datatypes • Less dependence on XML Schema datatypes (xsd:) • More number data types in OWL itself; some are outside of xsd: • Strings with a language specified using rdf:text datatype • Booleans, binary data, time instants, etc. • Range constraints had to be in xsd in OWL 1; OWL 2 can use facets: e.g., minInclusive • N-ary datatypes • Specifying constraints across values of two or more properties of the same object

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4. Metamodeling Features • Modeling with meta-classes • A set of instances (i.e., a class) is considered an instance of a meta-class • E.g., Tiger is a class of animals but an instance of the EndangeredSpecies class • OWL 2 allows “punning”: overloading a class name

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5. Extended Annotations • Anything can be annotated • Annotations of annotations, etc. • AnnotationPropertyDomain, AnnotationPropertyRange and SubAnnotationPropertyOf • Forward declarations of entities is required • Note: No backward compatibility of owl files! (see also no frame-like syntax below …) • Yet they say: The OWL Working Group intends to make OWL 2 be a superset of OWL 1, except for some small bug fixes and changes to the formal semantics of annotations. This means that OWL 2 will be backward compatible, and creators of OWL 1 documents need only move to OWL 2 when they want to make use of OWL 2 features. More details and advice concerning migration from OWL 1 to OWL 2 will be in future drafts.

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OWL 2 Profiles: The Big Difference • In place of the three languages • OWL Lite, OWL DL and OWL Full • OWL 2 has three "profiles" • OWL EL (aka OWL-EL++) • For large, simple ontologies with efficient reasoning • Complexity: Maximal language for polynomial reasoning • No value or cardinality constraints!!

• OWL QL (aka OWL-DL – Lite) • • • • •

Large datasets and interoperability with RDBMS and SQL No ObjectMinCardinality and ObjectExactCardinality Complexity: Maximal language for logspace reasoning (same as DB) Data in DBMS, queries translated directly to SQL IDEALLY SUITED FOR MOST OF OUR NEEDS!

• OWL RL (aka OWL-R) • Interoperable with rules engines where query answering is the primary task • Complexity: Polynomial reasoning

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Some Minor Differences • No frame-like syntax • This is a step backward (IMHO) towards predicate-logic-like syntax  • OWL 2 will not look object-oriented (not even as much as OWL 1) • Inverse property expressions • No need to name an inverse property since property chains are allowed

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Conclusion (i.e, Opinions) • Not a great path of evolution • Too much influence of logical reasoning communities • Most extensions are useless to us • Except the introduction of keys and better support for cardinality constraints • Disturbing to see how unstable OWL is as a standard • Too many versions; how will standard tools come up? • No backward compatibility • Major change in sublanguages of OWL (3 new languages!) • Nice to see that there is so much interest in OWL © 2009 Infosys Technologies Limited

Perhaps we need an OWL (or OWL 2?) ontology of OWLs! Thank You © 2009 Infosys Technologies Limited