OWL 2 Web Ontology Language

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/


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


So what?

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


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


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


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!


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


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


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.


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


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


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