WP1: Language Architecture

WonderWeb. Ontology Infrastructure for the Semantic Web. IST-2001-33052Project Review Meeting, 11th March, 2004.

WP1: Language Architecture

Sean Bechhofer

University of Manchester


Languages

• A language standard provides some of the “glue” that allows applications to interoperate.


WP1: Language Architecture

• Development of Ontology Language Layer [D1]– Participation in W3C Web Ontology Language working group

– Development of OWL standard

– Editorship of key documents

• Language Extensions – Query languages

– Rules languages [D2]

• WP1 has strong links with WP2: language design feeds into tool development and the development of tools is crucial to supporting language design.


Ontology Language

• OWL: a standard for a Web Ontology Language [OWL]• Now a W3C Recommendation (as of Feb 2004)

– Use Cases and Requirements – Overview– Guide– Reference– Semantics and Abstract Syntax– Test Cases

• Additional WG Notes– XML Concrete Syntax– Parsing OWL in RDF/XML


OWL Process

• August 2003: Candidate Recommendation– Exit criteria included implementation experience demonstrating that

the specifications are implementable.

• December 2003: Proposed Recommendation• Feb 2004: Recommendation

– WG Note on parsing


WonderWeb Contributions

• Members of the WonderWeb consortium made significant contributions to the work of WebOnt

• GS: co-chair of the working group– Use Cases and Requirements (RV)

– Overview (FvH)

– Guide (RV)

– Reference (FvH, IH, SB)

– Semantics and Abstract Syntax (IH)

– Test Cases (IH, SB)

– Parsing Note (SB)


OWL Layering

• OWL has a layered architecture with successive layers providing more expressivity.

• OWL Full corresponds to RDF.• OWL DL is OWL restricted to a DL/FOL fragment,

allowing the use of DL reasoning techniques.• OWL Lite has further restrictions intended to ease

implementation and provide easy entry for those familiar with frame-like languages.

• Layered syntax and semantics– DL semantics are normative

Full

DL

Lite


OWL Layering

• OWL Lite– Quantification; Simple number restrictions (0/1)– Subclass and Equivalence axioms relating class names

• OWL DL– Boolean expressions; Arbitrary number restrictions– Axioms relating arbitrary descriptions– Disjointness

• OWL Full– No restrictions on separation of interpretations (class-as-instance,

class-as-property etc.)– Redefinition of built-in vocabulary allowed


Layering and Species Recognition

• All OWL species are represented using RDF.• Thus a key task is species recognition – determining when an

RDF document is in the DL or Lite fragment.– Not just checking whether vocabulary is present but how

vocabulary is used.

• This allows applications to use appropriate reasoning technology.


WonderWeb Contributions

• Tools developed during the project (WP2) were crucial to the success of the standardisation activity.

• W3C standardisation requires demonstration of implementation experience, in particular:– Implementations of syntax checkers and recognisers.

• OWL API including OWL Validator

– Implementations of reasoners.

• FaCT++

• Hoolet (1st Order reasoner)


OWL API

• The OWL API provides programmatic access to OWL ontologies. [BVL03]

• Although this could be considered part of WP2 activity, the API has been important in promoting the use of OWL.

• Includes RDF Parser and Validator [BC04]– demonstration that the specifications are implementable – useful in education and explanation – why are ontologies not in

OWL DL?– framework for implementation of reasoners, again a key

requirement of the standardisation activity.

• Crossover interest from other communities– OMG’s RFP for Ontology Definition Metamodel


Rules: SWRL

• SWRL: A proposal for a Semantic Web Rule Language [HP04]• IH proposal editor.• Extends OWL with Horn-like rules• Rules can make use of OWL descriptions in both head and

body• Currently produced under the auspices of the Joint US/EU ad

hoc Agent Markup Language Committee• Soon to be W3C Note, which can then provide a starting point

for forthcoming W3C Semantic Web Rules WG• Model-theoretic semantics (extension of OWL DL semantics).


Rules

• Extends OWL expressivity, allowing inference of relations:– hasParent(?x1,?x2) hasBrother(?x2,?x3) hasUncle(?x1,?x3)

– An uncle is the brother of a parent.

• Extends rules to allow existential quantification in rule heads:– HighEarner(?x) spouse(?x, ?y) earns(?x, ?a) earns(?y, ?a)

owns.FastCar(?x)

– If you’re a high earner and you earn the same amount as your spouse, then you own a fast car.

spouse

earns

earns

owns


Rules: DLP

• An investigation of the use of logic programming for OWL reasoning. [GHVD03, V03, VSM03]

• Semantics-preserving translation of a fragment of OWL into Prolog.– SubClassOf( intersectionOf( Genius Composer)

restriction( hasComposed allValuesFrom ( Masterpiece ))

– Masterpiece(Y) :- Genius(X), Composer(X), hasComposed(X,Y)

• Is the fragment sufficiently expressive for realistic ontologies?

– Empirical analysis of ontologies available on the web.


Query Languages

• DQL (DAML Query Language) now updated as OWL Query Language

• IH proposal editor.• Will form input document to W3C’s Data Access WG to be

formed early 2004.• Query Example:

– Query: (“Who owns a red car?”)Query Pattern: {(owns ?p ?c) (type ?c Car) (has-color ?c Red)}Must-Bind Variables List: (?p)May-Bind Variables List: (?c)


Next Steps

• Further Working Groups– Semantic Web Best Practice (GS)– Data Access– Rules

• Prototype implementations of SWRL based on 1st order reasoners.

• Further Query Language investigations• Further language extensions:

– Complex roles [HS03]– Concrete datatype reasoning [PH03]– Keys [LAHS03]


Relevant Publications

• [D1] WonderWeb Deliverable D1: Ontology Language• [D2] WonderWeb Deliverable D2: Rules Language• [OWL] OWL Standardisation Documents

– Technical Reports– WG Notes

• [BC04] Sean Bechhofer and Jeremy J. Carroll. OWL DL: Trees or triples? To appear in WWW2004.

• [BVL03] Sean Bechhofer, Raphael Volz, and Phillip Lord. Cooking the Semantic Web with the OWL API, ISWC 2003

• [HP04] Ian Horrocks and Peter F. Patel-Schneider. A proposal for an OWL rules language. To appear in WWW2004.

• [HPH03] Ian Horrocks, Peter F. Patel-Schneider, and Frank van Harmelen. From SHIQ and RDF to OWL: The making of a web ontology language. Journal of Web Semantics, 1(1):7–26, 2003.

• [GHVD03] Benjamin N. Grosof, Ian Horrocks, Raphael Volz, and Stefan Decker. Description logic programs: Combining logic programs with description logic. WWW2003


Relevant Publications

• [HS03] Ian Horrocks and Ulrike Sattler. The effect of adding complex role inclusion axioms in description logics. IJCAI 2003

• [LAHS03] Carsten Lutz, Carlos Areces, Ian Horrocks, and Ulrike Sattler. Keys, nominals, and concrete domains. IJCAI 2003

• [PH03] Jeff Pan and Ian Horrocks. Web ontology reasoning with datatype groups. ISWC2003

• [V03] Raphael Volz. Web Ontology Reasoning with logic databases. PhD thesis, Universitaet Karlsruhe (TH), February 2004.

• [VSM03] Raphael Volz, Steffen Staab, and Boris Motik. Incremental maintenance of dynamic datalog programs. PSSS2003

• [VSM03a] Raphael Volz, Steffen Staab, and Boris Motik. Incremental Maintenance of Materialized Ontologies. ODBASE2003