1dagstuhl seminar "nonmonotonic reasoning, answer set programming and constraints "...

1Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "

Languages for the Semantic Web and Semantic Web Services

Current Efforts and Challenges for ASP

Axel [email protected]


Semantic Web• Publish machine-processable meta-data on the Web (semantic Web idea!)!• Provide the means to publish data on relations of resources and taxonomies of data on the Web• Provide standards on top of XML to describe the meaning of published knowledge• This meta-data shall ideally be consensual (Ontologies!).


Overview

• OWL Semantic Web– RDF(S), OWL, SWRL, SWRL FOL

• Semantic Web Services– SWSL-Rules – WSML

• Challenges


OWL Semantic WebRDF(S), OWL, SWRL, SWRL FOL

• RDF(S) W3C Recommendation, latest version,10 February 2004

– simple taxonomies, express structured knowledge in a graph made up of <subject predicate object> triples

• OWL W3C Recommendation, latest version,10 February 2004

• SWRL W3C member submission, 21 May 2004

– extends OWL DL by simple rules, quite restrictive, but undecidable already.

• This is not all! Several drawbacks, several alternative suggestions. Let's see the results of the current Rules Workshop going on this minute in Washington.


RDFS - OWL Lite/DL/Fullinappropriate Layering

• OWL DL is not properly layered on top of RDFS syntactically…

• … even worse: Given the same (OWL DL) ontology there are things semantically entailed by OWL Full, not entailed by OWL Lite…

… something's strange here!


OWL DL is not properly layered on top of RDFS syntactically:

• OWL does not smoothly integrate:• No meta-statements in OWL Lite and DL (i.e.

separate vocabulary:

RDF Triples:

hansi rdf:type eagle.

eagle rdf:type species.

Not possible in OWL Lite and DL!


Given the same (OWL DL) ontology there are things semantically entailed by OWL

Full, not entailed by OWL DL…• From OWL Semantics and abstract Syntax document:

• The "only if" direction is not true!

• I.e., OWL Full adds additional inferences on the same OWL DL ontology!

Theorem 2: Let O and O' be collections of OWL DL ontologies and axioms and facts in abstract syntax form that are imports closed, such that their union has a separated vocabulary. Given a datatype map D that maps xsd:string and xsd:integer to the appropriate XML Schema datatypes and that includes the RDF mapping for rdf:XMLLiteral, then the translation of O OWL Full entails the translation of O' with respect to D if the translation of O OWL DL entails the translation of O' with respect to D.


OWL DL entails less than OWL full:Axel friendOf Lea . ² OWL Full

Axel rdf:type :x .

: x owl:onProperty friend .

: x owl:minCardinality "1"xsd:nonNegativeInteger .

But:

Axel friendOf Lea . ² OWL DL

Axel rdf:type :x .x owl:onProperty friend .x owl:minCardinality "1"xsd:nonNegativeInteger .

Woulld need additionally:Axel rdf:type owl:Thing.Lea rdf:type owl:Thing .friend rdf:type owl:ObjectProperty .


SWRL• OWL unsatisfactory expressivity, not even simply rules like:

parent(?x,?y) brother(?y,?z) uncle(?x,?z)∧ ⇒

• SWRL extends OWL with a simple rule language which allows concept (unary) and role (binary) predicates as well as sameAs (equality) and differentFrom (inequality) as atoms.

• Undecidable, if not further restricted (e.g. to Description logic programs)

• SWRL FOL W3C member submission April 11th:Proposal to loosen some of the restrictions of SWRL to handle function-free handle unary/binary first-order logic, n-ary predicates only via reification.


RDF(S), OWL, SWRL syntax

• All of these standards offer XML exchange syntaxes

• RDF exchange syntaxes: OWL/RDF, SWRL/RDF exchange syntax are not really useful: puts OWL constructs inside an RDF graph themselves, causes problems with OWL DL and OWL Full semantic interoperability


Semantic Web Services- Requirements

• Semantic descriptions of the functional and behavioral aspects of Web Services to enable (semi-)automatic discovery, composition and execution of Web Services, build upon Web Services technologies (SOAP, WSDL, UDDI).

• pre-postconditions• Interfaces• Mediators• Capability• User goals, preferences

• Currently three major initiatives:– OWL-S … an OWL ontology to semantically describe Web Services (OWL)– WSMO/WSML/WSMX … a general framework for SWS description (own logical

language)– SWSL … a framework for SWS description based on OWL (but also own language)

• Another W3C workshop in June in Innsbruck:Frameworks for Semantic Web Services … deadline for position papers extended to Friday

April 29th!


Why OWL is not enough for SWS?

• Permit FOL for ontologies beyond DL/OWL: Need to express complex conditions, rules, trust policies for contracting, nonmon. features, prioritization, dynamics

• Integrate nonmon., frame/OO, DDB ontologies with mon. DL/FOL ontologies

• Cope robustly with conflict between ontologies, e.g., merging OWL ontologies from many sources

• OWL-S does not define the semantics of dynamics, proposes to allow DRS,KIF,SWRL for expressing pre-conditions, effects, etc. but does not specify the semantics.


SWSL-Rules• Semantic Web Service Language initiative

(Micheal Kifer, David Martin, Benjamin Grosof, … DAML founded plus European participants)

• Ontology/Rules Language:– SWSL Rules – LP with NAF; Courteous, Hilog extensions– SWSL FOL– Shared presentation syntax; builds on F-Logic– Markup syntax – TBD probably with RuleML committee

• W3C member submission planned


SWSL Language Layers:

• Strong Consensus: Need Nonmonotonic LP. And FOL.– “SWSL-Rules” = the LP KR. – “SWSL-FOL” = the FOL KR.

• Expressive Features for SWSL are similar to those desired for SW rules in general, but with bit different near-term importance/urgency:– Important in both: Prioritization, NAF (cf. Courteous LP)– Important in both, more urgent in SWS than SW overall:

Meta- power/convenience: Hilog, frame syntax (cf. F-Logic)– A bit more important in SWS than SW overall: Lloyd-Topor

(nested expressions)– Reification: meta-knowledge/modeling, mentioned already

in RDF, but no semantics for it.


SWSL Rules + SWSL FOL

• Semantics for rules part and FOL part separate• Exchange syntax RuleML• New fundamental KR theory is needed to unify nonmon. LP with FOL

– "A holy grail for SWS, and for SW generally"


WSML (joint work with Jos de Bruijn, Holger Lausen, Dieter Fensel, Michael Kifer) • Developed as joint effort in several EU Projects (dip, SEKT,

KnowledgeWeb)

• Based on Web Service Modeling Ontology WSMO

• Also has its own rules language, similarities with SWSL

• W3C member Submission pending


Web Service Modeling Language

• Four elements of WSMO:– Ontologies– Goals– Web Services– Mediators

• WSML provides a formal grounding for the conceptual elements of WSMO, based on:– Description Logics– Deductive Databases– First-Order Logic


Syntaxes for WSML

• Human-readable syntax– Layered syntax– Inspired by OIL/OWL and F-Logic– Two flavors:

• Conceptual syntax• Logical Expression Syntax

– Semantics is fixed in WSML variants

• XML syntax• RDF syntax• Mapping to OWL


Variants of WSML logical languages


Variants of WSML (contd.)

• WSML-Core– Based on the intersection of Description Logics and Datalog– Has (frame-based) conceptual syntax and logical expression

syntax

• WSML-Flight– Layered on top of WSML-Core

• Basic meta-class facility• Constraints• Non-monotonic features (default negation)

– Provides more intuitive modeling constructs (for people with DB/SE background)

– Preferred ontology modeling language– Based on Datalog with stratified negation and inequality


Variants of WSML (contd.)

• WSML-Rule– Based on Logic Programming with default negation and F-

Logic/HiLog syntactical extensions– Preferred goal/web service modeling language

• WSML-DL– Based on SHIQ

• WSML-Full– Combining FOL with minimal models and non-monotonicity


WSML Logical Expressions• Elements:

– Function symbols (e.g. f())– Variables (e.g. ?x)– The syntax is based on F-Logic style molecules, e.g.

Human subClassOf Animal.

Axel memberOf Human,

Axel[name hasValue “Axel Polleres”].– Predicates (e.g. distance(?x, ?y,?z))– Logical connectives (or, and, not, impliedBy, equivalent, implies,

forall, exists, :-, !-), – ASCII, but readable, more or less directly translatable to XML

tags.

• Example:?x memberOf Human equivalent ?x memberOf Animal and ?x

memberOf LegalAgent.


WSML-Core

• Allows conceptual modeling of ontologies• Based on Description Logic Programs subset of OWL

– i.e., efficient query answering– Should be easily adopted in existing implementations (e.g. XSB,

OntoBroker, SWI-Prolog, KAON, DLV)– Import/export OWL ontologies– Datatype support based on XML Schema datatypes and OWL-E

• Expressive enough for most current ontologies• Properly layered on top of RDF/RDFs worked upon, currently

restrictions on the use of RDF(S) vocabulary.• Can be used for limited goal/web service modeling


WSML-Flight

• Is an extension of WSML-Core– Adds limited support for nominals– Stays in LP world– Meta-modeling– Adds inequality (plus UNA!)– Adds constraints (wrt. local knowledge base)– Adds stratified non-monotonic negation

• Allows arbitrary safe Datalog rules (i.e. no function symbols; rules must be safe)

• Language is based on Datalog with inequality, constraints and stratified negation


WSML-Rule

• Extension of WSML-Flight

• Allows unrestricted use of function symbols• Non-stratified negation (current suggestion:

use wfs)• Possibly other features


WSML-DL

• Equivalent to SHIQ with datatype extension

• Open questions:– DL epistemology?– DL concrete syntax for logic expressions?


WSML-Full

• Based on a combination of First-Order Logic and minimal model semantics and default negation

• Unifies rule language with first-order based language

• Possible formalisms:– Autoepistemic Logic– Default Logic– Circumscription


WSML Goals and Web Services

• Goal / Web Service assumptions/effects and pre/post-conditions are defined through WSML logical expressions

• Logical expressions rely on ontologies

• Use of ontologies through– Ontology import– Mediation

• Open issue: semantics of dynamics, interfaces, grounding


WSML Conclusions

• WSML is concrete language for modeling:– Ontologies– Web Services– Goals– Mediators

• Variants:– WSML-Core– WSML-Flight– WSML-Rule– WSML-DL– WSML-Full

• Modular, Frame-based• Conceptual syntax vs. Logical Expressions• Syntaxes:

– Human readable– XML– RDF– Mapping to OWL

• More cautious, less variants than SWSL (which also doesn't explicitly tackle interoperability with OWL).


Summary

• The current W3C recommendations have some unresolved issues

• The discussion in the semantic Web community are often more about syntax than about semantics

• More expressivity than OWL is necessary, the standards discussion is not over!

• Challenges: For the "ontological reasoning" part:– Fix semantic layering mess on top of RDF/RDFS– Semantic Interoperability of Knowledge bases building on different paradigms. – Engines: Support/frontends for XSD/XPath built-ins, Frame-based syntax. Efficient, modular

reasoning support.– Implies: Accept incomplete reasoning in some cases.– How to unify the DL, FOL and LP worlds? What is the semantics of a unifying umbrella language!?!– Reasoning with networks of ontologies, rewriting, etc.For the Web Services Part:– Formalizing dynamic aspects of Semantic Web Services (pre-/postc., interfaces) in a way

interoperable with ontologies:Idea: use of action theories, action languages, mappings from/to process languages, etc.

Thank you!<questions?/>


Additional slide:

• Desirable features of a unified logical framework:– arbitrary use of neg/naf?– Combination of wfs/sm knowledge bases?– Nested expressions + quantifiers– Define easily checkable layered syntactical restrictions with

increasing expressive power and computational properties (also WITHIN LP, tight, etc.) in order to support modular reasoner-support.

1dagstuhl seminar "nonmonotonic reasoning, answer set programming and constraints "...

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