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 Polleres

2Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " 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!).

3Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " Overview OWL  Semantic Web – RDF(S), OWL, SWRL, SWRL FOL Semantic Web Services – SWSL-Rules – WSML Challenges

4Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " OWL  Semantic Web RDF(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 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.

5Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " RDFS - OWL Lite/DL/Full inappropriate 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!

6Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " 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!

7Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " 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.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 entailsOWL Full entails the translation of O' with respect to D if the translation of O OWL DL entailsOWL DL entails the translation of O' with respect to D.

8Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " 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.

9Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " 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.

10Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " 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

11Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " 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!

12Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " 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.

13Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " 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

14Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " 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.

15Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " 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"

16Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " 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

17Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " 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

18Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " 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

19Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " Variants of WSML logical languages

20Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " 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

21Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " 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

22Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " 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.

23Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " 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

24Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " 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

25Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " WSML-Rule Extension of WSML-Flight Allows unrestricted use of function symbols Non-stratified negation (current suggestion: use wfs) Possibly other features

26Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " WSML-DL Equivalent to SHIQ with datatype extension Open questions: – DL epistemology? – DL concrete syntax for logic expressions?

27Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " 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

28Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " 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

29Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " 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).

30Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " 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!

31Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints " 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.