Languages for the Semantic Web
and Semantic Web Services
Current Efforts and Challenges for ASP
Axel Polleres
[email protected]
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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Semantic Web
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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!).
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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Overview
• OWL  Semantic Web
– RDF(S), OWL, SWRL, SWRL FOL
• Semantic Web Services
– SWSL-Rules
– WSML
• Challenges
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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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 <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.
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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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!
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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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!
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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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:
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.
• The "only if" direction is not true!
• I.e., OWL Full adds additional inferences on the same
OWL DL ontology!
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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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:
Woulld need additionally:
Axel rdf:type owl:Thing.
Lea rdf:type owl:Thing .
friend rdf:type owl:ObjectProperty .
Axel friendOf Lea .
² OWL DL
Axel rdf:type :x .
x owl:onProperty friend .
x owl:minCardinality "1"xsd:nonNegativeInteger .
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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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.
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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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
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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Semantic Web Services
- Requirements
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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).
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Currently three major initiatives:
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pre-postconditions
Interfaces
Mediators
Capability
User goals, preferences
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!
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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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.
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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SWSL-Rules
• Semantic Web Service Language initiative
(Micheal Kifer, David Martin, Benjamin Grosof, … DAML
founded plus European participants)
• Ontology/Rules Language:
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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
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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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.
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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SWSL Rules + SWSL FOL
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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"
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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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
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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Web Service Modeling Language
• Four elements of WSMO:
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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
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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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
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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Variants of WSML logical
languages
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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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
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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Variants of WSML (contd.)
• WSML-Rule
– Based on Logic Programming with default negation and FLogic/HiLog syntactical extensions
– Preferred goal/web service modeling language
• WSML-DL
– Based on SHIQ
• WSML-Full
– Combining FOL with minimal models and non-monotonicity
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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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.
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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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
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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WSML-Flight
• Is an extension of WSML-Core
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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
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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WSML-Rule
• Extension of WSML-Flight
• Allows unrestricted use of function symbols
• Non-stratified negation (current suggestion:
use wfs)
• Possibly other features
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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WSML-DL
• Equivalent to SHIQ with datatype extension
• Open questions:
– DL epistemology?
– DL concrete syntax for logic expressions?
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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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
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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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
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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WSML Conclusions
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WSML is concrete language for modeling:
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Variants:
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WSML-Core
WSML-Flight
WSML-Rule
WSML-DL
WSML-Full
Modular, Frame-based
Conceptual syntax vs. Logical Expressions
Syntaxes:
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Ontologies
Web Services
Goals
Mediators
Human readable
XML
RDF
Mapping to OWL
More cautious, less variants than SWSL (which also doesn't explicitly tackle interoperability
with OWL).
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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Summary
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The current W3C recommendations have some unresolved issues
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The discussion in the semantic Web community are often more about syntax than about
semantics
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More expressivity than OWL is necessary, the standards discussion is not over!
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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?/>
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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Additional slide:
• Desirable features of a unified logical framework:
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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.
Dagstuhl Seminar "Nonmonotonic Reasoning, Answer Set Programming and Constraints "
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