state of the art in semantic web standards and technologies andreas duscher

State of the Art in Semantic Web standards

and technologies

Andreas Duscher

State of the Art in Semantic Web Standards

and Technologies

General Overview RDF / RDF Schema OWL OWL-S Final words

Today´s web It is designed for human consumption Information retrieval is mainly supported by keyword-based

search engines Some problems with information retrieval:

High recall, low precision Low or no recall Results are highly sensitive to vocabulary

Web content is not machine-proccessable„I am a professor of computer science.“-- or --

„I am a professor of computer science, you may think. Well….“

Semantic Web Vision

„The Semantic Web provides a common framework that allows data to be shared and reused across application, enterprise, and community boundaries.“[http://www.w3.org/2001/sw/]

Problem „Business-to-consumer electronic commerce“• Manually retrieving the best offers from different online-shops is too time-consuming.• Tools for shoping are available in the form of shop bots.• For every online shop a wrapper is needed (information is extracted through text analysis).

Vision „Business-to-consumer electronic commerce“• The user asks a autonomously acting piece of software for a certain product.• The software retrieves all offers and compares them with the user‘s preferences.• If needed the sofware negotiates with the shop for a special discount or tries to get a trusted rating for the shop.

Semantic Web Technologies

Ontologies An ontology describes formally a domain of

discourse. It is a finite list of terms and the relationship

between these terms. Types of relationship: subclass hierarchy,

properties, value restrictions, logical relationships between objects

„In the context of web an ontology provide a shared understanding of a domain.“


Logic „A discipline that studies the principles of reasoning.“ Automated reasoners allow to draw conclusions from

given knowledge, make implicit knowledge explicit.

prof(X) -> faculty(X)faculty(X) -> staff(X)prof(michael)


Logic „A discipline that studies the principles of reasoning.“ Automated reasoners allow to draw conclusions from

given knowledge, make implicit knowledge explicit.

prof(X) -> faculty(X)faculty(X) -> staff(X)prof(michael)

faculty(michael)staff(michael)prof(X) -> staff(X)

This example involves knowledge typically found in ontologies.


Agents Agents are pieces of software that work autonomously

and proactively. A personal agent would recieive some task and

preferences from the user, communicate with other agents, compare information and select certain choices.

Web services collection of protocols and standard for exchanging

data between various applications


How it fits together? Onotologies can be used to represent knowledge, interpret

the retrieved information and communicate with other agents.

Logic can be used for processing the retrieved information and for drawing conclusions.

Agent / Web service technologies allow the communication between different systems and the composition of complexer services from simple ones.


How it fits together? Onotologies can be used to represent knowledge, interpret

the retrieved information and communicate with other agents. => RDF / RDF Schema / OWL

Logic can be used for processing the retrieved information and for drawing conclusions. => Defining an OWL-based language is in progress!

Agent / Web service technologies allow the communication between different systems and the composition of complexer services from simpler ones. => OWL-S

RDF Motivation

The Resource Description Framework (RDF) is a language for representing resources in the World Wide Web.

RDF is intended for situations in which this information needs to be processed by applications, rather than being only displayed to people.

RDF is based on the idea of identifying things using Web identifiers (URIs).

RDF Basic Concepts

the thing the statement describes (the web page`s URL) a specific property of the thing (e.g. creator) the concrete message the statement wants to give,

in other words the value of the property (John Smith)

RDF basic ideas Things being described have properties, which have values Resources can be described by making statements

(similar to the above example)

http://www.example.org/index.html has a creator whose value is John Smith

Example

„Imagine trying to state that someone named John Smith created a particular Web page.“

RDF Basic Concepts

RDF terminology the part that identifies the thing the statemant is about is called subject the part that identifies the property is called predicate the part that identifies the value of the property is called object

Subject ObjectPredicate

RDF Basic Concepts

RDF terminology the part that identifies the thing the statemant is about is called subject the part that identifies the property is called predicate the part that identifies the value of the property is called object

Subject ObjectPredicate


the subject is the URL „http://www.example.org/index.html“ the predicate is the word „creator“ the object is the name „John Smith“

RDF Basic Concepts

To make these statements machine-proccessable

two things are needed: a system of machine-processable identifiers (for subjects, predicates

and objects) without any possibilty of confusion between similar looking identifiers

RDF Basic Concepts




a machine-processable language for representing these statements and exchanging them between machines

RDF Basic Concepts





Uniform Resource Identifiers (URI) allow to identify and uniquely name things - even if they have no network-accessible location.

RDF Basic Concepts





RDF defines a XML markup language, named RDF/XML, which allows to represent RDF statements.

Uniform Resource Identifiers (URI) allow to identify and uniquely name things - even if they have no network-accessible location.

RDF Model

As mentioned: RDF makes statements about resources Each statement consists of a subject, a predicate and an object


http://www.example.org/staffid/5232

http://purl.org/dc/elements/1.1/creator

http://www.example.org/index.html

RDF Model

As mentioned: RDF makes statements about resources Each statement consists of a subject, a predicate and an object




http://www.example.org/index.htmlsubject

object

predicate

RDF Model

Conclusion RDF documents are „nodes-and-arcs diagrams

interpreted as statements about things identified by URIrefs“.

So subjects, predicates and objects can be identified by URIrefs.

RDF Syntax

http://www.example.org/terms/creation-date


August 16, 1999

<?xml version="1.0"?><rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:exterms="http://www.example.org/terms/">

<rdf:Description rdf:about="http://www.example.org/index.html"><exterms:creation-date>August 16, 1999</exterms:creation-

date> </rdf:Description>

</rdf:RDF>

RDF Syntax

http://www.example.org/terms/creation-date

August 16, 1999

<?xml version="1.0"?><rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"

xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:exterms="http://www.example.org/terms/">

<rdf:Description rdf:about="http://www.example.org/index.html">

<exterms:creation-date>August 16, 1999</exterms:creation-date> <dc:creator rdf:resource="http://www.example.org/staffid/85740"/> </rdf:Description>

</rdf:RDF>




RDF SyntaxAbbreviating and Organizing RDF URIrefs

<?xml version="1.0"?><!DOCTYPE rdf:RDF [<!ENTITY xsd "http://www.w3.org/2001/XMLSchema#">]>

<rdf:RDF xmlns:rdf=http://www.w3.org/1999/02/22-rdf-syntax-ns# xmlns:exterms="http://www.example.com/terms/">

<rdf:Description rdf:ID="item10245"><exterms:model rdf:datatype="&xsd;string">Overnighter</exterms:model><exterms:sleeps rdf:datatype="&xsd;integer">2</exterms:sleeps><exterms:weight rdf:datatype="&xsd;decimal">2.4</exterms:weight> <exterms:packedSize rdf:datatype="&xsd;integer">784</exterms:packedSize>

</rdf:Description>

rdf:about and rdf:ID are strictly speaking the same. rdf:about is often used for talking about resources that have been defined elsewhere.

The value of rdf:ID can only appear once in a document. The fragment identifier item10245 will be interpreted relative to a base URI.

RDF SyntaxTyping in RDF



<rdf:Description rdf:ID="item10245"><rdf:type rdf:resource="http://www.example.com/terms/Tent"/><exterms:model rdf:datatype="&xsd;string">Overnighter</exterms:model><exterms:sleeps rdf:datatype="&xsd;integer">2</exterms:sleeps>…

</rdf:Description>

RDF allows to classify resources with the special attribute rdf:type. The resource item10245 is called a tpyed node. It is similar to the programming language concept of objects and classes

=> RDF Schema (RDFS)

RDF SyntaxTyping in RDF (abbreviated form)



<exterms:Tent rdf:ID="item10245">

<exterms:model rdf:datatype="&xsd;string">Overnighter</exterms:model><exterms:sleeps rdf:datatype="&xsd;integer">2</exterms:sleeps>…

</exterms:Tent>

RDF allows to classify resources with the special attribute rdf:type. The resource item10245 is called a tpyed node. It is similar to the programming language concept of objects and classes

=> RDF Schema (RDFS)

RDFS Basics

What it is not RDF Schema does not make any assumptions about any

application domain, nor does it define the semantics.

What it is RDF Schema provides a vocabulary to describe classes of

things and/or resources. Vocabulary descriptions written in RDF Schema language

are legal RDF graphs.

It is up to the user to devolpe a RDF Schema (RDFS) for the needed application domain.

RDFS Basics Own namespace http://www.w3.org/2000/01/rdf-schema# Core classes

rdfs:Resource, the class of all resouces rdfs:Class, the class of all classes rdf:Property, the class of all properties

Properties can be used to characterize concrete classes

In RDF Schema, a class is any resource having an rdf:type property whose value is the resource rdfs:Class.

RDFS Basics Core properties

rdf:type, relates a resource to its classSo the resource is an instance of the class

rdfs:subClassOf, relates a class to one of its superclasses rdfs:subPropertyOf, relates a property to one of its superproperties

The above properties are instances of the class rdf:Property. Own properties can be defined by assigning the type rdf:Property to any kind of resource.

RDFS Example (Classes)<?xml version="1.0"?><!DOCTYPE rdf:RDF [<!ENTITY xsd "http://www.w3.org/2001/XMLSchema#">]>

<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#" xml:base="http://example.org/schemas/vehicles">

<rdf:Description rdf:ID="MotorVehicle"> <rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-chema#Class"/> </rdf:Description>

<rdf:Description rdf:ID="PassengerVehicle"> <rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/> <rdfs:subClassOf rdf:resource="#MotorVehicle"/></rdf:Description>

<rdf:Description rdf:ID="Truck"> <rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/> <rdfs:subClassOf rdf:resource="#MotorVehicle"/></rdf:Description>

<rdf:Description rdf:ID="Van"> <rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/> <rdfs:subClassOf rdf:resource="#MotorVehicle"/> </rdf:Description>

<rdf:Description rdf:ID="MiniVan"> <rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/> <rdfs:subClassOf rdf:resource="#Van"/> <rdfs:subClassOf rdf:resource="#PassengerVehicle"/></rdf:Description>

</rdf:RDF>



<rdf:Description rdf:ID="MotorVehicle"> <rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-chema#Class"/> </rdf:Description>

<rdf:Description rdf:ID="PassengerVehicle"> <rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/> <rdfs:subClassOf rdf:resource="#MotorVehicle"/></rdf:Description>

<rdf:Description rdf:ID="Truck"> <rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/> <rdfs:subClassOf rdf:resource="#MotorVehicle"/></rdf:Description>

<rdf:Description rdf:ID="Van"> <rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/> <rdfs:subClassOf rdf:resource="#MotorVehicle"/> </rdf:Description>

<rdf:Description rdf:ID="MiniVan"> <rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/> <rdfs:subClassOf rdf:resource="#Van"/> <rdfs:subClassOf rdf:resource="#PassengerVehicle"/></rdf:Description>

</rdf:RDF>

resource with an unique id

property „rdf:type“ that defines this resource as „Class“

property „rfds:subClassOf“ with a resource as value, in this case a formerly defined class



<rdfs:Class rdf:ID="MotorVehicle"/>

<rdfs:Class rdf:ID="PassengerVehicle"> <rdfs:subClassOf rdf:resource="#MotorVehicle"/></rdfs:Class>

<rdfs:Class rdf:ID="Truck"> <rdfs:subClassOf rdf:resource="#MotorVehicle"/></rdfs:Class>

<rdfs:Class rdf:ID="Van"> <rdfs:subClassOf rdf:resource="#MotorVehicle"/></rdfs:Class>

<rdfs:Class rdf:ID="MiniVan"> <rdfs:subClassOf rdf:resource="#Van"/> <rdfs:subClassOf rdf:resource="#PassengerVehicle"/></rdfs:Class>

</rdf:RDF>

Abbreviated form

The value of the property „rdf:type“ can be used for naming the whole resource.

RDFS Example (Properties)<?xml version="1.0"?><!DOCTYPE rdf:RDF [<!ENTITY xsd "http://www.w3.org/2001/XMLSchema#">]>


…

<rdf:Property rdf:ID="registeredTo"> <rdfs:domain rdf:resource="#MotorVehicle"/> <rdfs:range rdf:resource="#Person"/></rdf:Property>

<rdf:Property rdf:ID="rearSeatLegRoom"> <rdfs:domain rdf:resource="#PassengerVehicle"/> <rdfs:range rdf:resource="&xsd;integer"/></rdf:Property>

<rdfs:Class rdf:ID="Person"/>

</rdf:RDF>

„rdfs:range“ indicates, that the values of that property are instances of a certain class

„rdfs:domain“ indicates, that the values of the particular property applies to a designated class

RDFS Example (Instances)<?xml version="1.0"?> <!DOCTYPE rdf:RDF [<!ENTITY xsd

"http://www.w3.org/2001/XMLSchema#">]>

<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:ex="http://example.org/schemas/vehicles#" xml:base="http://example.org/things">

<ex:PassengerVehicle rdf:ID="johnSmithsCar"> <ex:registeredTo rdf:resource="http://www.example.org/staffid/85740"/> <ex:rearSeatLegRoom rdf:datatype="&xsd;integer">

127 </ex:rearSeatLegRoom></ex:PassengerVehicle>

</rdf:RDF>

an instance of the class „PassengerVehicle“

the defined properties that can be applied to this class

RDF / RDFS Conclusion

Expressivity of RDF and RDF Schema is limited Local scope of properties Disjointness of classes Boolean combination of classes Cardiniality restrictions Special characteristics of properties

Need for standardized ontology languagethat builds upon existing concepts of RDF / RDFS

=> OWL Web Ontology Language

OWL Hierarchy

rdfs:Resource

rdfs:Class rdf:Property

owl:Class owl:ObjectProperty owl:DatatypeProperty

OWL Syntax

Class elements Classes are defined using owl:Class

<owl:Class rdf:ID=“associateProfessor“><rdfs:subClassOf rdf:resource=“#academicStaffMember“ />

</owl:Class>

Disjoint classes<owl:Class rdf:about=“#associateProfessor“>

<owl:disjointWith rdf:resource=“#professor“ /> <owl:disjointWith rdf:resource=“#assistantprofessor“ /></owl:Class>

Equivalence of classes<owl:Class rdf:ID=“faculty“> <owl:equivalentClass rdf:resource=“#academicStaffMember“/></owl:Class>

OWL Syntax

Property elements Datatype properties relate objects to datatype values

<owl:DatatypeProperty rdf:ID=“age“><rdfs:range rdf:resource=“http://www.w3.org/2001/XMLSchema

#nonNegativeInteger“ /></owl:Class>

Object properties relate objects to other objects<owl:ObjectProperty rdf:ID=“isTaughtBy“>

<rdfs:domain rdf:resource=“#course“/><rdfs:range rdf:resource=“#academicStaffMember“/>

</owl:Class>

<owl:ObjectProperty rdf:ID=“teaches“><rdfs:domain rdf:resource=“#academicStaffMember“/><rdfs:range rdf:resource=“#course“/><owl:inverseOf rdf:resource=“#isTaughtBy“/>

</owl:Class>

OWL Syntax

Property restrictionsAllow to specify constraints on classes and properties

<owl:Class about:ID=“#firstYearCourse“><rdfs:subClassOf> <owl:Restriction>

<owl:onProperty rdf:resource=“#isTaughtBy“/><owl:allValuesFrom rdf:resource=“#Professor“/>

</owl:Restriction></rdfs:subClassOf>

</owl:Class>

<owl:Class about:ID=“#course“><rdfs:subClassOf> <owl:Restriction>

<owl:onProperty rdf:resource=“#isTaughtBy“/><owl:minCardinality

rdf:datatype=“#&xsd;nonNegativeInteger“/> </owl:Restriction></rdfs:subClassOf>

</owl:Class>

Web Services and the Semantic Web

The Semantic Web should enable users to locate, select, employ, compose, and monitor Web-based services automatically.

Computer-interpretable description of the service is needed.

OWL-S defines an ontology for describing web services.

Web Services and the Semantic Web

What does the service provide?ServiceProfileA profile is used for advertizing the service.

How is it used?ServiceModelA model describes how a service works.

How to interact with it?ServiceGroundingA grounding provides the needed details about transport protocols.

Web Services and the Semantic Web (ServiceProfile)<owl:Class rdf:ID="Profile">

<rdfs:label>Profile</rdfs:label><rdfs:subClassOf rdf:resource="&service;#ServiceProfile" /><rdfs:comment> Definition of Profile </rdfs:comment>

</owl:Class>

<owl:ObjectProperty rdf:ID="hasInput"><rdfs:subPropertyOf rdf:resource="#hasParameter"/><rdfs:range rdf:resource="&process;#Input"/>

</owl:ObjectProperty>

Web Services and the Semantic Web (ServiceModel)

Web Services and the Semantic Web (ServiceModel)

<owl:Class rdf:ID="SimpleProcess"> <rdfs:subClassOf rdf:resource="#Process"/> <owl:disjointWith

rdf:resource="#AtomicProcess"/></owl:Class>

<owl:ObjectProperty rdf:ID="realizedBy"> <rdfs:domain rdf:resource="#SimpleProcess"/> <rdfs:range rdf:resource="#AtomicProcess"/> <owl:inverseOf rdf:resource="#realizes"/></owl:ObjectProperty>

<owl:ObjectProperty rdf:ID="realizes"> <rdfs:domain rdf:resource="#AtomicProcess"/> <rdfs:range rdf:resource="#SimpleProcess"/> <owl:inverseOf rdf:resource="#realizedBy"/></owl:ObjectProperty>

Conclusion

Today‘s web and its problems A vision for a possible semantic web application Overview of important standards

RDF / RDF Schema OWL OWL-S

The basic technologies exist but - standards have to mature and

- more practical problems have to be solved (tool support, ontology matching, …)

Bibliography

D. Martin et al., „OWL-S Semantic Markup for Web Services“http://www.daml.org/services/owl-s/1.1/overview/

F. Manola and E. Miller, eds. „RDF Primer“, February 10, 2004. http://www.w3.org/TR/rdf-primer/

M. Smith et al., „OWL Web Ontology Language Guide“, http://www.w3.org/TR/owl-guide/

G. Antoniou, F. van Harmelen, „A Semantic Web Primer“, MIT Press, London, England, 2004.

state of the art in semantic web standards and technologies andreas duscher

Documents

semantic web standards

context of web

agent web service technologies

retrieved information

implicit knowledge explicit

online shop

rdf rdf schema owllogic

profx facultyxfacultyx