Intro to Semantic Web, Web Services, and Semantic Web Services

Ben Vandervalk

James Hogg Research Centre, Heart + Lung Institute

University of British Columbia

ben.vvalk@gmail.com

June 2012

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The Semantic Web

• Idea
• a global, machine-readable "web of data"
• analogous to the WWW
• WWW: web pages link to other web pages
• Semantic Web: data sets link to other data sets
• a general-purpose framework for data integration and data processing�
• W3C Standards
• Representing Data: Resource Description Framework (RDF)
• Query Language: SPARQL Protocol and RDF Query Language (SPARQL)
• Vocabularies and Logical Reasoning: Web Ontology Language (OWL)

Tim Berners-Lee, Wikimedia Commons, picture taken by Paul Clarke, 2010.

The Semantic Web, Scientific American, 2001.

Resource Description Framework (RDF)

• Universal language for data on the Semantic Web
• An RDF document is a set of triples
• A triple makes a statement
• A triple consists of 3 parts: subject, predicate, object
• every subject/predicate/object has a globally unique identifier (URI)

An example RDF document, in N3 format.

RDF is a Graph-Based Data Model

Merging RDF Datasets

facebook:BenVandervalk

foaf:Person

mailto:ben@gmail.com

rdf:type

foaf:mbox

facebook:PeterVandervalk

foaf:Person

mailto:peter@gmail.com

rdf:type

foaf:mbox

facebook:BenVandervalk

foaf:knows

facebook:BenVandervalk

foaf:Person

mailto:ben@gmail.com

rdf:type

foaf:mbox

facebook:PeterVandervalk

foaf:Person

mailto:peter@gmail.com

rdf:type

foaf:mbox

foaf:knows

Merging RDF Datasets

merged with

yields

Just concatenate the two lists of triples! (and remove duplicates)

SPARQL Protocol and RDF Query Language (SPARQL)

dataset

query

solutions

Web Ontology Language (OWL)

• OWL is a language for describing logical constraints in RDF data
• Many types of axioms are supported. Examples:
• Subproperties:
• (x, brother, y) => (x, related, y)
• brother is a subproperty of related
• note: "property" is synonym "predicate"
• Classes:
• (x, is a, father) <=> (x, sex, male) AND (∃y (x, child, y))
• fathers are male and have at least one child
• A formal ontology is a set of logical axioms that describes a certain domain
• At the same time, it provides vocabulary (URIs) for encoding data in that domain
• An OWL reasoner answers questions about an ontology and related data, e.g.:
• Which individuals are fathers?
• Are John and Joe related?

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Web Ontology Language (OWL)

<owl:Class rdf:ID="Father">

<owl:equivalentClass> <!-- if and only if -->

<owl:Class>

<owl:intersectionOf rdf:parseType="Collection"> <!-- logical AND -->

<!-- sex is male -->

<owl:Restriction>

<owl:onProperty rdf:resource="#sex"/>

<owl:hasValue rdf:resource="#male"/>

</owl:Restriction>

<!-- at least one child -->

<owl:Restriction>

<owl:onProperty rdf:resource="#child"/>

<owl:minCardinality rdf:datatype="&xsd;integer">1</owl:minCardinality>

</owl:Restriction>

</owl:intersectionOf>

</owl:Class>

</owl:equivalentClass>

</owl:Class>

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...

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(x, is a, father) <=> (x, sex, male) AND (∃y (x, child, y))

"Fathers are male and have at least one child."

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That's all about the Semantic Web for now.

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Topic #2: Web Services

(short)

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Web Services

• Web Service: an interface that enables software to remotely access a resource (e.g. a program, a database) over the web
• analogous to subroutines
• example: Google Maps API
• RESTful services (Representation State Transfer)
• encode request parameters in an URL:
• request: http://helloworld.com/hello?name=John
• response: "Hello, John!"
• WSDL services (Web Service Description Language)
• WSDL is an XML syntax for describing web services:
• supported "operations" of a web service (e.g. deposit, withdraw)
• messages exchanged between client and service for each operation (usually XML)
• advantage:
• automatically generate code for clients and services (Apache Axis)

Semantic Web Services

• OWL-S
• formally models stateful web services:
• preconditions - state of the world before
• effects - state of the world after
• world state is modeled using an AI planning type of approach
• world states are expressed as boolean formulae over input/output variables and state variables
• complicated standard!
• input and output data is not necessarily RDF
• best-documented use case is WSDL/XML!
• WSMO: competitor of OWL-S, very similar in approach

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