Ontology languages and OWL

RDF / RDF Schema

F. Corno, L. Farinetti - Politecnico di Torino 2

RDF Schema example


RDFS problems
bl
RDFS is too “weak” to describe resources
with a suitable level of details
range and domain cannot be localized (e.g.
the range of hasChild is a person when
applied to a person, elephant when applied to
an elephant)
no constraints on existence or cardinality (e.g.
all instances of persons have one and only
one mother which is a person, and have
exactly two parents)

RDFS problems
it is not possible to define transitive, inverse
or symmetrical statements (e.g. p of is a
y ( g part
transitive property, hasPart is the inverse of
isPartOf, touches is symmetrical)
, y )
Reasoning is not well supported
Non t d d
N standard semantics, no native reasoner
ti ti
exists


Requirements for an ontology
language
g g
Extend existing Web standards
XML, RDF, RDFS, ...
Easy t understand and t use
E to d t d d to
based on well known KR languages
Formally specified
Adequate expressive power
Adeq ate e pressi e po er
Automatic support for reasoning
pp g

Stack of Web languages

W3C
IST EU project
OntoKnowledgeg OWL
bioinformatics DARPA
University of OIL DAML+OIL
community
Washington
XOL SHOE OML RDF(S)

University of
y
Maryland XML


Ontology Web Language (OWL)
O t l W bL
4th level on the semantic web cake
le el eb
Built on top of
p
XML
RDF/S
Three versions
Lite
DL (maps to Description Logic)
Full (not fully tractable)
Serializable
S i li bl as XML

Ontology Web Language (OWL)


OWL-DL
OWL DL
Based on Description Logic
Well defined formal semantics
well defined rules to treat sentence meaning
well defined assumptions on the world being
p g
modeled
Well known reasoning/inferencing
g g
algorithms
tractable,
tractable conclusions can be derived in finite
time
Widely available reasoning systems

Building blocks in OWL
Ontology declaration (XML syntax)
<rdf:RDF xmlns:owl =http://www.w3.org/2002/07/owl#"
xmlns:rdf ="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
l df "h // 3 /1999/02/22 df #"
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
xmlns:xsd ="http://www.w3.org/2001/XMLSchema#">

Ontology metadata (information about the
ontology)
<owl:Ontology rdf:about="">
<rdfs:comment>An example OWL ontology</rdfs:comment>
<owl:priorVersion
< l i V i
rdf:resource="http://www.mydomain.org/uni-ns-old"/>
<owl:imports
rdf:resource="http://www.mydomain.org/persons"/>
p y g p
<rdfs:label>University Ontology</rdfs:label>
</owl:Ontology>

Classes
Cl
Every class i a d
E l is descendant of
d t f
owl:Thing
g
Classes are defined using
owl:Class
<owl:Class rdf:ID="Vehicle"/>

Equivalence
owl:equivalentClass
<owl:Class rdf:ID="Car">
<owl:equivalentClass rdf:resource="#Automobile"/>
q
</owl:Class>

Subsumption
Provided by
owl:subClassOf

<owl:Class rdf:ID="2-Wheel-Drive">
< l Cl df ID "2 Wh l D i ">
<rdfs:subClassOf rdf:resource="#Car"/>
</owl:Class>


Partitions
Disjoint partition
owl:disjointWith

<owl:Class rdf:about="#2-Wheel-Drive">
<owl:disjointWith
rdf:resource="#4-Wheel-Drive"/>
</owl:Class>


Partitions
Exhaustive partition
owl:oneOf

<owl:Class rdf:ID="Car">
<owl:oneOf rdf:parseType="Collection">
<owl:Thing rdf:about="#2-Wheel-Drive"/>
g # /
<owl:Thing rdf:about="#4-Wheel-Drive"/>
</owl:oneOf>
</owl:Class>


Attributes
A ib
Known also as “properties”
Datatype properties
Attributes that specify a class features by
means of data (XSD datatype)
Phone, title, age
Object properties
Attributes that define relationships between
classes (Relations)
isTaughtBy(Class(course), Class(professor))


Datatype properties
Allow to describe a specific aspect of a
concept
Based on XSD data types
The range specifies the data type
The domain specifies the class to which the
property is referred
<owl:DatatypeProperty rdf:ID="age">
<rdfs:domain rdf:resource="#Person"/>
< df d i df "#P "/>
<rdfs:range rdf:resource="http://www.w3.org/2001/
XMLSchema#nonNegativeInteger"/>
</owl:DatatypeProperty>


Relationships
Directed
From one concept to another, no viceversa
Defined through object properties
Domain: the class(es) from which the relation
departs
Range: the relation destination(s)
Subsumption between relationships is
possible

Relationships
Example
<owl:ObjectProperty rdf:ID="isTaughtBy">
<rdfs:domain rdf:resource="#course"/>
<rdfs:range rdf:resource="#academicStaffMember"/>
<rdfs:subPropertyOf rdf:resource="#involves"/>
</owl:ObjectProperty>


Instances (individuals)
I t (i di id l )
No i
N unique name assumption i OWL
ti in
If two instances have a different name or ID
this does not imply that they are different
individuals
E.g.: “Queen Elizabeth”, “The Queen” and
“Elizabeth Windsor” might all refer to the
g
same individual
It must be explicitly stated that individuals are
the same as each other, or different to each
other

Instances (individuals)
I t (i di id l )
Defined by
D fi d b means of
f
rdf:Description + rdf:Type
p yp

<academicStaffMember rdf:ID="949352">
rdf:ID= 949352 >
<uni:age rdf:datatype="&xsd;integer">
39
</uni:age>
</academicStaffMember>
<rdf:Description rdf:ID="949353">
<rdf:type rdf:resource="#academicStaffMember"/>
</rdf:Description>


Advanced constructs
Ad d t t
OWL supports several advanced constructs to
define classes and relationships
Intensional definition of classes
By defining constraints on attribute values (
y g (either
object or datatype properties)
<owl:Class rdf:about="#academicStaffMember">
<rdfs:subClassOf>
<owl:Restriction>
<owl:onProperty rdf:resource="#teaches"/>
<owl:someValuesFrom
< l V l F
rdf:resource="#undergraduateCourse"/>
</owl:Restriction>
</rdfs:subClassOf>
</owl:Class>


Advanced constructs
Ad d t t
Cardinality
Used to fix the number of instances that can be
related
E.g.: a department should have at least 10 members

<owl:Class rdf:about="#department">
<rdfs:subClassOf>
<owl:Restriction>
<owl:onProperty rdf:resource="#hasMember"/>
<owl:minCardinality
rdf:datatype="&xsd;nonNegativeInteger"> 10
</owl:minCardinality>
</owl:Restriction>
</rdfs:subClassOf>
</owl:Class>


Special properties
S i l ti
owl:TransitiveProperty
defines a transitive property, such as “has better
grade than”, “is taller than”, or “is ancestor of”
owl:SymmetricProperty
defines a symmetric property, such as “has same
grade as” or “is sibling of”
owl:FunctionalProperty
defines a property that has at most one value for
each object, such as “age”, “height”, or
“directSupervisor”


Special properties
owl:InverseFunctionalProperty
defines a property for which two different
objects cannot have the same value
j
E.g.: the property
“isTheSocialSecurityNumberFor”: a social
isTheSocialSecurityNumberFor :
security number is assigned to one person
only


OWL class constructors


OWL axioms


OWL: h t f ?
OWL what for?
To build an ontology
define classes and provide information on them
p
define properties and provide information on
them
To express facts about a domain
provide information on instances (individuals)
To reason about ontologies and facts
discover consequences of what is expressly
stated

Example of ontology
p gy
building in OWL
with P té é
ith Protégé

Protégé
Open source ontology editor
Developed by Stanford Center for
Biomedical Informatics Research at the
Stanford University School of Medicine
http://protege.stanford.edu/
htt // t t f d d /


Project steps
0. Conceptual design of the ontology
1. Classes definition
2.
2 Properties definition
3. Individuals definition
4. Restrictions definition


Step 0
Conceptual design of the ontology


Bed&Breakfast OneStarRating
hasRating Accommodation
Rating
Accommodation TwoStarRating

Camping
Budget ThreeStarRating
Accommodation
Hotel
int
hasZipCode
hasAccommodation
Luxury Hotel string
Backpacker Contact
Hiking Destination hasStreet
Surfing hasEMail
Sheraton hasContact
C string

Sports isOffered
Destination
Yoga Beach
Activity Caprera
hasCity
Relaxation
hasActivity
Quiet string
Destination Family
Destination Gallipoli
Sunbathing
Adventure Sightseeing

Bardonecchia
Rimini
Safari
BunjeeJumping
Museums


Step 1a
Define classes and subclasses
Is-a relationship, or subsumption


Bed&Breakfast
Accommodation
Rating
Accommodation
Camping
Budget
Accommodation
Hotel

Luxury Hotel
Backpacker Contact
Hiking Destination

Surfing

Sports
Destination
Yoga Beach
Activity

Relaxation
Quiet
Destination Family
Destination
Sunbathing

Safari
BunjeeJumping
Museums


OWL code
<owl:Class rdf:ID="Camping">
<rdfs:subClassOf>
<owl:Class rdf:ID="Accommodation"/>
rdf:ID Accommodation />
</rdfs:subClassOf>
</owl:Class>
<owl:Class rdf:ID="BudgetAccommodation">
<rdfs:subClassOf rdf:resource #Accommodation />
rdf:resource="#Accommodation"/>
</owl:Class>
<owl:Class rdf:ID="BedAndBreakfast">
<rdfs:subClassOf rdf:resource="#Accommodation"/>
</owl:Class>
<owl:Class rdf:ID="LuxuryHotel">
<rdfs:subClassOf>
<owl:Class rdf:ID="Hotel"/>
</rdfs:subClassOf>
</owl:Class>
<owl:Class rdf:about="#Hotel">
</owl:Class>


Step 1b
Disjoint
Define disjoint Bed&Breakfast

classes Accommodation
Camping
Budget
Accommodation
Hotel

<owl:Class rdf:about="#Hotel">
<owl:disjointWith rdf:resource="#Camping"/>
<owl:disjointWith>
<owl:Class rdf:about="#BedAndBreakfast"/>
</owl:disjointWith>
</owl:Class>


Step 1c Activity


Define multiple
Safari

inheritance BunjeeJumping
Museums


Step 2a Accommodation
Range

Define object properties hasAccommodation

relationships Domain

Destination

<owl:ObjectProperty rdf:ID="hasAccommodation">
<rdfs:domain rdf:resource="#Destination"/>
<rdfs:range rdf:resource="#Accommodation"/>


Step 2b
Define object properties characteristics
E.g:
E g: define an inverse object property

isOffered
Destination

Activity
hasActivity

<owl:ObjectProperty rdf:about #isOffered >
rdf:about="#isOffered">
<rdfs:range rdf:resource="#Destination"/>
<owl:inverseOf rdf:resource="#hasActivity"/>
<rdfs:domain rdf:resource="#Activity"/>


int

Step 2c
hasZipCode

string
Contact
hasStreet

hasEMail string

Define datatype properties
The range specifies the
data type
yp

<owl:DatatypeProperty rdf:ID="hasEMail">
<rdfs:range rdf:resource="http://www.w3.org/2001/XMLSchema#string"/>
<rdfs:domain rdf:resource="#Contact"/>
/owl:DatatypeProperty
<owl:DatatypeProperty rdf:ID="hasZipCode">
<rdfs:range rdf:resource="http://www.w3.org/2001/XMLSchema#int"/>
<rdfs:domain rdf:resource="#Contact"/>


Step 3 Accommodation
OneStarRating

Rating
g
TwoStarRating

Define individuals ThreeStarRating

instances Family
Destination

Bardonecchia
Rimini

<AccommodationRating rdf:ID="OneStarRating"/>
<AccommodationRating rdf:ID="ThreeStarRating"/>
<AccommodationRating rdf:ID="TwoStarRating"/>
<FamilyDestination rdf:ID="Bardonecchia"/>
<F il D ti ti df ID "B d hi "/>
<FamilyDestination rdf:ID="Rimini"/>


Step 4
Define restrictions
To restrict the individuals that belong to a
class
Quantifier restrictions (existential, universal
quantifiers)
Cardinality restrictions
hasValue
h V l restrictions
t i ti


Step 4 – example 1 Accommodation

hasAccommodation >= 1
Activity
FamilyDestination is a Destination
Destination ith t least one
D ti ti with at l t hasActivity >= 2

accommodation and at least
Family
2 activities Destination

Necessary and
sufficient condition

Cardinality restriction

FamilyDestination is a
Step
St 4 – example 1
l Destination with at least
one accommodation and
<owl:Class rdf:ID= FamilyDestination >
rdf:ID="FamilyDestination"> at least 2 activities
<owl:equivalentClass>
<owl:Class>
<owl:intersectionOf rdf:parseType="Collection">
<owl:Restriction>
<owl:onProperty>
<owl:ObjectProperty rdf:ID="hasAccommodation"/>
</owl:onProperty>
<owl:minCardinality rdf:datatype="http://www.w3.org/2001/XMLSchema#int"
>1</owl:minCardinality>
/ i i i
</owl:Restriction>
<owl:Restriction>
<owl:minCardinality rdf:datatype="http://www.w3.org/2001/XMLSchema#int"
>2</owl:minCardinality>
<owl:onProperty>
<owl:ObjectProperty rdf:ID="hasActivity"/>
</owl:onProperty>
</owl:Restriction>
<owl:Class rdf:about="#Destination"/>
</owl:intersectionOf>
</owl:Class>
</owl:equivalentClass>
</owl:Class>
/


Step 4 – example 2
QuietDestination is a Destination Destination

that is t h
th t i not chosen b noisy
by i
families Quiet
Destination Family
Destination

Complement restriction


Step
St 4 – example 2
l QuietDestination is a
Destination that is not
chosen by noisy families
<owl:Class rdf:ID="QuietDestination">
<owl:Class>
<owl:Class>
<owl:complementOf rdf:resource="#FamilyDestination"/>
</owl:Class>
</owl:Class>
</owl:Class>


OneStarRating

Step
St 4 – example 3
l Accommodation
Rating
TwoStarRating

Bed&Breakfast
ThreeStarRating

Budget
hasRating
Accommodation Camping

Accommodation

BudgetAccommodation is
Luxury Hotel
an Accommodation that has Hotel

either one or two star rating
g Sheraton

Existential restriction

Step
St 4 – example 3
l BudgetAccommodation
is an Accommodation
that has either one or
<owl:Class rdf:ID "BudgetAccommodation">
rdf:ID="BudgetAccommodation"> two star rating
t t ti
<owl:Class>
<owl:Restriction>
<owl:onProperty>
<owl:ObjectProperty rdf:ID="hasRating"/>
</owl:onProperty>
<owl:someValuesFrom>
<owl:Class>
<owl:oneOf rdf:parseType="Collection">
<AccommodationRating rdf:ID="OneStarRating"/>
<AccommodationRating rdf:ID= TwoStarRating />
rdf:ID="TwoStarRating"/>
</owl:oneOf>
</owl:Class>
</owl:someValuesFrom>
</owl:Restriction>
<owl:Class rdf:about="#Accommodation"/>
</owl:Class>
</owl:Class>


Bed&Breakfast
B d&B kf t
Hotel

Step 4 – example 4 Camping

Accommodation
Budget
BackpackerDestination Accommodation
hasAccommodation

is a Destination that Backpacker
Sports Destination
hasActivity
provides budget
accommodation and Relaxation Activity Destination

offers sport or
ff t
Adventure
adventure activities Sightseeing


BackpackerDestination is
p
Step 4 – example 4 a Destination that provides
budget accommodation
<owl:Class rdf:ID="BackpackerDestination"> and offers sport or
<owl:equivalentClass> adventure activities
<owl:Class>
<owl:Restriction>
<owl:onProperty>
<owl:ObjectProperty rdf:ID="hasAccommodation"/>
</owl:onProperty>
<owl:someValuesFrom rdf:resource="#BudgetAccommodation"/>
</owl:Restriction>
< l Cl df b t "#D ti ti "/>
<owl:Restriction>
<owl:onProperty>
<owl:ObjectProperty rdf:ID="hasActivity"/>
</owl:onProperty>
<owl:someValuesFrom>
<owl:Class>
<owl:unionOf rdf:parseType="Collection">
<owl:Class rdf:about="#Sports"/>
<owl:Class rdf:about #Adventure />
rdf:about="#Adventure"/>
</owl:unionOf>
</owl:Class>
</owl:someValuesFrom>
</owl:Restriction>
</owl:Class>
F. Corno, L. Farinetti - Politecnico di Torino
</owl:Class> 52

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Ontology languages and OWL

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