RDF Resource Description Framework

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RDF ResourceDescription Framework
Una piattaforma inferenziale per il Web
Semantico Jena2 Roma, 2006 Web Semantico

• M. Missikoff F. Taglino
• LEKS, IASI-CNR

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Summary

• Motivations
• RDF knowledge model
• RDF(S) modelling notions
• XML/RDF, an XML syntax for RDF(S)
• Using RDF for inferring knowledge
• Extending RDF(S) OWL
• Conclusions

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Motivations

• Semantic Web Information needs to be processed
  by applications, rather than being only displayed
  to people (e.g., like with HTML)
• Common convention about semantics, syntax, and
  structure required
• Define a language for representing meta-data,
  with a semantic flavor

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What is RDF?

• The Resource Description Framework is a
• Flexible language for representing knowledge over
  the web
• It models conceptual knowledge (schemas) and
  factual knowledge (data)
• Enables the encoding, exchange, and reuse of
  structured knowledge about defined entities
  (identified by URI)
• Enables data interoperability through the design
  of mechanisms that support common conventions of
  semantics, syntax, and structure.
• The
• W3C Recommendation
• http//www.w3.org/TR/rdf-syntax-grammar/

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URI - Uniform Resource Identifier

• Used to identify items on the Web
• Anyone can create a URI
• A URI can be associated to anything
• Network-accessible things, e.g., electronic
  document, image, service,
• Not network-accessible things, e.g., human
  beings, corporation, book, car,
• Abstract concepts, e.g., creator, member,
• Uniform Resource Locator (URL) one form of URI
  e.g., http//www.w3.org

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RDF (Meta)Model

• Knowledge is represented as collections of Triples

resource (subject)
property (predicate)
value (object)

• Resource thing that can be identified on the
  Web
• Property specific aspect, characteristic,
  attribute, or relation used to describe a
  resource
• Value - Propertys value resource or literal
• Connecting triples together we can represent a
  graph

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RDF abstract (graph) representation
mary
has_sister
john
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has_age
type
john
Person
subClassOf
Student
Prof
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Statement (triple)

• The following sentence
• http//www.example.org/index.html has creator
  whose_value_is John Smith
• Can be expressed by an RDF statement having
• Subject http//www.example.org/index.html
• Predicate http//purl.org/dc/creator
• Object John Smith

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Basic RDF Model - example

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

http//www.example.org/ index.html
http//purl.org/dc/creator
John Smith
resource (subject)
property (predicate)
value (object)
RDF Statement
Please Note when the object is a Literal it is
represented by a BOX
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RDF Schema

• The purpose is to define an XML vocabulary to
• Denote classes (subclasses) and their
  relationships
• Define properties and associate them with classes
• The benefit is that
• Data are modeled according to schema (as usual in
  DB)
• inferencing on data, and enhanced searching are
  facilitated
• http//www.w3.org/TR/rdf-schema/

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Knowledge model of RDF(S)
RDF(S) Meta-model
property
Class
range
Application specific schema (RDF(S) Model)
has_age
Person
Int
Application specific actual data (RDF Instances)
has_age
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John
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RDF(S) modelling notions

• The following RDF(S) modelling notions are
  presented
• Schema definition useful for defining new
  vocabularies (i.e., names of classes, of
  attributes, )
• Utility additional notions useful to enrich the
  entity definiton
• Label and Comment
• Complex Structures, e.g., Container
• Synonyms
• Note we will present a simplified version of
  RDF(S). In particular, reification will be
  omitted

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Schema definition (RDFS vocabulary)

• rdfsClass
• Set of resources
• rdfssubClassOf
• For defining specialisation among Classes
• rdfsdomain and rdfsrange
• Constraints on properties definition
• rdfssubProperty
• For defining specialisation among Properties
• rdfProperty
• Borrowed from RDF (ground), but used at schema
  level

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rdfsClass

• Class is a resource denoting a set of resources
  (instances)
• Examples
• Person
• River

Person
rdfsClass
River
ltrdfsClass rdfIDRiver/gt ltrdfsClass
rdfIDPerson/gt
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rdfssubClassOf Example
NaturallyOccurringWaterSource
rdfssubClassOf
BodyOfWater
Stream
Lake
Ocean
Sea
Brook
River
Tributary

• ltrdfsClass rdfIDNaturallyOccurringWaterSource
  /gt
• ltrdfsClass rdfIDBodyOfWater/gt
• ltrdfssubClassOf rdfresourceNaturallyOccurr
  ingWaterSource/gt
• lt/rdfsClassgt
• ltrdfsClass rdfIDStreamgt
• ltrdfssubClassOf rdfresourceNaturallyOccurr
  ingWaterSource/gt
• lt/rdfsClassgt
• ltrdfsClass rdfIDRivergt
• ltrdfssubClassOf rdfresourceStream/gt
• lt/rdfsClassgt

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rdfssubClassOf Properties

• Allows specialisation hierarchy among Classes to
  be defined
• Multiple subClassOf properties may be specified
• Transitive (i.e., any River is a
  NaturallyOccurringWaterSource too)

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Definition of a Property at intensional level
rdfsdomain and rdfsrange

• Used to define properties at intentional level
• rdfsdomain, restricts the set of resources that
  may have a given property
• rdfsrange, restricts the set of valid values for
  a property.
• A property may have multiple domain, and more
  than one range

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Example of definition of a Property at
intensional level (1)
A River has a name expressed as a Literal
rdfsClass
River
rdfsdomain
rdfProperty
hasName
Class representing literals (strings)
rdfsLiteral
rdfsrange

• ltrdfsClass rdfIDRiver/gt
• ltrdfProperty rdfIDhasNamegt
• ltrdfsdomain rdfresourceRiver/gt
• ltrdfsrange rdfresourcehttp//www.w3.org/200
  0/01/rdfschemaLiteral/gt
• lt/rdfPropertygt

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Example of definition of a Property at
intensional level (2)
rdfsClass
River
rdfsdomain
rdfProperty
emptiesInto
rdfsClass
BodyOfWater
rdfsrange

• ltrdfProperty rdfIDemptiesIntogt
• ltrdfsdomain rdfresourceRiver/gt
• ltrdfsrange rdfresourceBodyOfWater/gt
• lt/rdfPropertygt

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rdfssubPropertyOf

• Allows specialisation hierararchy among
  properties to be defined
• Multiple subPropertyOf properties for the same
  property can be specified

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Example of a subProperty definition
The hasMother property is a sub property of the
hasParent property
rdfsdomain
rdfProperty
rdfsClass
Person
hasParent
rdfssubPropertyOf
rdfsrange
hasMother
rdfProperty

• ltrdfProperty rdfIDhasParentgt
• ltrdfsdomain rdfresourcePerson/gt
• ltrdfsrange rdfresourcePerson/gt
• lt/rdfProperty gt
• ltrdfProperty rdfIDhasMothergt
• ltrdfssubPropertyOf rdfresourcehasParent/gt
  
• lt/rdfPropertygt

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Class instance definitionrdftype

• Instances are defined through the rdftype
  property

The Yangtze is an instance of the River Class
rdftype
Yangtze
River
ltrdfDescription rdfIDYangtzegt ltrdftype
rdfresourceRiver/gt lt/rdfDescriptiongt
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Utility notions

• rdfslabel and rdfscomment
• For describing resources with human readable text
• rdfsContainer
• Collections of resources
• rdfsseeAlso and rdfsisDefinedBy
• For referring to alternative descriptions of
  resources

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rdfslabel and rdfscomment

• Describe a resource with human readable text in
  addition to pure RDF properties
• rdfslabel, to assign a human readable name to a
  resource
• rdfscomment, to give a longer natural language
  description to a resource

Yangtze
rdfslabel
http//www.china.org/geography/ rivers/Yangtze
The biggest Chinese river
rdfscomment
ltrdfDescription rdfIDhttp//www.china.org/geog
raphy/rivers/Yangtzegt ltrdfslabelgtYangtzelt/rdf
slabelgt ltrdfscommentgtThe biggest Chinese
riverlt/rdfscommentgt lt/rdfPropertygt
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Summary of RDF(S) modeling constructs

• rdfsClass and rdftype
• rdfssubClassOf
• rdfsdomain and rdfsrange
• rdfssubProperty and rdfproperty
• rdfslabel and rdfscomment
• rdfsContainer
• rdfsseeAlso and rdfsisDefinedBy
• rdfResource (the generic entity)
• rdfID (when the entity is first introduced)
• rdfabout (when the entity is referred to)
• rdfDescription

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How RDF can be implemented

• Diagrammatic representation is an Abstract Syntax
• Several Concrete Sintaxes usually RDF/XML syntax
• However, other notations possible
• e.g., Notation3 (N3)
• ltpatgt ltlovesgt ltjoegt
• lthttp//xyz.org/agt lthttp//xyz.org/bgt
  lthttp//xyz.org/cgt
• lthttp//xyz.org/Seangt lthttp//xyz.org/namegt
  Sean
• RDF/XML Abreviated syntax

Note the symbol is a separator between the
Prefix and the resource ID
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Using RDF(S) for Ontologies and Inferencing
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An extract of an RDF ontology

• lt?xml version1.0/gt
• ltrdfRDF xmlnsrdfhttp//www.w3.org/1999/02/22-r
  df-syntax-ns
• xmlnsrdfshttp//www.w3.org/2000/
  01/rdf-schema
• xmlbasehttp//www.geodesy.org/wa
  ter/naturally-occurringgt
• ltrdfsClass rdfIDNaturallyOccurringWaterSource
  /gt
• ltrdfsClass rdfIDBodyOfWater/gt
• ltrdfssubClassOf rdfresourceNaturallyOccur
  ringWaterSource/gt
• lt/rdfsClassgt
• ltrdfsClass rdfIDStreamgt
• ltrdfssubClassOf rdfresourceNaturallyOccur
  ringWaterSource/gt
• lt/rdfsClassgt
• ltrdfsClass rdfIDRivergt
• ltrdfssubClassOf rdfresourceStream/gt
• lt/rdfsClassgt
• ltrdfProperty rdfIDemptiesIntogt
• ltrdfsdomain rdfresourceRiver/gt
• ltrdfsrange rdfresourceBodyOfWater/gt
• lt/rdfPropertygt
• ltrdfProperty rdfIDlengthgt

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What inferences can be made?
NaturallyOccurringWaterSource
rdfssubClassOf
Inference Engine
BodyOfWater
Stream
Brook
River
Tributary
Lake
Ocean
Sea
- length Literal - emptiesInto BodyOfWater
ltRiver rdfIDhttp//www.china.org/geography/rive
rs/Yangtze xmlnsrdfhttp//www.w3.org/1999/02
/22-rdf-syntax-ns xmlnshttp//www.geodesy.o
rg/rivergt ltlenghtgt6300 kilometerslt/lenghtgt ltem
ptiesInto rdfresourcehttp//www.china.org/EastC
hinaSea/gt lt/Rivergt
Inferences - Yangtze is a Stream - Yangtze is
a NaturallyOccurringWaterSource -
http//www.china.org/EastChinaSea is a BodyOfWater
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How does a taxonomy facilitate searching?
NaturallyOccurringWaterSource
rdfssubClassOf
BodyOfWater
Stream
Brook
River
Tributary
Lake
Ocean
Sea
Inference Engine
- length Literal - emptiesInto BodyOfWater
Show me all the documents that contain info
about streams
ltRiver rdfIDhttp//www.china.org/geography/rive
rs/Yangtze xmlnsrdfhttp//www.w3.org/1999/02
/22-rdf-syntax-ns xmlnshttp//www.geodesy.o
rg/rivergt ltlenghtgt6300 kilometerslt/lenghtgt ltem
ptiesInto rdfresourcehttp//www.china.org/EastC
hinaSea/gt lt/Rivergt
Results - Yangtze is a Stream, so this document
is relevant to the query
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Desire more expressiveness

• Two classes, same concept
• Equivalent classes Airplane and Plane
• Cardinality constraints
• Ocean has one maxDepth

OWL
RDF Schema Building Block to more expressive
Ontology Languages like OWL
RDF(S)
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OWL (Ontology Web Language)

• Provides three increasingly expressive languages
• OWL Full maximum expressiveness with no
  computational guarantees
• OWL DL, expressiveness of Description Logics
  without losing computational completeness
• OWL Lite, simple constraint features. E.g.,
  cardinality constraints

OWL Full
OWL DL
OWL Lite
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Rules

• Allows for derivations, by introducing a
  special binary predicate
• logimplies gt
• Subject and Object are RDF triples, called
  formulae
• Derivation (n3 syntax)
• X parent Y brother Z gt Z uncle Y

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Conclusions

• RDF has potential to make web contents machine
  readable
• RDF describes resources and anything can be seen
  as a Resources
• RDF(S) comprises vocabulary for describing also
  the intentional level
• Several serializations (i.e., RDF/XML, N3)
• Suitable for applying inference
• Limited features (i.e., no cardinality
  constraints)
• OWL for a more expressiveness

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Sources

• http//www.w3.org/TR/rdf-primer/
• http//www.w3.org/TR/rdf-syntax-grammar/
• http//www.w3.org/TR/rdf-schema/
• P.A. Champin, RDF Tutorial - http//www710.univ-ly
  on1.fr/champin/rdf-tutorial/rdf-tutorial.pdf
• A. Wilk, RDF(S) - www.ida.liu.se/labs/iislab/cours
  es/LW/slides/RDFS.pdf

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Qualche esercizio rapido

• Rappresentare in RDF alcune frasi di linguaggio
  naturale
• Ugo ama Anna
• Il treno parte alle 1040
• Il treno è partito in ritardo
• Le auto vecchie inquinano
• Il corso di Knowledge Extraction è interessante
• La prima domenica del mese cè il mercatino in
  Piazza Dante

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Passaggio a RDF/Diagramma

• Ugo ama Anna

Persona
RdfssubClassOf
Studente
rdftype
Exama
ExUgo
ExAnna