resource description framework building the semantic web

Cornell CS 502

Resource Description FrameworkBuilding the Semantic Web

CS 502 – 20020226Carl Lagoze – Cornell University

Acknowledgements:Eric MillerDieter Fensel

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Motivating the “Semantic Web”

M. Doe illustrated the book “Best

Stories”Mary Doe

animated the cartoon “Best Stories – the

movie”

Illustration is a type of

contribution

animation is a type of

contribution

M. Doe and Mary Doe are

pseudonyms forSusan Mann

Show me the works to which Susan

Mann contributed?

Cartoons and Books are types

of Works

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Modeling & Encoding Metadata Components: RDF

• RDF (Resource Description Format)• The instantiation of the Warwick Framework on

the Web– Support for and integration of multiple independent

metadata vocabularies

• Provides enabling technology for richly-structured metadata

• Rich data model supporting notions of distinct entities and properties

• Primitives permit semantic inferencing • Expressible in machine readable manner (e.g.,

XML)

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RDF Components

• Formal data model• Syntax for interchange of data• Schema Type system (schema model)• Syntax for machine-understandable schemas• Query and profile protocols

• Ontologies layered on top

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RDF Data Model

• Imposes structural constraints on the expression of application data models– for consistent encoding, exchange and processing of

metadata– Provides for structural interoperability

• Enables resource description communities to define their own semantics

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RDF Data Model

• Directed labeled graphs• Model elements

– Resource– Property– Value– Statement– Containers

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RDF Model Primitives

ResourceProperty

ValueResource

Statement

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Simple Example

ResourceAuthor

“Eric”

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RDF Syntax

• RDF Model defines a formal relationships among resources, properties and values

• Syntax is required to...– Store instances of the model into files– Communicate files from one application to another

• XML is one well-supported syntax• There are syntax alternatives

– Relational databases– Triple Stores

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RDF Model Example #1

URI:R“CIMI Presentation”

Title

Creatordc:

dc:

“Eric Miller”

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RDF Syntax Example #1

URI:R“CIMI Presentation”

Title

Creatordc:

dc:

“Eric Miller”

<RDF xmlns = “http://www.w3.org/TR/WD-rdf-syntax#” xmlns:dc = “http://purl.org/dc/elements/1.0/”> <Description about = “URI:R”> <dc:Title> CIMI Presentation </dc:Title> <dc:Creator> Eric Miller </dc:Creator> </Description></RDF>

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“Eric Miller”

RDF Model Example #2

URI:R

URI:ERIC

“emiller@oclc.org”“Eric Miller”

“OCLC”

bib:Emailbib:Affbib:Name

URI:OCLC

“CIMI Presentation”Title

Creatoroa:

dc:

Cornell CS 502

<RDF xmlns = “http://www.w3.org/TR/WD-rdf-syntax#” xmlns:dc = “http://purl.org/dc/elements/1.0/” xmlns:bib = “http://www.bib.org/persons#”> <Description about = “URI:R”> <dc:Title> CIMI Presentation </dc:Title> <oa:Creator> <Description> <bib:Name> Eric Miller </bib:Name> <bib:Email> emiller@oclc.org </bib:Email> <bib:Aff resource = “http://www.oclc.org” /> </Description> </oa:Creator> </Description></RDF>

RDF Syntax Example #2

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“Eric Miller”

RDF Model Example #3

URI:R

URI:ERIC

“emiller@oclc.org”“Eric Miller”

“OCLC”

bib:Emailbib:Affbib:Name

URI:OCLC

“CIMI Presentation”Title

Creator admin:By

admin:On

“LOC”

“03-09-99”

admin:For“...”

dc:

dc:

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RDF Containers

• Permit the aggregation of several values for a property

• Express multiple aggregation semantics– unordered– sequential or priority order– alternative

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RDF Containers

• Permit the aggregation of several values for a property

• Express multiple aggregation semantics– unordered– sequential or priority order– alternative

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RDF Containers

• Bag– unordered grouping

• Sequence– ordered grouping

• Alternatives– alternate values

• need to choose

– at least one value– first value is default or preferred value

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RDF - Bag

• Unordered group• “Carl Lagoze and Stuart Weibel are co-authors”

<BIB:Author> <Bag> <li> Carl Lagoze </li> <li> Stuart Weibel </li></Bag></BIB:Author>

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RDF - Sequence

• Ordered or priority group• “Carl Lagoze is primary author and Stuart Weibel is

second author”

<BIB:Author> <Seq> <li> Carl Lagoze </li> <li> Stuart Weibel </li></Seq></BIB:Author>

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RDF - Alt

• Client chooses one of several values• First value is default• “The distance is 15 kilometers or 9.3 miles”

<DC:Coverage> <Alt> <li> 15KM </li> <li> 9.3M </li></Alt></DC:Coverage>

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Formalizing the RDF model – Thinking in triples

• RDF basic types– rdf:Resource – everything that can be identified (with

a URI)– rdf:Property – specialization of a resource expressing

a binary relation between two resources– rdf:statement – a triple with properties rdf:subject,

rdf:predicate, rdf:object

• An RDF statement is a triple consisting of a resource (subject), a property and a second resource (object)– (:s :p :o)

• Expressible also as binary relations– P(S,O) – e.g., Title(R, “War & Peace”)

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RDF triple model

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RDF statements and basic types

WYA

creator

DigitalLibraries

rdf:s

ubje

ct

rdf:p

red

icate

rdf:object

rdf:statement

rdf:property

“CL says ‘WYA wrote Digital Libraries’”

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Reification – Statements about statements

“CL says ‘WYA wrote Digital Libraries’”

WYA

creator

DigitalLibraries

rdf:s

ubje

ct

rdf:p

red

icate

rdf:object

rdf:statement

rdf:property

CLassertedBy

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From Graphs to Triples

alice

betty

charles

doris

eve

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Expressing Collection Primitives in Binary Relations

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RDF Schemas

• Declaration of vocabularies– properties defined by a particular community– characteristics of properties and/or constraints on

corresponding values

• Schema Type System - Basic Types– Property, Class, SubClassOf, Domain, Range– Minimal (but extensible) at this time– Expressible in the RDF model and syntax

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Schema Vocabularies

• Enables communities to share machine readable tokens and locally define human readable labels.

dc:Creator“Nom”rdfs:label

“Author”rdfs:label

“$100 $a”

rdfs:label

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Relationships among vocabularies

dc:Creator

ms:director

marc:100

bib:Author

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Relationships among vocabulary elements

URI:R “John Smith”ms:director

dc:Creatorms:director

rdfs:subPropertyOf

rdfs:label“Director”

dc:Creator

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RDF Schema: Specializing Properties

• rdfs:subPropertyOf – allows specialization of relations– E.g., the property “father” is a subPropertyOf the

property parent

• subProperty semantics

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Sub-Property Semantics

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Constraints on Properties

• Force objects to be of a certain type• rdfs:domain

– Restricts the type of resources that may have a specific property

• rdfs:range– Restricts the type of resources that may be the value

of a specific property

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Inferences from Constraints

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Class Hierarchy

• rdfs:Class– Resources denoting a set of resources; range of

rdf:type

• rdfs:subClassOf– Create class hierarchy

rdf:type rdf:typerdfs:class

rdfs:subClassOf

rdf:type

rdf:class

rdf:type

rdf:class

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Sub-Class Inferencing

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Sub-class Inferencing Example

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Storing and querying RDF models – Relational DB

• Issues– Scalability: potentially huge # of triples– Tables: number, sparseness, joins– Queries: how and how expensive– Reification?

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Storing and querying RDF models – SQUISH

SELECT ?sal, ?t, ?x FROM http://ilrt.org/discovery/2000/11/rss-query/jobs-rss.rdf, http://ilrt.org/discovery/2000/11/rss-query/jobs.rss WHERE (job::advertises ?x ?y) (job::salary ?y ?sal) (job::title ?y ?t) AND ?sal > 55000 USING job for http://ilrt.org/discovery/2000/11/rss-query/jobvocab.rdf#

http://swordfish.rdfweb.org:8085/rdfquery/index.html

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Where do you stop?

• Model provides enabling technology• Degree of metadata simplicity/complexity is a matter of:

– Resource description communities needs, best-practice and experience

– Organization/Institution’s Policy– Economics– Goals and requirements of implementation