Cornell CS 502 Resource Description Framework Building the Semantic Web CS 502 – Carl Lagoze – Cornell University Acknowledgements: Eric Miller Dieter Fensel

Cornell CS 502 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 for Susan Mann Show me the works to which Susan Mann contributed? Cartoons and Books are types of Works

Cornell CS 502 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)

Cornell CS 502 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

Cornell CS 502 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

Cornell CS 502 RDF Data Model Directed labeled graphs Model elements –Resource –Property –Value –Statement –Containers

Cornell CS 502 RDF Model Primitives Resource Property Value Resource Statement

Cornell CS 502 Simple Example Resource Author “Eric”

Cornell CS 502 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

Cornell CS 502 RDF Model Example #1 URI:R “CIMI Presentation” Title Creator dc: “Eric Miller”

Cornell CS 502 RDF Syntax Example #1 URI:R “CIMI Presentation” Title Creator dc: “Eric Miller” <RDF xmlns = “ xmlns:dc = “ CIMI Presentation Eric Miller

Cornell CS 502 “Eric Miller” RDF Model Example #2 URI:R URI:ERIC oclc.org” “Eric Miller” “OCLC” bib: bib:Aff bib:Name URI:OCLC “CIMI Presentation” Title Creator oa: dc:

Cornell CS 502 <RDF xmlns = “ xmlns:dc = “ xmlns:bib = “ CIMI Presentation Eric Miller RDF Syntax Example #2

Cornell CS 502 “Eric Miller” RDF Model Example #3 URI:R URI:ERIC oclc.org” “Eric Miller” “OCLC” bib: bib:Aff bib:Name URI:OCLC “CIMI Presentation” Title Creator admin:By admin:On “LOC” “ ” admin:For “...” dc:

Cornell CS 502 RDF Containers Permit the aggregation of several values for a property Express multiple aggregation semantics –unordered –sequential or priority order –alternative

Cornell CS 502 RDF Containers Permit the aggregation of several values for a property Express multiple aggregation semantics –unordered –sequential or priority order –alternative

Cornell CS 502 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

Cornell CS 502 RDF - Bag Unordered group “Carl Lagoze and Stuart Weibel are co-authors” Carl Lagoze Stuart Weibel

Cornell CS 502 RDF - Sequence Ordered or priority group “Carl Lagoze is primary author and Stuart Weibel is second author” Carl Lagoze Stuart Weibel

Cornell CS 502 RDF - Alt Client chooses one of several values First value is default “The distance is 15 kilometers or 9.3 miles” 15KM 9.3M

Cornell CS 502 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”)

Cornell CS 502 RDF triple model

Cornell CS 502 RDF statements and basic types WYA creator Digital Libraries rdf:subject rdf:predicate rdf:object rdf:statement rdf:property “CL says ‘WYA wrote Digital Libraries’”

Cornell CS 502 Reification – Statements about statements “CL says ‘WYA wrote Digital Libraries’” WYA creator Digital Libraries rdf:subject rdf:predicate rdf:object rdf:statement rdf:property CL assertedBy

Cornell CS 502 From Graphs to Triples alice betty charles doris eve

Cornell CS 502 Expressing Collection Primitives in Binary Relations

Cornell CS 502 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

Cornell CS 502 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

Cornell CS 502 Relationships among vocabularies dc:Creator ms:director marc:100 bib:Author

Cornell CS 502 Relationships among vocabulary elements URI:R “John Smith” ms:director dc:Creatorms:director rdfs: subPropertyOf rdfs:label “Director” dc:Creator

Cornell CS 502 RDF Schema: Specializing Properties rdfs:subPropertyOf – allows specialization of relations –E.g., the property “father” is a subPropertyOf the property parent subProperty semantics

Cornell CS 502 Sub-Property Semantics

Cornell CS 502 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

Cornell CS 502 Inferences from Constraints

Cornell CS 502 Class Hierarchy rdfs:Class –Resources denoting a set of resources; range of rdf:type rdfs:subClassOf –Create class hierarchy rdf:type rdfs:class rdfs:subClassOf rdf:type rdf:class rdf:type rdf:class

Cornell CS 502 Sub-Class Inferencing

Cornell CS 502 Sub-class Inferencing Example

Cornell CS 502 Storing and querying RDF models – Relational DB Issues –Scalability: potentially huge # of triples –Tables: number, sparseness, joins –Queries: how and how expensive –Reification?

Cornell CS 502 Storing and querying RDF models – SQUISH SELECT ?sal, ?t, ?x FROM rss.rdf, WHERE (job::advertises ?x ?y) (job::salary ?y ?sal) (job::title ?y ?t) AND ?sal > USING job for

Cornell CS 502 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