Semantic web course – Computer Engineering Department – Sharif Univ. of Technology – Fall 2005 1Semantic web course – Computer Engineering Department – Sharif Univ. of Technology – Fall 2005 1

RDF & RDF SchemaMachine Understandable Metadata for the Web

Semantic Web - Fall 2005Computer Engineering Department

Sharif University of Technology

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Semantic web course – Computer Engineering Department – Sharif Univ. of Technology – Fall 2005 2Semantic web course – Computer Engineering Department – Sharif Univ. of Technology – Fall 2005 2

Outline

• Metadata• RDF• RDFS• RDF(S) Tools

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Semantic Web: Problems

• Too much Web information– around 1,000,000,000 (1109) resources– Many different types of resources

• text, images, graphics,• audio, video, multimedia, • databases, Web applications, …

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Semantic Web: Problems (2)

• Information not indexable– No common “scheme” for doing so– Short-lived, dynamic resources– Differing relationships between authors, publishers, info

intermediaries, users• Each community uses their own approach

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Semantic Web: Problems (3)

• Information not shareable– Difficult to share information– Difficult to share information about information

• no common cataloging schemes

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Main Issues:

• Metadata – Information about information– Structured data about data

• Many types/forms of metadata, dependent on role:

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Types of Metadata:

Web Resource discovery

Content ratings (PICS)

Product &ServicesDescriptions

Document management administration

Security & Userauthentication

(Intellectual)property rightsmanagement

Database / data schemas

Archivalinformation /status

Process description &control

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Second Issue:

• Language for expressing metadata must be:– universal (so all can understand)– flexible (to incorporate different types) – extensible (flexible to custom types)– simple (to encourage adoption)– modular (so that schemes can be mixed, extended)

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RDF

• RDF stands for Resource Description Framework

• It is a machine understandable metadata• RDF is graphical formalism ( + XML syntax

+ semantics)– for representing metadata– for describing the semantics of information in a

machine- accessible way

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RDF in SW Architecture

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Example (generated by RDFPic)

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RDF Model

• A model is a collection of statements• Statement := (predicate,subject,object)• Predicate is a resource• Subject is a resource• Object is either a resource or a literal

Subject Object

Predicate

Statement

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Example shown in triples view

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RDF model and natural language

• Subject. In grammar, this is the noun or noun phrase that is the doer of the action. In the sentence “The company sells batteries,” the subject is “the company.”

• Predicate. In grammar, this is the part of a sentence that modifies the subject and includes the verb phrase. In our sentence, the predicate is the phrase “sells”

• Object. In grammar this is a noun that is acted upon by the verb. In our sentence, the object is the noun “batteries.”

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XML vs. RDF

• RDF is not just an XML dialect.– XML:

• Has a tree structure data model.• Only nodes are labeled.

– RDF:• Has a graph structure data model.• Both edges (properties) and nodes (subjects/objects) are

labeled.

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Linking Statements

• The subject of one statement can be the object of another

• Such collections of statements form a directed, labeled graph

Ganji CEstudentOF

Sharif http://ce.sharif.edu

departmentOF hasHomePage

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RDF Graph: ‘anonymous’ nodes

Person12345

Jonathan

Borden

person.name

first

last

value

value

PersonName LiteralPerson

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Using XPointer to name

Person12345

/1/1/1/1/1

/1/1/2

Jonathan

Borden

person.name

first

last

value

value

PersonName LiteralPerson

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How can RDF be implemented

• Usually RDF/XML syntax• However other notations are possible

– e.g. Notation3:• Buddy Belden owns a business.• The business has a Web site accessible at

http://www.c2i2.com/~budstv.• Buddy is the father of Lynne.

• <#Buddy> <#owns> <#business>.• <#business> <#has-website>

<http://www.c2i2.com/~budstv>.• <#Buddy> <#father-of> <#Lynne>.

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Converting N3 to RDF

• Jena toolkit can do such conversion

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XML Syntax for RDF• RDF has an XML syntax that has a specific meaning:• Every Description element describes a resource• Every attribute or nested element inside a Description is a

property of that Resource• We can refer to resources by using URIs

<rdf:Description about="some.uri/person/ganji"> <studentOf resource="some.uri/Sharif/CE"/></Description><Description about="some.uri/Sharif/CE"> <hasHomePage>http://ce.sharif.edu</hasHomePage>

<departmentOf resource="some.uri/~Sharif"/></rdf:Description>

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RDF type• RDF predifined property• Its value – a resource that represent a category or class• Its subject – Instance of that category or class

prefix ex: URI: http://www.example.org/terms

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Containers

• Containers are collections– they allow grouping of resources (or literal values)

• It is possible to make statements about the container (as a whole) or about its members individually

• It is also possible to create collections based on URI patterns– for example, all files in a particular web site

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RDF containers

• Bag: (A resource having type rdf:Bag)– Represents an unordered list of resources or

literals– Duplicated values are prermitted

• Sequence: (A resource having type rdf:Seq)– Represents ordered list of resources or literal– Duplicated values are permitted

• Alternatives: (A resource having type rdf:Alt)– Represents group of resources or literals that are

alternatives

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Sequence example

http://www.w3.org/TR/REC-rdf-syntax

“Ora Lassila”

rdf:_1

rdf:Seq

dc:Creator

rdf:Type

“Ralph Swick”

rdf:_2

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Bag example

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RDF reification

• association of a statement and a specific resource representing the statement• used to make statements about statements• Vocabulary:

• type rdf:asserts• properties

• rdf:subject• rdf:predicate• rdf:object

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Reification example

• In N3:

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Reification example (cont.)

• In RDF:

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Reification example (cont.)

• RDF Graph (by IsaViz):

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RDF Schema (RDFS)

• RDF gives a formalism for meta data annotation, and a way to write it down in XML, but it does not give any special meaning to vocabulary such as subClassOf or type– Interpretation is an arbitrary binary relation

• RDF Schema allows you to define vocabulary terms and the relations between those terms– it gives “extra meaning” to particular RDF predicates and

resources– this “extra meaning”, or semantics, specifies how a term should

be interpreted

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Core Classes & Properties

Core Classes

Core Properties

rdfs:Resource

rdfs:Literal

rdfs:XMLLiteral

rdfs:Class

rdfs:Property

rdfs:Type

rdfs:SubClassOf

rdfs:SubPropertyOf

rdfs:Domain

rdfs:Range

rdfs:Label

rdfs:Comment

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RDFS Examples

<Person,type,Class><hasColleague,type,Property><Professor,subClassOf,Person><Carole,type,Professor><hasColleague,range,Person><hasColleague,domain,Person>

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RDF/RDFS “Liberality”• No distinction between classes and instances

(individuals)<Species,type,Class><Lion,type,Species><Leo,type,Lion>

• Properties can themselves have properties<hasDaughter,subPropertyOf,hasChild><hasDaughter,type,familyProperty>

• No distinction between language constructors and ontology vocabulary, so constructors can be applied to themselves/each other<type,range,Class><Property,type,Class><type,subPropertyOf,subClassOf>

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Problems with RDFS• RDFS too weak to describe resources in sufficient detail

– No localised range and domain constraints• Can’t say that the range of hasChild is person when applied to

persons and elephant when applied to elephants– No existence/cardinality constraints

• Can’t say that all instances of person have a mother that is also a person, or that persons have exactly 2 parents

– No transitive, inverse or symmetrical properties• Can’t say that isPartOf is a transitive property, that hasPart is

the inverse of isPartOf or that touches is symmetrical– …

• Difficult to provide reasoning support– No “native” reasoners for non-standard semantics– May be possible to reason via FO axiomatisation

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RDF(S) tools• Read RDF data

– Parsers: Jena, Redland, SWI-Prolog– Validators: W3C RDF validation service– Editors: IsaViz, RDF Author, RDFEd, InferEd

• Store RDF data (XML format, tripples or relational/oo DB)– Sesame, RSSDB, RDFLib

• Use RDF data (applications, RSS news, etc.)• Manipulate RDF data (inference, query, etc.)

– Jena RDQL, etc.– Example:

SELECT ?person, ?knowsWHERE (?x <http://xmlns.com/foap/knows> ?z),(?x <http://xmlns.com/foap/name> ?person), (?z <http://xmlns.com/foap/name> ?knows)

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RDF Validators

• RDF Validation Service– http://www.w3.org/RDF/Validator/

• In general all the RDF parsers do some kind of validation

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References

• RDF Resource Guide:– http://www.ilrt.bris.ac.uk/discovery/rdf/resources/

• http://www.w3.org/RDF• http://www.w3.org/RDF/Validator/• Chapter 5 of the book