RDF: Resource Description Framework

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RDF Resource Description Framework
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Contents

• Ontology
• Syntax of RDF and RDFS
• Axiomatic Semantics for RDF and RDFS
• Applications of RDF and RDFS
• Querying of RDF/RDFS Documents using RQL

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Web Today
Hendler Miller 02
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The semantic web
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Ontology

• Long history coming from Philosophy, introduced
  by Aristotle
• a branch of metaphysics concerned with the
  nature and relations of being --Webster
• Picked up by the Artificial Intelligence
• For AI systems, what exists is that which can
  be represented
• a shared and common understanding of some domain
  that can be communicated between people and
  application systems Gruber

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What is Ontology

• An ontology is a formal, explicit specification
  of a shared conceptualization - Gruber
• Conceptualization refers to an abstract model of
  phenomena.
• Explicit means that the type of concepts used,
  and the constraints on their use are explicitly
  defined.
• Formal refers to the fact that the ontology
  should be machine readable.
• Shared reflects that ontology should capture
  consensual knowledge accepted by the communities.

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Main components of an Ontology

• Classes concepts of the domain or tasks, which
  are usually organized in taxonomies
• in a university ontology, student and professor
  are two classes
• Relations a type of interaction between concepts
  of the domain
• such as subclass-of, is-a
• Axioms model sentences that are always true
• such as if the student attends both A and B
  course, then he or she must be a second year
  student
• Instances to represent specific elements
• such as a student called Peter is the instance
  of Student class

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Contents

• Ontology
• Syntax of RDF and RDFS
• Semantics
• Applications

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RDF is a Web Standard

• RDF Model and Syntax Specification became a W3C
  (World Wide Web Consortium) Recommendation in
  February 1999.
• It has a long history, considering that XML
  became W3C Recommendation in 1998.
• The purpose of RDF (Resource Description
  Framework) is to give a standard way of
  specifying data "about" something.

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XML vs. RDF

• ltcompanygt
• lttreatmentOfferedgtPhysiotherapylt/treatmentOffered
  gt
• ltcompanyNamegtAgilitas Physiotherapy Centre
  lt/companyNamegt
• ltstaffgt
• lttherapistgtLisa Davenportlt/therapistgt
• lttherapistgtSteve Matthewslt/therapistgt
• ltsecretarygtKelly Townsendlt/secretarygt
• lt/staffgt
• lt/companygt
• How to answer the query give me the list of
  staff members in this company?
• XML provides semantic information as a by-product
  of defining the structure of the document
• XML prescribes a tree structure for documents and
  the different leaves of the tree have a
  well-defined tag and context the information can
  be understood with.
• That is, structure and semantics of documents are
  interwoven
• there is no intended meaning associated with the
  nesting of tags
• It is up to each application to interpret the
  nesting.

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RDF and RDF Schema

• RDF

isA
therapist
Lisa

• RDF Schema

How to encode the rule that a person can not be
a secretary and therapist at the same time?
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OWL (Web Ontology Language)
isA
therapist
Lisa
RDF
secretary
isA
staff
RDFS
subClassOf
therapist
secretary
OWL
Disjoint (therapist, secretary)
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Basic Ideas of RDF

• David Billington is a lecturer of Discrete Maths
• ltcourse name"Discrete Maths"gt
• ltlecturergtDavid Billingtonlt/lecturergt
• lt/coursegt
• ltlecturer name"David Billington"gt
• ltteachesgtDiscrete Mathslt/teachesgt
• lt/lecturergt
• Opposite nesting, same information!
• Basic building block of rdf subject-predicate-obj
  ect triple
• It is called a statement
• Sentence about Billington is such a statement
• ltrdfRDF xmlnsrdf"http//www.w3.org/1999/02/22-r
  df-syntax-ns"
• xmlnsmydomain"http//www.mydomai
  n.org/my-rdf-ns"gt
• ltrdfDescription rdfabout"http//www.cit.gu.edu
  .au/DiscreteMath"gt
• ltmydomaintaughtBygt David Billington
  lt/mydomaintaughtBygt
• lt/rdfDescriptiongt

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Statements

• Statements assert the properties of resources
• A statement is an resource-property-value triple
• Sometimes also called subject, predicate, and
  value
• Values can be resources or literals
• Literals are atomic values (strings)

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Resource in a statement

• We can think of a resource as a thing we want
  to talk about
• E.g. authors, books, publishers, places, people,
  hotels
• Every resource has a URI, a Universal Resource
  Identifier
• A URI can be
• a URL (Web address), or
• some other kind of unique identifier.
• In the following example, http//www.cit.gu.edu.a
  u/DiscreteMath is a resource that we want to
  talk about.
• ltrdfRDF xmlnsrdf"http//www.w3.org/1999/02/22-r
  df-syntax-ns"
• xmlnsmydomain"http//www.mydomai
  n.org/my-rdf-ns"gt
• ltrdfDescription rdfabout"http//www.cit.gu.edu
  .au/DiscreteMath"gt
• ltmydomaintaughtBygt David Billington
  lt/mydomaintaughtBygt
• lt/rdfDescriptiongt
• lt/rdfRDFgt

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Properties

• Properties are a special kind of resources
• They describe relations between resources
• E.g. taught by, written by, age, title,
  etc.
• Properties are also identified by URIs.
• Advantages of using URIs
• ? global, worldwide, unique naming scheme
• Reduces the homonym problem of distributed data
  representation.

ltrdfRDF xmlnsrdf"http//www.w3.org/1999/02/22-r
df-syntax-ns" xmlnsmydomain"http
//www.mydomain.org/my-rdf-ns"gt
ltrdfDescription rdfabout"http//www.cit.gu.edu.
au/DiscreteMath"gt ltmydomaintaughtBygt
David Billington lt/mydomaintaughtBygt
lt/rdfDescriptiongt lt/rdfRDFgt
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Three Views of a Statement

• A triple
• A piece of a graph
• A piece of XML code
• Thus an RDF document can be viewed as
• A set of triples
• A graph (semantic net)
• An XML document.

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RDF Parser

• There is a nice RDF parser at the W3 Web site
• http//www.w3.org/RDF/Validator
• This RDF parser will tell you if your XML is in
  the proper RDF format.
• Altova semanticworks support RDF, RDFS, and OWL.