The RDF meta model

1
The RDF meta model

• Basic ideas of the RDF
• Resource instance descriptions in the RDF format
• Application-specific RDF schemas
• Limitations of XML compared w/ RDF
• The Dublin Core Standard
• Metadata, ontology and information registry

2
Definations

• RDF
• a uniform format for resource instance as well as
  schema description
• Dublin Core
• a conceptual schema about web resources
• Ontology
• data about real world objects classes,
  inheritance, attribute, relationship
• Information Registry
• data about data various aspects of on-line
  documents
• Information Resource Schema
• schema for the information registry
• All three (the ontology, the registry and the
  schema) will be specified in the RDF format

3
Basic Ideas

• Its all about machine-understandability and
  automation
• resource discovery in building and maintaining
  search engines
• cataloging for certain Web sites or a digital
  library
• sophisticated communication between intelligent
  software agents
• Standardization at two levels
• instance level all resource instances can be
  described in a uniformed format
• schema level class, attribute and relationship
  definitions also in a uniformed format.
• Conventional meta models only enforce the
  standardized format in the schema level, e.g.
  O.O., ER or other semantic models.

4
Resource instance descriptions

• The RDF meta model contains the following three
  basic concepts
• A resource can be anything describable using RDF.
  E.g., an entire web page, a whole collection of
  web pages, a web site. Even an object in the
  physical world such as a book.
• A property is a specific aspect of a resource.
  It can be a characteristic that belongs to a
  resource, or a relationship that links one
  resource with another.
• A statement is a piece of description about a
  particular resource in the RDF format.
• All RDF keywords for the instance-level
  description start w/ rdf

5
Resource instance descriptions (contd)

• A statement about a resource instance has
• the resources identifier
• one of the resources property (defined in an RDF
  schema)
• the value for that property (can be either a
  literal, or a resource)
• ltrdfRDFxmlnswchttp//www.scit.wlv.ac.uk/ex12
  53/wc/schemagt
• ltrdfDescription abouthttp//www.cnn.com/2000/H
  EALTH/cancer/12/06/ colon.cancer.ap/index.htmlgt
  
• ltwcTitlegtCigarette smoking linked to colorectal
  cancer lt/wcTitlegt
• lt/rdfDescriptiongt
• lt/rdfRDFgt

6
Resource instance descriptions (contd)

• A property can be a collection of elements
• RDF provides three types of collection
  ltrdfBaggt, ltrdfSeqgt and ltrdfAltgt
• Collection specification at the instance rather
  than the schema level
• ltrdfRDF
• xmlnswchttp//www.scit.wlv.ac.uk/ex1253/wc/sc
  hemagt
• ltrdfDescription abouthttp//www.cnn.com/2000/H
  EALTH/cancer/12/06/ colon.cancer.ap/index.html
  gt
• ltwcTitlegt
• ltrdfAltgt
• ltrdfli xmllangengtCigarette smoking
  lt/rdfligt
• ltrdfli xmllangitgtlt/rdfligt
• lt/rdfAltgt
• lt/wcTitlegt
• lt/rdfDescriptiongt
• lt/rdfRDFgt

7
Resource Schema Specification

• A schema in RDF is comparable to that in the O.O.
  model (a set of class definitions) or the ER
  model (a set of entity specifications)
• User can define classes as well as an inheritance
  hierarchy.
• Attributes of classes are separately defined as
  properties a major difference from
  conventional modeling methods.
• All RDF keywords for the schema-level description
  start w/ rdfs

8
Resource Schema Specification Classes

• A class definition in RDF is more like a class
  declaration in O.O. languages.
• Keywords ltrdfsClassgt, ltrdfssubClassOfgt.
• An example
• ltrdfRDFgt
• ltrdfsClass rdfIDMedicalDocumentsgt
• ltrdfscommentgtThe set of all medical related
  documents. lt/rdfcommentgt
• ltrdfssubClassOf rdfresourcehttp//www.w3.o
  rg/2000/
• 01/rdf-schemaResource/gt
• lt/rdfsClassgt
• ltrdfsClass rdfIDPatientRecordsgt
• ltrdfscommentgtThe set of patients records.
  lt/rdfcommentgt
• ltrdfssubClassOf rdfresourceMedicalDocumen
  ts/gt
• lt/rdfsClassgt
• lt/rdfRDFgt

9
Resource Schema Specification Properties

• Unlike conventional modeling methods where
  attributes are subordinates of classes, the
  Property concept in RDF is at the same level
  with the Class concept.
• Properties are linked w/ classes via the domain
  construct.
• Benefit more flexibility and extensibility.
• An example
• ltrdfsProperty rdfIDpatientIDgt ltrdfsdomai
  n rdfresourcePatientRecords/gt ltrdfsrange
  rdfresourcePatientIDClass/gtlt/rdfsPropertygt
  

Comparable to class PatientRecords
MedicalDocument PatientIDClass patientID
10
Limitations of XML compared w/ RDF

• XML provides a standardized syntax for
  interoperability purpose, not on the semantics.
• an XML parser can be reused anywhere
• the XML parser cannot understand the semantics
  behind the syntax
• for two parties to communicate, they have to
  agree upon certain semantic aspects of the
  document

ltabc cdgt lt/abcgt
App1
App2
XML Parser (encoder)
XML Parser (decoder)
11
Limitations of XML (contd)

• No fixed XML syntax to describe one fact

• DTD 1
• lt!ELEMENT Resource (property)gt
• lt!ATTLIST Resource id (CDATA)gt
• lt!ELEMENT property (Value)gt
• lt!ATTLIST property name (CDATA)gt
• lt!ELEMENT Value (CDATA)gt
• XML Instance
• ltResource idhttp//www.cnn.com//gtltproperty
  nameTitlegt ltValuegtCigarette
  lt/Valuegtlt/propertygt lt/Resourcegt

• DTD 2
• lt!ELEMENT property (Resource, Value)gt
• lt!ATTLIST property name (CDATA)gt
• lt!ELEMENT Resource (CDATA)gt
• lt!ELEMENT Value (CDATA)gt
• XML Instance
• ltproperty nameTitlegtltResourcegthttp//www.cnn.c
  om/lt/ResourcegtltValuegtCigarette lt/Valuegt
• lt/propertygt

12
The Dublin Core Standard

• The Dublin Core Metadata Intiative
• A standardized conceptual schema for describing
  web resources
• Dublin Core Elements (attributes of a web
  resource)
• 15 pre-defined elements Title, Creator, Subject,
  Description, Publisher, Contributor, Date, Type,
  Format, Identifier, Source, Language, Relation,
  Coverage, Rights.
• Dublin Core also defines a set of keywords
  associated with each element, called Dublin Core
  Qualifiers, to make the element instances more
  specialized
• E.g. a Date element may be further refined as
  Created Date, Valid Date, Available Date,
  Issued Date or Modified Date. The encoding
  scheme can be DCMI period or W3C-DTF.
• The Dublin Core standard can be specified as an
  RDF Schema

13
Metadata, Ontology and Information Registry

• Ontology
• concept (class) graph about real world objects
• Metadata
• conceptually, people are committed to this de
  facto definition - structured data about data
• practically, metadata is used to denote both
  data about data and data about real world
  objects (the ontology)
• Information Registry
• to free ourselves from the ambiguity of
  metadata, we can use information registry to
  denote structured data about data.
• Both the ontology and the information registry
  can be represented in the RDF format

14
Ontology and Information Registry (contd)
Dublin Core Schema
Extended Schema
All specified in the RDF format
Resource Schema
abstraction
Ontology
Information Registry
described by
abstraction
Information Resources
described by
Real World Objects