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The RDF meta model: a closer look

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RDF provides three types of collection: rdf:Bag , rdf:Seq and rdf:Alt ... ELEMENT Value (#CDATA) XML Instance Resource id='http://www.cnn.com ... – PowerPoint PPT presentation

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Title: The RDF meta model: a closer look


1
The RDF meta model a closer look
  • 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
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.

3
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

4
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)
  • ltrdfRDF xmlnswchttp//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

5
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 xmllangitgt……lt/rdfligt
  • lt/rdfAltgt
  • lt/wcTitlegt
  • lt/rdfDescriptiongt
  • lt/rdfRDFgt

6
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

7
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

8
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/gt lt/rdfsPropertygt
    …

Comparable to class PatientRecords
MedicalDocument PatientIDClass patientID
9
RDF v.s. UML(OMG)
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 cd…gt … 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/…/gt ltproperty
    nameTitlegt ltValuegtCigarette …
    lt/Valuegt lt/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 nameTitlegt ltResourcegthttp//www.cnn.c
    om/…lt/Resourcegt ltValuegtCigarette … 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
15
Current Tasks
  • Make a decision about which medical ontology to
    use. See if it is already specified in the RDF
    format
  • Design the information registrys structure make
    extensions to existing schemas such as Dublin
    Core
  • Based on the medical ontology and the registry
    structure
  • design an algorithm for automatic resource
    registration
  • design an ontology-enhanced algorithm for content
    correlation

16
Conclusion
  • 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
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