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RDF

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The overall design goals emphasise generality and precision in expressing ... Lion,type,Species Leo,type,Lion Properties can themselves have properties ... – PowerPoint PPT presentation

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Title: RDF


1
RDF RDF SchemaMachine Understandable Metadata
for the Web
  • Semantic Web - Spring 2006
  • Computer Engineering Department
  • Sharif University of Technology

2
Outline
  • Metadata
  • RDF
  • RDFS
  • RDF(S) Tools

3
Semantic Web Problems
  • Too much Web information
  • around 1,000,000,000 (1?109) resources
  • Many different types of resources
  • text, images, graphics,
  • audio, video, multimedia,
  • databases, Web applications,

4
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

5
Semantic Web Problems (3)
  • Information not shareable
  • Difficult to share information
  • Difficult to share information about information
  • no common cataloging schemes

6
Main Issues
  • Metadata
  • Information about information
  • Structured data about data
  • Many types/forms of metadata, dependent on role

7
Types of Metadata
Web Resource discovery
(Intellectual) property rights management
Document management administration
Archival information / status
Security User authentication
Content ratings (PICS)
Process description control
Product Services Descriptions
Database / data schemas
8
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)

9
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

10
RDF (cont.)
  • RDF is an assertional language intended to be
    used to express propositions using precise formal
    vocabularies.
  • It is intended to provide a basic foundation for
    more advanced assertional languages with a
    similar purpose
  • The overall design goals emphasise generality and
    precision in expressing propositions about any
    topic, rather than conformity to any particular
    processing model

11
RDF in SW Architecture
12
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
13
Example (generated by RDFPic)
14
Example shown in triples view
15
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.

16
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.

17
Linking Statements
  • The subject of one statement can be the object of
    another
  • Such collections of statements form a directed,
    labeled graph

studentOF
departmentOF
hasHomePage
18
RDF Graph anonymous nodes
Person
PersonName
Literal
Person12345
person.name
value
Jonathan
first
last
value
Borden
19
Using XPointer to name
Person
PersonName
Literal
Person12345
person.name
value
Jonathan
/1/1/1
first
/1/1
last
value
/1/1/2
Borden
20
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.
  • .
  • udstv.
  • .

21
Converting N3 to RDF
  • Jena toolkit can do such conversion

22
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
  • http//ce.sharif.edu

23
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

24
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

25
RDF containers
  • Bag (A resource having type rdfBag)
  • Represents an unordered list of resources or
    literals
  • Duplicated values are prermitted
  • Sequence (A resource having type rdfSeq)
  • Represents ordered list of resources or literal
  • Duplicated values are permitted
  • Alternatives (A resource having type rdfAlt)
  • Represents group of resources or literals that
    are alternatives

26
Sequence example
27
Bag example
28
RDF reification
  • association of a statement and a specific
    resource representing the statement
  • used to make statements about statements
  • Vocabulary
  • type rdfasserts
  • properties
  • rdfsubject
  • rdfpredicate
  • rdfobject

29
Reification example
  • In N3

30
Reification example (cont.)
  • In RDF

31
Reification example (cont.)
  • RDF Graph (by IsaViz)

32
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

33
Core Classes Properties
rdfsResource rdfsLiteral rdfsXMLLiteral rdfsCl
ass rdfsProperty
Core Classes
rdfsType rdfsSubClassOf rdfsSubPropertyOf rdfs
Domain rdfsRange rdfsLabel rdfsComment
Core Properties
34
RDFS Examples

35
RDF/RDFS Liberality
  • No distinction between classes and instances
    (individuals)
  • Properties can themselves have properties
  • No distinction between language constructors and
    ontology vocabulary, so constructors can be
    applied to themselves/each other

36
Problems with RDFS
  • RDFS too weak to describe resources in sufficient
    detail
  • No localised range and domain constraints
  • Cant say that the range of hasChild is person
    when applied to persons and elephant when applied
    to elephants
  • No existence/cardinality constraints
  • Cant 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
  • Cant 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

37
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, ?knows
  • WHERE (?x ?z),
  • (?x ?person),
  • (?z ?knows)

38
RDF Validators
  • RDF Validation Service
  • http//www.w3.org/RDF/Validator/
  • In general all the RDF parsers do some kind of
    validation

39
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
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