Semantic Web Basics cont'

1
Semantic Web Basics (cont.)

• CS 431 March 30, 2005
• Carl Lagoze Cornell University

2
Acknowledgements for various slides and ideas

• Ian Horrocks (Manchester U.K.)
• Eric Miller (W3C)
• Dieter Fensel (Berlin)
• Volker Haarslev (Montreal)

3
Semantic Web and the W3C

• http//www.w3.org/2001/sw/
• http//www.w3.org/RDF/

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RDF Data Model

• Directed Graph
• Primitives
• resource
• property
• literal
• statement
• Other constructs
• container
• collection
• reification
• URIs for everything except literals

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RDF Model Primitives
Resource
Property
Value
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Typed Literals
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RDF Containers

• Permit aggregation of several values for a
  property
• Different container semantics
• Bag
• unordered grouping (e.g., students in this class)
• Sequence
• ordered grouping (e.g., authors of a paper)
• Alternatives
• alternate values (e.g., measurement in different
  units)

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RDF Collections

• Containers are not closed
• open world assumption in all of them
• Collections use lisp-like primitives (first,
  rest, nil) to express a close list.

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RDF Reification

• Treat a statement as a first-class object
  (resource)
• It then can become a graph element (and be used
  as subject and object of statements)

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Formalizing RDF

• There is a meta-model that bootstraps RDF
• Set of basic types and properties
• Allows basic inferencing

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RDF meta-model basis elements

• All defined in rdf namespace
• http//www.w3.org/1999/02/22-rdf-syntax-ns
• Properties
• rdftype - subject is an instance of that
  category or class defined by the value
• rdfsubject, rdfpredicate, rdfobject relate
  elements of statement tuple to a resource of type
  statement.
• Types (or classes)
• rdfResource everything that can be identified
  (with a URI)
• rdfProperty specialization of a resource
  expressing a binary relation between two
  resources
• rdfstatement a triple with properties
  rdfsubject, rdfpredicate, rdfobject

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Use of rdftype

• Resource named http//foo.org/inst is member of
  class http//foo.org/classes/cl1
• lthttp//foo.org/instgt ltrdftypegt
  lthttp//foo.org/classes/cl1gt

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Formalizing a statement

• An RDF statement is a triple consisting of
• subject ? rdftype resource
• property ? rdftype property
• object ? rdftype resource literal
• Examples
• lthttp//www.cs.cornell.edu/lagozegt
  lthttp//purl.org/dc/elements/creatorgtCarl
  Lagoze
• lthttp//www.cs.cornell.edu/lagozegt
  lthttp//purl.org/dc/elements/creatorgt
  ltmailtolagoze_at_cs.cornellgt
• Expressible as
• triple (ns1s ns2p ns3o)

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RDF statements and basic types
rdfstatement
rdfobject
rdfsubject
rdfpredicate
WYA
DigitalLibraries
creator
rdfproperty
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Simple type inferencing
explicit triple
Allows inference
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Reification Statements about statements
rdfstatement
CL
assertedBy
rdfobject
rdfsubject
rdfpredicate
WYA
DigitalLibraries
creator
rdfproperty
CL says WYA wrote Digital Libraries
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Beyond binary relations

• Note mapping of RDF statements to binary
  relations that could be stored in a database
• (s p o) maps to P(S,O) e.g., Title(R, War
  Peace)
• But the world is more complex
• Carl Lagoze has his office at 301 College Ave.,
  Ithaca, NY 14850
• (Carl Lagoze hasOffice 301 College Ave,
  Ithaca, NY 14850)
• (Carl Lagoze address 301 College Ave
  Ithaca NY 14850)

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Expressing n-ary relationswith blank nodes
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Another n-ary relation example
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Expressing Collection Primitives in Binary
Relations
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RDF Collections and blank nodes
The students in course 6.001 are Amy, Mohamed,
and Johann
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Reification Structure
Staff member 85740 said the weight of item 10245
is 2.4 units
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Reification XML
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Why Schema (1)?

• Enables communities to share machine readable
  tokens and locally define human readable labels.

dcCreator
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Why Schema (2)?Relationships among vocabularies
dcCreator
marc100
msdirector
bibAuthor
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Why Schema(3)?Relationships among vocabulary
elements
URIR
msdirector
John Smith
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RDF Schemas

• Declaration of vocabularies
• classes, properties, and structures defined by a
  particular community
• relationship of properties to classes
• Provides substructure for inferences based on
  existing triples
• NOT prescriptive, but descriptive
• Schema language is an expression of basic RDF
  model
• uses meta-model constructs
• schema are legal rdf graphs and can be
  expressed in RDF/XML syntax

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RDFs Namespace

• Class-related
• rdfsClass, rdfssubClassOf
• Property-related
• rdfssubPropertyOf, rdfsdomain, rdfsrange

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RDF Schema Specializing Properties

• rdfssubPropertyOf allows specialization of
  relations
• E.g., the property father is a subPropertyOf
  the property parent
• subProperty semantics

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Sub-Property Semantics
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Property-based semantics

• Provide basis for type inference from properties
• rdfsdomain
• classes of resources that have a specific
  property
• rdfsrange
• classes of resources that may be the value of a
  specific property

range
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Inferences from Constraints
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Class Declaration

• rdfsClass
• Resources denoting a set of resources range of
  rdftype

rdftype
rdftype
rdfsclass
exMotorVehicle rdftype rdfsClass exthingscompa
nyCar rdftype exMotorVehicle
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Class Hierarchy

• rdfssubClassOf
• Create class hierarchy

rdfssubClassOf
rdftype
rdftype
rdfclass
rdfclass
exMotorVehicle rdftype rdfsClass exSUV
rdftype rdfsClass exSUV rdfsubClassOf
exMotorVehicle exthingscompanyCar rdftype
exSUV
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Sub-Class Inferencing
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Sub-class Inferencing Example
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Jena Toolkit

• Robust tools for building and manipulating RDF
  models
• HP Labs Bristol
• Capabilities
• Model construction
• XML and N3 parsing
• Model persistence (DB foundation)
• Model querying
• Ontology building
• Inferencing
• http//www.hpl.hp.com/semweb/jena2.htm

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IsaViz

• Visualizing and constructing RDF models
• http//www.w3.org/2001/11/IsaViz/