1 ICS-FORTH & Univ. of Crete SeLene November 15, 2002 A View Definition Language for the Semantic Web Maganaraki Aimilia

November 15, 2002 1

ICS-FORTH & Univ. of Crete SeLene

A View Definition Language for the Semantic Web

Maganaraki Aimilia

2

ICS-FORTH & Univ. of Crete SeLene November 2002

Maganaraki Aimilia

The Semantic Web…

The objectivesTo facilitate the reuse and

integration of information resources by machines over the Web

WorkplaceBusiness

Health

Science

Education

Culture

The Community WebsGroups of people sharing a

domain of discourse and a set of information resources (e.g., documents, data, images etc)

Use of the Web as a medium to exchange information and knowledge

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…and why bother with views on Semantic Web?

For the good old reasons from the relational world

Data IndependencePersonalization (end user, developer, application..)Data Protection Mechanism- Access RestrictionsIntegrity Constraints Verification

from the object-oriented worldIntegration of Heterogeneous DatabasesVersioning/ Schema Evolution

from the semistructured and XML worldStructuring schema-less dataPublishing Relational Databases for the Web

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…and why bother with views on Semantic Web?

…and for new ones!Web Resource Personalization

Subjective ontologiesPersonalized navigation mapsSmart bookmarks…

Mediation of heterogeneous web resources Translation of structures between different schemas

Ontology managementModularity VersioningEvolution

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Resource Description Framework/Schema

A standard representation language for resource description and domain modeling

Humanly readable/ machine understandable syntax

Interpreted within or across communities using extensible descriptive schemas

Communities can define their own semantics and share the same infrastructure for consistent encoding, exchange and processing of resource descriptions

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Resource Description Framework/Schema

RDFS: Schema VocabulariesAbstraction Mechanisms

(multiple)class/property specialization(multiple) classification of resources

Restriction Mechanismsdomain and range restriction

Documentation Facilitieslabel, comment, seeAlso, isDefinedBy

Global identifiers for schema labels: allow their reuse across different data sources

&r1title “The Thinker”

creator “Rodin”

PaintingPaintingtitle

creatorrdfs:Literalrdfs:Literal

RDF: Resource DescriptionsData Model: Directed Label Graph

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RD

RS

A Formal Data Model for RDF/S

PropertyClass< < SN

subject predicate Dobject

resources Vcontainersliterals names

ωωω

[[ . ]] [[ . ]]

TCL P { }[ ]M ( )

U

σ σ

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On Designing a View Language

Must take into account:The underlying data model

An object-oriented approach adapted to semistructured dataClass and Properties are first-class citizens

Must take advantage of:The RQL functional query language

To allow for functional composition of operationsThe underlying type system

To ensure the validity of operationsTo infer the type of view constructs

…to outputA well defined virtual RDF/S schema that can be reused by

others

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External Level

Conceptual Level

The View Definition Process

Virtual Schema

Virtual Base

Root Bases

Root Schemas

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RDF/S views: Target Functionality

Basic RDF/S namespaces New classes/properties Formulation of hierarchies

Top-down (specialization) Bottom-up (generalization)

Reuse Class/Property DAG

Filter/Restructure a hierarchy

Customized population of classes and properties Transformations

Instance Schema Schema Metaschema

Combination of above

Input Output

ClassProperty

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Proposed View Definition Language- RVL

Integrates in a uniform way the functionality needed, while taking into account the peculiarities of the underlying data model

Instantiation OperatorCreate virtual (meta)classes and propertiesPopulate virtual (meta)classes and propertiesChange the semantic level of a base element

Subsumption operatorCreate hierarchies of virtual (meta)classes and propertiesFilter and reorganize base hierarchies of (meta)classes and

properties

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Declaring an RDF/S View

[ VIEW operator

FROM RQL_path_expression

WHERE filtering_conditions

USING NAMESPACE root_schema_namespace]………………………………………USING NAMESPACE root_schema_namespace

CREATE NAMESPACE RVL_view_namespace

Everything defined in the view is identified by the namespace of the view Everything reused by a root schema is identified by the namespace of the view

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Instantiation Operator

General Syntax for (meta)classes: VIEW Symbol(Symbol)

{FROM RQL_path_expression}{WHERE filtering_conditions}{USING NAMESPACE ns1=URI, ..}

General Syntax for properties: VIEW Symbol(Symbol, Symbol, Symbol)

{FROM RQL_path_expression}{WHERE filtering_conditions}{USING NAMESPACE ns1=URI, ..}

where Symbol=Constant|Variable

Constraint: Unique name assumption

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Instantiation Operator (2)

creates instances

creates classes& properties

creates metaclasses

Metaschema Level

rdfs:Class rdf:Property

MyMetaClass MyMetaProperty

Data Level

Schema Level

RVL Levelrvl:MetaClass rvl:MetaProperty

MyClass1 MyClass2

&Resource1 &Resource2

rdf:typeOf link

property link

subsumption link

P1

P1

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Define a virtual metaclass of classesVIEW rvl:MetaClass(“MyMetaClass”)

Define a virtual metaclass of propertiesVIEW rvl:MetaProperty(“MyMetaProperty”)

Define a virtual classVIEW rdfs:Class(“MyClass1”)

Define a virtual propertyVIEW rdf:Property(“P1”,MyClass1,MyClass2)

Populate a virtual classVIEW MyClass1(&Resource1)

Populate a virtual propertyVIEW P1(&Resource1,&Resource2)

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Reuse and rename a set of root classesVIEW Class(“Greek_”+X)FROM subClassOf(ns1:Artist,2){X}USING ns1=&www.art.com/schema.rdf#

{Greek_Sculptor, Greek_ Painter, Greek_ Cubist, Greek_ Flemish}

Reuse a set of root propertiesVIEW Property(@P,Class(“Creator”),range(@P))FROM {;Artist}@P

{Creator, last_name, xml:string}, {Creator, first_name, xml:string},

{Creator, creates, Artifact}

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Populate a virtual classVIEW PicassoWorksInOil(Y)FROM ns1:Artist{X}.paints{Y}.technique{Z},

{X}last_name{W}WHERE W like “*Picasso” and Z like “*oil*”USING ns1=&www.art.com/schema.rdf#

Populate a virtual propertyVIEW last_modified(X,2002-11-15)FROM ns1:Museum{X}.ns2:last_modified{Y}WHERE Y>2000-01-01USING ns1=&URI1,ns2=&URI

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Subsumption Operator

General Syntax:

VIEW Symbol<Symbol>{FROM RQL_path_expression}{WHERE filtering_conditions}{USING NAMESPACE ns1=URI, ..}

Constraints: Acyclic hierarchiesCompatibility in terms of subsumption for domain and range of

properties in property hierarchies

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Subsumption Operator (2)

Relate two virtual classes with subsumptionVIEW Person<Artist>

Create subclasses for a virtual class VIEW Museum_Artifact<X>FROM subClassof(ns1:Artifact){X}USING NAMESPACE ns1=&www.art.com/schema.rdf#

Create superclasses for a virtual class VIEW X<PhD_Student>FROM bag(Teaching_Stuff,Graduate_Student){X}

Person

Artist

Museum_Artifact

Sculpture Painting

PhD_Student

Teaching_Stuff Graduate_Student

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Properties of the View Definition Language

Minimality and SimplicityDefines a minimum set of constructors without reducing the

expressive power of the languageView declarations are simple and view results are clear

Generality Preserves the underlying data modelSupports all primitives of the underlying data model

Logical IndependenceThe view declarations are isolated from those of the stored data

Real schemas do not use view definitionsView constructs are virtual support of virtual and not

materialized views Closure

Everything used by the view is also defined in the view

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Thank you

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Typing Rules

Instantiation operatorClasses

τMc (string | τC | τMc | τMp | τp | τU) τC

τC τU τU

PropertiesτMp (string|τp, τC|τMc|τMp, τC|τMc|τMp|τL) [τp, τC|τMc|τMp, τC|

τMc|τMp|τL]

τP (τU|τC|τP, τU|τC|τP|string|integer|real|date|boolean)

[τU|τC|τP, τP, τU|τC|τP|string|integer|real|date|boolean]

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Typing Rules

Subsumption operator

τC τC [ τC, τC ]

τP τP [ τP, τP ]

τMc τMc [ τMc, τMc ]

τMp τMp [ τMp, τMp ]

Documents

1 ICS-FORTH & Univ. of Crete SeLene November 15, 2002 A View Definition Language for the Semantic Web Maganaraki Aimilia