Viewing the Semantic Web

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October 20-23, 2003 1

ICS-FORTH & Univ. of Crete ISWC’03

D. Plexousakis

Viewing the Semantic Web through RVL Lenses

Aimilia Maganaraki, Val Tannen*, Vassilis Christophides, Dimitris Plexousakis

University of Crete and Institute of Computer Science - FORTHHeraklion, Crete, Greece

*University of Pennsylvania, Philadelphia, PA, USA

Ack: This work has been supported in part by the IST Project “SeLeNe”

RVL

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D. Plexousakis

The Semantic Web…

Community Webs–Groups sharing a domain of

discourse and a set of information resources(documents, data, services, etc.)

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

The objective:gto provide a single point of useful,

ubiquitous, comprehensive, and integrated access to community information resources

WorkplaceBusiness

Health

Science

Education

Culture

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…and why bother with views on the SW?

n For the good old reasonsData IndependencePersonalizationData Protection Mechanism - Access ControlIntegration of Heterogeneous DatabasesIntegrity Constraint VerificationVersioning / Schema EvolutionStructuring schema-less dataPublishing Relational Databases on the Web

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Still, why bother with views on the SW?

n …and for a bunch of new ones!gWeb Resource Personalization

•Subjective ontologies•Personalized schema navigation maps•Smart bookmarks

gMediation of heterogeneous web resources•Translation of structures according to different schemas•Ontology Integration / Interoperation

gOntology management•Modularity •Versioning•Evolution

SW

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Our Vision: Combine DB and KR Approaches

n Provide a comprehensive, high-level access to community resourcesgOntologies as shared, formal conceptua-

lizations of domains of interestn Build scalable technologies for managing

semantically rich data and metadata gDeclarative Query / View LanguagesgEfficient Storage for Voluminous

Descriptive Informationn Support an expressive SW Integration

MiddlewaregEstablish Mapping / Translation RulesgReformulate Conceptual QueriesgExploit data semantics for Query

Optimization and Consistency Checking

Community Web Ontologies

Archives

Virtual SW Integration

Documents

Databases

Web

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Example Application: Web Personalization

Home Business

Regional

.….…

News

SocietyArts

.….…

.….…

Computers

Health

Recreation

Sports

.….…

Science

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Example Application: Ontology Integration

Product Catalog of Buyer 2

Sellers Buyers

Β2Β MARKETPLACE

Product Catalog of Seller 1

Product Catalog of Seller 2

Product Catalog of Seller n

…

… ……

… …

…

… ……

… …



Product Catalog of Buyer n

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The RDF/S View Language: RVL

n Declarative view definition language for virtual RDF description bases and schemasgrelies on the RQL typed data modelgalso follows a functional approach (object construction

operators)gensures logical data independence

• view specifications are independent from those of the source schemas and bases

• the semantics of existing virtual schemas is not altered by the definition of new ones

gsupports object-preserving and object-generating viewsgprovides heavy-duty data restructuring facilitiesgallows users to query and create views using both source

and virtual schemas

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External Level

Conceptual Level

RVL Approach

Source Bases

Source Schemas

Virtual Schema

Virtual Base

ƒ

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The View Definition Process

Virtual Schema

Root Schema

Root Base

Extended Schema

Virtual Base

n A view defines a set of virtual names and their hierarchies. Everything defined in the view is identified by the view namespace. Everything reused from a root schema is identified by the root schema namespace.

Extended schema: the root schema is extended with new constructsVirtual schema: a projection over the extended schema for selecting the constructs of the view

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RVL Functionality

Combination of above

g Customized population of classes and propertiesg Transformations

• Instance ↔ Schema• Schema ↔ Metaschema

g Reuse• Class/Property• DAG

g Filter/Restructure a hierarchy

g New classes/propertiesg New subsumption hierarchies

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

Basic RDF/S namespacesInput Output

Class Property

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The RVL Syntax

[ VIEW operatorFROM RQL_path_expressionWHERE filtering_conditionsUSING NAMESPACE source_schema_namespace]

………………………………………CREATE NAMESPACE RVL_view_namespace

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RVL Operators

n RVL integrates in a uniform way the functionality needed, whilst taking into account the peculiarities of the RDF/S data model

n Instantiation OperatorgCreates virtual (meta-) classes and propertiesgPopulates virtual (meta-) classes and propertiesgUp- (Down-) grades the abstraction level of a source entity

n Subsumption OperatorgCreates new subsumption hierarchies of virtual (meta-)

classes and propertiesgReorganizes source subsumption hierarchies of (meta-)

classes and properties

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An RVL virtual RDF/S schema and base

Fine_Art_Museum

Painting_MuseumSculpture_Museum

name StringArtifact

SculpturePainting

exhibitedString

creator

sculpture_exhibited

painting_exhibited

Artist

Sculptor

StringArtifact

Sculpture

Painting

sculpts

createsfname

lname

paints

StringMuseumexhibited

techniqueStringPainter

denomString

Vir

tual sc

hem

aS

ou

rce S

chem

a

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VIEW Class(“Fine_Art_Museum”), Class(“Painting_Museum”), Class(“Sculpture_Museum”), Class(“Artifact”), Class(“Painting”), Class(“Sculpture”)

CREATE NAMESPACE myview=&http://www.ics.forth.gr/mycult.rdf#

VIEW Property(“name”, Fine_Art_Museum, xsd:string), Property(“title”, Artifact, xsd:string), Property(“creator”, Artifact, xsd:string), Property(“exhibited”, Artifact, Fine_Art_Museum),Property(“sculpture_exhibited”,Sculpture, Sculpture_Museum),Property(“painting_exhibited”, Painting, Painting_Museum)

VIEW Fine_Art_Museum<Sculpture_Museum>, Fine_Art_Museum<Painting_Museum>,Artifact<Painting>, Artifact<Sculpture>exhibited<sculpture_exhibited>,exhibited<painting_exhibited>

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VIEW Painting(X), painting_exhibited(X,Y), Painting_Museum(Y), name(Y,W), title(X,K), creator(X,Z)

FROM {Z}n1:creates{X; n1:Painting}.n1:exhibited{Y}.n1:denom{W}, {X}n1:title{K}

USING NAMESPACE n1=&http://www.culture.mus/cult.rdf#

VIEW Sculpture(X), sculpture_exhibited(X,Y), Sculpture_Museum(Y), name(Y,W), title(X,K), creator(X,Z)

FROM {Z}n1:creates{X; n1:Sculpture}.n1:exhibited{Y}.n1:denom{W},{X}n1:title{K}

USING NAMESPACE n1=&http://www.culture.mus/cult.rdf#

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RVL Design Issues

What is a good specification of a view language for the RDF/S data model?

How are the virtual schema (meta-) classes and properties of a view related to the source description schema(s)?

How are the virtual base resources and property values of a view related to source description base(s)?

What is the expressiveness of the input/output transformationssupported by the view specification language?

How can the output of view specifications be used in queries and other views?

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RVL Design Choices

Logical Data Independence: the view specifications should be independent from those of the source schemas and bases, while the semantics of existing virtual schemas should not be altered by the definition of new onesgthe scope of virtual (meta-) class and property definitions is

determined by the namespace of the viewgvirtual subsumption hierarchies instead of global hierarchies

View Instantiation Capabilities: population of virtual (meta-) classes and propertiesgobject-preserving views vs object-generating views

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RVL Design Choices

Transformation Expressiveness: provide the ability to both create and reconcile different conceptual representationsgheavy-duty data restructuring facilities enabling users to change

the abstraction level in which a particular view construct is defined

Closure of View Language: ability to query and create views using both source and virtual schemasgthe namespace of a view can be used to formulate RQL queries

and define views

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RVL vs other View Languages

n ODMG-compliant view definition languages: O2Views, MultiView, Chimera, K2gDifferences in data models and underlying design choicesgRVL is capable of creating virtual classes and properties using RQL

queries on (meta-) schema and data information

n RDF view definition languages: gKAON Views: violates the logical data independence of views (one

global hierarchy), while restructuring constructs for subsumptionhierarchies are not supported

gTriple Views: relies on F-Logic rules to define only virtual description bases

gSeRQL: proposes a variation of RQL in order to produce resource description graphs

gRVL is the only full-fledged RDF/S view definition language

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Advanced Semantic Web Infrastructure

XML Server

S2

ODBCServer

S1

SW Middleware Server

Q1

RQL

R2R1

HTML/WAP

Q2

+ mapping rulesconstraints

HTML/WAP

RVLRVL

RQL

RDF RDF/S

RQL

RDF

RQL

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Advanced Semantic Web Services

n Semantic Integration of Heterogeneous Resourcesg Consistency Checking of Mappings

n Semantic Query OptimizationgMinimization of RQL Queries

n Semantic Query MediationgReformulation of RQL to SQL/XQuery

n Peer-to-Peer Personalization gUnconstrained RVL/RQL Composition

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The SW Layer Cake

RQL

RVL

Constraints

Datalog

First Order

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Conclusions

n Re-use of existing data/schemas/ontologies is crucial in describing the semantics of various information sources in the Semantic Web

n Defining views is a way of reusing the same information set by different users

n RVL is a declarative, functional view definition language integrating into a uniform way all the functionality needed with the minimal set of operators. gIt takes advantage of the underlying data model, the underlying

type system and the expressiveness of the RQL query language to constitute a simple, yet very expressive view definition language.

Viewing the Semantic Web

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