1 ICS-FORTH & Univ. of Crete SeLene November 15, 2002 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 objectives To facilitate the reuse and integration of information resources by machines over the Web Workplace Business Health Science Education Culture The Community Webs Groups 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

3 ICS-FORTH & Univ. of Crete SeLene November 2002 Maganaraki Aimilia …and why bother with views on Semantic Web? For the good old reasons from the relational world Data Independence Personalization (end user, developer, application..) Data Protection Mechanism- Access Restrictions Integrity Constraints Verification from the object-oriented world Integration of Heterogeneous Databases Versioning/ Schema Evolution from the semistructured and XML world Structuring schema-less data Publishing Relational Databases for the Web

4 ICS-FORTH & Univ. of Crete SeLene November 2002 Maganaraki Aimilia …and why bother with views on Semantic Web? …and for new ones! Web Resource Personalization Subjective ontologies Personalized navigation maps Smart bookmarks… Mediation of heterogeneous web resources Translation of structures between different schemas Ontology management Modularity Versioning Evolution

5 ICS-FORTH & Univ. of Crete SeLene November 2002 Maganaraki Aimilia 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

6 ICS-FORTH & Univ. of Crete SeLene November 2002 Maganaraki Aimilia Resource Description Framework/Schema RDFS: Schema Vocabularies Abstraction Mechanisms (multiple)class/property specialization (multiple) classification of resources Restriction Mechanisms domain and range restriction Documentation Facilities label, comment, seeAlso, isDefinedBy Global identifiers for schema labels: allow their reuse across different data sources &r1 title The Thinker creator Rodin Painting title creator rdfs:Literal RDF: Resource Descriptions Data Model: Directed Label Graph

7 ICS-FORTH & Univ. of Crete SeLene November 2002 Maganaraki Aimilia RD RS A Formal Data Model for RDF/S PropertyClass << S N subjectpredicate D object resources V containersliteralsnames ωω ω [[. ]] [[. ]] T C L P { } [ ] M ( ) U σ σ

8 ICS-FORTH & Univ. of Crete SeLene November 2002 Maganaraki Aimilia On Designing a View Language Must take into account: The underlying data model An object-oriented approach adapted to semistructured data Class and Properties are first-class citizens Must take advantage of: The RQL functional query language To allow for functional composition of operations The underlying type system To ensure the validity of operations To infer the type of view constructs …to output A well defined virtual RDF/S schema that can be reused by others

9 ICS-FORTH & Univ. of Crete SeLene November 2002 Maganaraki Aimilia External Level Conceptual Level The View Definition Process Virtual Schema Virtual Base Root Bases Root Schemas

10 ICS-FORTH & Univ. of Crete SeLene November 2002 Maganaraki Aimilia 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 Class Property

11 ICS-FORTH & Univ. of Crete SeLene November 2002 Maganaraki Aimilia 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 Operator Create virtual (meta)classes and properties Populate virtual (meta)classes and properties Change the semantic level of a base element Subsumption operator Create hierarchies of virtual (meta)classes and properties Filter and reorganize base hierarchies of (meta)classes and properties

12 ICS-FORTH & Univ. of Crete SeLene November 2002 Maganaraki Aimilia 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

13 ICS-FORTH & Univ. of Crete SeLene November 2002 Maganaraki Aimilia 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

14 ICS-FORTH & Univ. of Crete SeLene November 2002 Maganaraki Aimilia Instantiation Operator (2) creates instances creates classes & properties creates metaclasses Metaschema Level rdfs:Classrdf:Property MyMetaClassMyMetaProperty Data Level Schema Level RVL Level rvl:MetaClass rvl:MetaProperty MyClass1MyClass2 &Resource1&Resource2 rdf:typeOf link property link subsumption link P1

15 ICS-FORTH & Univ. of Crete SeLene November 2002 Maganaraki Aimilia Instantiation Operator (3) Define a virtual metaclass of classes VIEW rvl:MetaClass(MyMetaClass) Define a virtual metaclass of properties VIEW rvl:MetaProperty(MyMetaProperty) Define a virtual class VIEW rdfs:Class(MyClass1) Define a virtual property VIEW rdf:Property(P1,MyClass1,MyClass2) Populate a virtual class VIEW MyClass1(&Resource1) Populate a virtual property VIEW P1(&Resource1,&Resource2)

16 ICS-FORTH & Univ. of Crete SeLene November 2002 Maganaraki Aimilia Instantiation Operator (4) Reuse and rename a set of root classes VIEW Class(Greek_+X) FROM subClassOf(ns1:Artist,2){X} USING ns1=& {Greek_Sculptor, Greek_ Painter, Greek_ Cubist, Greek_ Flemish} Reuse a set of root properties VIEW FROM {Creator, last_name, xml:string}, {Creator, first_name, xml:string}, {Creator, creates, Artifact}

17 ICS-FORTH & Univ. of Crete SeLene November 2002 Maganaraki Aimilia Instantiation Operator (5) Populate a virtual class VIEW PicassoWorksInOil(Y) FROM ns1:Artist{X}.paints{Y}.technique{Z}, {X}last_name{W} WHERE W like *Picasso and Z like *oil* USING ns1=& Populate a virtual property VIEW last_modified(X, ) FROM ns1:Museum{X}.ns2:last_modified{Y} WHERE Y> USING ns1=&URI1,ns2=&URI

18 ICS-FORTH & Univ. of Crete SeLene November 2002 Maganaraki Aimilia Subsumption Operator General Syntax: VIEW Symbol {FROM RQL_path_expression} {WHERE filtering_conditions} {USING NAMESPACE ns1=URI,..} Constraints: Acyclic hierarchies Compatibility in terms of subsumption for domain and range of properties in property hierarchies

19 ICS-FORTH & Univ. of Crete SeLene November 2002 Maganaraki Aimilia Subsumption Operator (2) Relate two virtual classes with subsumption VIEW Person Create subclasses for a virtual class VIEW Museum_Artifact FROM subClassof(ns1:Artifact){X} USING NAMESPACE ns1=& Create superclasses for a virtual class VIEW X FROM bag(Teaching_Stuff,Graduate_Student){X} Person Artist Museum_Artifact Sculpture Painting PhD_Student Teaching_StuffGraduate_Student

20 ICS-FORTH & Univ. of Crete SeLene November 2002 Maganaraki Aimilia Properties of the View Definition Language Minimality and Simplicity Defines a minimum set of constructors without reducing the expressive power of the language View declarations are simple and view results are clear Generality Preserves the underlying data model Supports all primitives of the underlying data model Logical Independence The view declarations are isolated from those of the stored data Real schemas do not use view definitions View constructs are virtual support of virtual and not materialized views Closure Everything used by the view is also defined in the view

21 ICS-FORTH & Univ. of Crete SeLene November 2002 Maganaraki Aimilia Thank you

22 ICS-FORTH & Univ. of Crete SeLene November 2002 Maganaraki Aimilia Typing Rules Instantiation operator Classes τ Mc (string | τ C | τ Mc | τ Mp | τ p | τ U ) τ C τ C τ U τ U Properties τ Mp (string|τ p, τ C |τ Mc |τ Mp, τ C |τ Mc |τ Mp |τ L ) [τ p, τ C |τ Mc |τ Mp, τ C |τ Mc |τ Mp |τ L ] τ P (τ U |τ C |τ P, τ U |τ C |τ P |string|integer|real|date|boolean) [τ U |τ C |τ P, τ P, τ U |τ C |τ P |string|integer|real|date|boolean]

23 ICS-FORTH & Univ. of Crete SeLene November 2002 Maganaraki Aimilia Typing Rules Subsumption operator τ C τ C [ τ C, τ C ] τ P τ P [ τ P, τ P ] τ Mc τ Mc [ τ Mc, τ Mc ] τ Mp τ Mp [ τ Mp, τ Mp ]