A View Definition Language for the Semantic Web

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A View Definition Language for the Semantic Web

• Maganaraki Aimilia

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The Semantic Web

• 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

• The objectives
• To facilitate the reuse and integration of
  information resources by machines over the Web

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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

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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

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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

• RDF Resource Descriptions
• Data Model Directed Label Graph

• 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

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A Formal Data Model for RDF/S
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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

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The View Definition Process
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RDF/S views Target Functionality
Input
Output
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 Combination of above
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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 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

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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 ns1URI, ..
• General Syntax for properties
• VIEW Symbol(Symbol, Symbol, Symbol)
• FROM RQL_path_expression
• WHERE filtering_conditions
• USING NAMESPACE ns1URI, ..
• where SymbolConstantVariable
• Constraint Unique name assumption

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Instantiation Operator (2)
creates metaclasses
creates classes properties
creates instances
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Instantiation Operator (3)

• Define a virtual metaclass of classes
• VIEW rvlMetaClass(MyMetaClass)
• Define a virtual metaclass of properties
• VIEW rvlMetaProperty(MyMetaProperty)
• Define a virtual class
• VIEW rdfsClass(MyClass1)
• Define a virtual property
• VIEW rdfProperty(P1,MyClass1,MyClass2)
• Populate a virtual class
• VIEW MyClass1(Resource1)
• Populate a virtual property
• VIEW P1(Resource1,Resource2)

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Instantiation Operator (4)

• Reuse and rename a set of root classes
• VIEW Class(Greek_X)
• FROM subClassOf(ns1Artist,2)X
• USING ns1www.art.com/schema.rdf
• ?Greek_Sculptor, Greek_ Painter, Greek_
  Cubist, Greek_ Flemish
• Reuse a set of root properties
• VIEW Property(_at_P,Class(Creator),range(_at_P))
• FROM Artist_at_P
• ?Creator, last_name, xmlstring, Creator,
  first_name, xmlstring,
• Creator, creates, Artifact

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Instantiation Operator (5)

• Populate a virtual class
• VIEW PicassoWorksInOil(Y)
• FROM ns1ArtistX.paintsY.techniqueZ,
• Xlast_nameW
• WHERE W like Picasso and Z like oil
• USING ns1www.art.com/schema.rdf
• Populate a virtual property
• VIEW last_modified(X,2002-11-15)
• FROM ns1MuseumX.ns2last_modifiedY
• WHERE Ygt2000-01-01
• USING ns1URI1,ns2URI

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Subsumption Operator

• General Syntax
• VIEW SymbolltSymbolgt
• FROM RQL_path_expression
• WHERE filtering_conditions
• USING NAMESPACE ns1URI, ..
• Constraints
• Acyclic hierarchies
• Compatibility 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 subsumption
• VIEW PersonltArtistgt
• Create subclasses for a virtual class
• VIEW Museum_ArtifactltXgt
• FROM subClassof(ns1Artifact)X
• USING NAMESPACE ns1www.art.com/schema.rdf
• Create superclasses for a virtual class
• VIEW XltPhD_Studentgt
• FROM bag(Teaching_Stuff,Graduate_Student)X

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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

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Thank you
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Typing Rules

• Instantiation operator
• Classes
• tMc ? (string tC tMc tMp tp tU) ? tC
• tC ? tU? tU
• Properties
• tMp ? (stringtp, tCtMctMp, tCtMctMptL)?
  tp, tCtMctMp, tCtMctMptL
• tP ? (tUtCtP, tUtCtPstringintegerrealdate
  boolean)?
• tUtCtP, tP, tUtCtPstringintegerrealdate
  boolean

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Typing Rules

• Subsumption operator
• tC ? tC ? tC, tC
• tP ? tP ? tP, tP
• tMc ? tMc ? tMc, tMc
• tMp ? tMp ? tMp, tMp