Sesame

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Sesame

• Querying the Semantic Web

By Arjohn Kampman Jeen Broekstra E-mail
sesame_at_aidministrator.nl
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Goal of this presentation

• To give an impression of
• what Sesame is,
• what it can do and
• where it is heading.

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Contents

• What is Sesame?
• RDF and RDF Schema
• Sesame architecture
• Sesames components
• Our demo server
• Scalability
• Future work
• Some pointers

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What is Sesame?

• Sesame is RDF Schema-based repository
• and querying facility that offers
• persistent storage of RDF data
• export of repository contents in RDF format
• data administration functionality
• query engine (RQL-like)

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RDF

• Statements about resources
• Triples (subject, predicate, object)
• Chaining graph model

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RDF Schema

• Defines vocabulary for RDF
• Organizes this vocabulary in a typed hierarchy
• Class, subClassOf, type
• Property, subPropertyOf
• domain, range

Person
subClassOf
subClassOf
range
domain
Student
Researcher
hasSuperVisor
type
type
hasSuperVisor
Jeen
Frank
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Sesame architecture
Communication with the outside world
Sesame
Abstracts from actual repository being used
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Repository RAL

• Repository Abstraction Layer (RAL) hides the
  repository implementation from the rest of Sesame
• Any type of repository can be used
• Database
• In-memory data structure
• Other RDF repositories
• Currently only PostgreSQL

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RDF Admin

• Functionality for data administration
• Incremental uploads of RDF data
• Clearing of repositories
• TODO more fine-grained update/delete
  functionality
• SQL-like? (delete-from-where)
• Pattern matching?
• E.g. delete ltmyClass, rdfslabel, gt

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Query engine

• RQL RDF Schema Query Language
• Developed by ICS-FORTH
• Currently only available query language
  supporting RDF Schema
• Currently support for subset of RQL
• Some differences with ICS-FORTH
• Small changes to grammar (needs standardization)
• W3C interpretation of RDF Schema
• Domain, range, data typing

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RDF Schema interpretation

• RDF Schema specification not easy to understand
• No formal semantics, only NL description
• Defined using RDF Schema itself
• No clear distinction between levels and
  meta-levels, i.e. data/ontology/language
• RDF Schema not only language for, but also part
  of ontologies
• http//sesame.aidministrator.nl/doc/rdf-interpreta
  tion.html (.pdf)

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Deviations from RDF Schema

• Multiple range-restrictions allowed
• Intersection semantics for multiple domain/range
  restrictions
• Union semantics leads to inconsistencies

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RQL

• Based on OQL (standard QL for object-relational
  DBMS)
• Set of core queries
• Class, Property, subClassOf,
• Filters
• Select-from-where
• Boolean expressions
• Functional composition of queries
• Path expressions
• For navigating the RDF model
• Allowing bridging between RDF data and schema
  information
• Tailored to the RDF graph model
• Fully aware of RDF Schema semantics

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Some example RQL queries

• Basic queries

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Example RQL queries (2)

• More advanced

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RDF Export

• Exports data in RDF format
• Output contains actual contents of repository,
  query engine outputs query results
• Ontology- and data statements can be exported
  separately

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Communicating with Sesame

• Communication is handled by Protocol Handlers
• Currently only via HTTP
• Relatively easy to support other protocols
• http//sesame.aidministrator.nl/doc/communication.
  html (.pdf)

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Sesame web server

• Demo server running at http//sesame.aidministrato
  r.nl/
• Web server supports multiple repositories
• Administration tasks (adding/deleting data)
  protected by password
• Querying and data extraction unrestricted
• E-mail sesame_at_aidministrator.nl for your own
  repository

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Scalability

• Targets for OnToKnowledge
• O(105) statements
• O(103) classes
• Currently tested with
• Wordnet nouns
• Small ontology
• Approx. 400,000 statements
• Skills ontology
• Ontology approx. 1000 classes
• No instances yet

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Future work

• Convergence of the RQL versions
• Add more fine-grained update/delete functionality
• Support for DAMLOIL

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Pointers

• This presentation
• http//sesame.aidministrator.nl/doc/sesame-dissemi
  nation.ppt
• Demo server and more documentation
• http//sesame.aidministrator.nl/
• E-mail addresses
• sesame_at_aidministrator.nl
• Arjohn.Kampman_at_aidministrator.nl
• Jeen.Broekstra_at_aidministrator.nl