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«OBJECT DATABASES AND THE SEMANTIC WEB A THESIS SUBMITTED IN FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY. ING. JAKUB ...»

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Semantic Web as an Object-oriented Database 74 B.4 SKOS AND SWAD-EUROPE SKOS (Simple Knowledge Organization System, http://www.w3.org/2004/02/skos/core.rdf) is one of the activities of SWAD-Europe (Semantic Web Advanced Development, http://www.w3.org/2001/sw/Europe/), EU IST-supported project and part of the W3C Semantic Web Activity. The SKOS-Core specification is intended for connecting various thesaurus activities and the Semantic Web.

SKOS provides a basic framework for building concept schemes, but it does not carry the strictly defined semantics of OWL, which makes it ideal for representing those types of knowledge organization systems, such as thesauri, that cannot be mapped directly to an OWL ontology. SKOS is also easier to use, and harder to misuse than OWL, providing an ideal entry point for those wishing to use the Semantic Web for knowledge organization [MRB04].

SKOS-Core contains a RDF/S vocabulary for linking concepts to the words and phrases (labels) used by people to refer to them — this includes support for definitions, scope notes and examples; several degrees of subsumption (broader/narrower, generic, instantive and partitive relationships), relatedness (related, PART-OF related); high-level categories and internationalization. Other parts of the activity involve work integrating existing RDF solutions and providing frameworks for multilingual thesauri and inter-thesaurus mappings.

B.5 PRISM PRISM (Publishing Requirements for Industry Standard Metadata, [PRISM02]) was developed by a working group in the publishing industry. Its goal is to specify metadata for reusing existing content in a variety of situations, including online versions of magazine articles, licensing to aggregators, retrospective articles, book compilations or licensing to outsiders. In order to meet these needs, the standard focuses on: discovery of content within the industry, which includes metadata about multiple subject taxonomies, topics, origins and context; rights tracking such as needed for stock photo agencies which defines issues like royalty payments or limitations on use in different industries; and end-to-end metadata that can carry over multiple stages of the production pipeline without being lost in processing the files in question.

RDF was chosen as a carrier because DTDs for plain XML cannot adequately describe the many possibilities of defining PRISM metadata (from plain text attributes to structured vocabularies). Apart from using controlled vocabulary such as Dublin Core or ISO standards, PRISM also defines three

new RDF vocabularies:

PRISM core vocabulary (PRISM) subclasses several Dublin Core terms and gives them a more specific meaning — an example is dc:date, which breaks down to prism:publicationTime, prism:releaseTime or prism:expirationTime.

PRISM Rights Language (PRL) is an interim standard in an environment struggling to come up with an agreement in the area of digital rights management. It helps encode basic information about how content may or may not be used depending on information about industry, time and place.

PRISM Controlled Vocabulary (PCV) helps build shared lists of controlled keywords. It assists with defining terms, relations between them and alternate names.

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C.1 GETDATA GetData2 is a very simple query language intended to provide basic RDF retrieval with as little load on the server as possible, making it useful for large-scale deployment. It is also very easy to implement, although its capabilities are extremely limited. Given the object (or subject) and predicate of a given triple, it provides its subject (or object). For data exploration, a query can also return all triples with a given subject.

C.2 RQL RQL [KCA02] was first proposed as part of RDF Suite [ACK01]. It is a declarative language that works with RDF and RDF Schema. Its basic building blocks are functions that query RDF Schema (such as Class, Property, domain and range) and path expressions. RQL contains a SELECT–FROM–WHERE statement that differs from its SQL counterpart in several aspects. The SELECT part gives a list of variables that are returned as a result. FROM specifies a filter in form of path expressions that allows querying a RDF subgraph, and WHERE gives additional conditions such as comparison of literals, variables and results of embedding queries; these can be composed using logical connectives.

This query returns the first names of all Person objects with surname that starts with “D”:

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C.3 SERQL SeRQL1 [MKvH02] used in the Sesame framework is a declarative language similar to RQL, but with several important extensions. First, it supports the CONSTRUCT statement that is identical to SELECT but it returns a portion of RDF graph because a structure of triples replaces the variable list. Second, schema information that cannot be obtained from the RDF graph due to automatic entailments is supported by predicates like serql:directSubClassOf or serql:directType. Third, literals in the WHERE clause must be assigned XSD types; and fourth, SeRQL supports optional path expressions that may or may not return results depending on whether they are found.

This query returns triples of names and possible titles of all PhD students called “Smith”:





CONSTRUCT {name} rdf:type {my:Smith}, {name} my:studentTitle {title} FROM {X} serql:directType {my:PhDStudent};

my:hasSurname {surname};

[ my:hasTitle {title} rdf:type {my:EnumTitles} ];

my:hasFirstName {name}, WHERE surname = "Smith“^^xsd:String C.4 RDQL RDQL was originally developed for the Jena [CDD03] tool. It works with RDF Schema, and supports path expressions. It also supports the SELECT statement, but the FROM part is missing; the subgraph filter is given in WHERE clause as a list of triples, and partial paths can be bound together by variables.

RDQL also supports an AND part of Select, where the results are filtered by value through comparisons and logical connectives.

The following query returns information about all adults called “Smith”.

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C.5 PERLRDF QUERIES PerlRDF Suite was developed by Ginger Alliance (http://www.gingerall.com/), and contains an array of tools for the Perl programming language, including a query processor. Again, the SELECT–FROM– WHERE statement is available with support for namespace use, functions, path expressions, comparisons and logical connectives.

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This query retrieves names and surnames of Person objects. It demonstrates several ways of

returning values, using class membership (::) and namespace with or without abbreviations:

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Figure 2.1 A timeline of milestones in OODB data modeling ______________________________ 5 Figure 2.

2 Metatypes in the ODMG model ____________________________________________ 8 Figure 2.3 The full ODMG built-in type hierarchy (abstract classes italicized) __________________ 9 Figure 2.4 Example of an LDM schema category_______________________________________ 24 Figure 3.1 An example from Tim Berners-Lee’s original WWW proposal [BernersLee89] ________ 29 Figure 3.2 An example of a RDF graph (description of Eric Miller) [MM04] __________________ 31 Figure 3.3 Example of a RDF list collection — books on a library shelf _____________________ 34 Figure 3.4 An example of RDF model theory _________________________________________ 35 Figure 4.1 The worlds of object-oriented databases and ontologies _________________________ 44 Figure 5.1 Domain of discourse division _____________________________________________ 50 Figure 5.2 Attribute path from a tuple to class instance __________________________________ 55 Figure 5.3 Overview of the example — development stages ______________________________ 56 Figure 5.4 UML class diagram of the online magazine ___________________________________ 57 Figure 5.5 RDF database contents (instances) for the online magazine _______________________ 59 Figure 6.1 Reachability in part of a RDF graph ________________________________________ 64 Semantic Web as an Object-oriented Database 80

E GLOSSARY OF ABBREVIATIONS

CDL Concept Definition Language, language that describes schemas in the G2 object database CT Category Theory, a very general and abstract formalism for declarative modeling DC Dublin Core Metadata, a standard for specifying basic metadata about various resources DTD Document Type Definition, a context-free grammar based definition format for XML files JDO Java Data Objects, Java standard for object-oriented and object-relational database models JDOQL Java Data Objects Query Language, part of JDO specification, a language similar to OQL ODMG Object Data Management Group, a standard for object-oriented models of persistent data OID Object Identifier, mandatory unique identification of objects in object-oriented databases OODB Object-Oriented Database, a database that stores objects connected by relationships OQL Object Query Language, SQL-like query language specification in the ODMG standard OWL Ontology Web Language, a high-level ontology language (now a W3C recommendation) P-C Persistence Capable classes, classes in the JDO framework that are stored in the database RDF Resource Description Framework, a W3C standard for structuring Semantic Web data RDFS RDF Schema, a lightweight ontological standard for further structuring RDF graphs RSS Remote Site Syndication 1.0 is a popular RDF-based format for syndicating online content SemWeb Semantic Web, a W3C standard for next-generation Web with computer-readable semantics SODA Semantic Object-Oriented Database, model presented in this thesis, based on CDL+RDF URI Universal Resource Identifier, a widely used standard for assigning global identifiers uriref URI reference, used as a unique object identifier (OID) XML EXtensible Markup Language, a standard for exchanging syntactically structured data XMP Extensible Metadata Protocol, an Adobe standard for embedding RDF metadata in files XSD XML Schema Datatypes, a standard for typing XML data and forming new datatypes W3C World Wide Web Consortium, an organization that develops Semantic Web standards



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