Zugl.: Bamberg, Universiẗat, Diss., 2008.

Techn. University, Diss., 2004--Darmstadt.

Universiẗat, Diss., 2004--Bielefeld.

Universiẗat, Diss., 2006--Frankfurt (Main).

abstract: The Resource Description Framework (RDF) is a specification that aims to support the conceptual modeling of metadata or information about resources in the form of a directed graph composed of triples of knowledge (facts). RDF also provides mechanisms to encode meta-information (such as source, trust, and certainty) about facts already existing in a knowledge base through a process called reification. In this thesis, an extension to the current RDF specification is proposed in order to enhance RDF triples with an application specific weight (cost). Unlike reification, this extension treats these additional weights as first class knowledge attributes in the RDF model, which can be leveraged by the underlying query engine. Additionally, current RDF query languages, such as SPARQL, have a limited expressive power which limits the capabilities of applications that use them. Plus, even in the presence of language extensions, current RDF stores could not provide methods and tools to process extended queries in an efficient and effective way. To overcome these limitations, a set of novel primitives for the SPARQL language is proposed to express Top-k queries using traditional query patterns as well as novel predicates inspired by those from the XPath language. Plus, an extended query processor engine is developed to support efficient ranked path search, join, and indexing. In addition, several query optimization strategies are proposed, which employ heuristics, advanced indexing tools, and two graph metrics: proximity and sub-result inter-arrival time. These strategies aim to find join orders that reduce the total query execution time while avoiding worst-case pattern combinations. Finally, extensive experimental evaluation shows that using these two metrics in query optimization has a significant impact on the performance and efficiency of Top-k queries. Further experiments also show that proximity and inter-arrival have an even greater, although sometimes undesirable, impact when combined through aggregation functions. Based on these results, a hybrid algorithm is proposed which acknowledges that proximity is more important than inter-arrival time, due to its more complete nature, and performs a fine-grained combination of both metrics by analyzing the differences between their individual scores and performing the aggregation only if these differences are negligible. / Dissertation/Thesis / M.S. Computer Science 2010

There is an emerging demand on efficiently archiving and (temporal) querying different versions of evolving semantic Web data. As novel archiving systems are starting to address this challenge, foundations/standards for benchmarking RDF archives are needed to evaluate its storage space efficiency and the performance of different retrieval operations. To this end, we provide theoretical foundations on the design of data and queries to evaluate emerging RDF archiving systems. Then, we instantiate these foundations along a concrete set of queries on the basis of a real-world evolving dataset. Finally, we perform an empirical evaluation of various current archiving techniques and querying strategies on this data that is meant to serve as a baseline of future developments on querying archives of evolving RDF data.

En el año 2004 el Consorcio de la World Wide Web (W3C) estandarizó un lenguaje de metadatos para la descripción de recursos en la Web denominado Resource Description Framework (RDF). La motivación fue el definir un lenguaje que sirva de base para el modelado extensible de dominios altamente interconectados y redes de información. La especificación de RDF puede ser vista, desde un punto de vista de base de datos, como un modelo de base de datos. En efecto, subyacente a RDF se encuentra un modelo el cual trae a la mente la noción de datos con estructura de grafo. Por otra parte, la cifra creciente de información representada en el lenguaje RDF ha sido acompañada de varias propuestas sobre como almacenar y consultar datos RDF. En consecuencia, resulta natural el estudio de RDF desde un punto de vista de base de datos. El objetivo principal de esta tesis es el estudio de RDF desde una perspectiva de base de datos. Nuestro estudio se concentra en el modelado de base de datos, enfocándonos en modelos particulares llamados modelos de base de datos de grafo, los cuales parecen estar más relacionados a RDF desde un punto de vista teórico. Asimismo, esta tesis sigue de cerca los desarrollos del W3C en el sentido de entregar a RDF el soporte de un modelo de base de datos, en particular desarrollar los aspectos de consulta de datos. La principal contribución de la tesis es haber clarificado y desarrollado la relación entre RDF y los modelos de base de datos de grafo. Para esto, primero se estudió RDF como un modelo de base de datos. Segundo, se realizó una conceptualización del área de modelos de base de datos de grafo. Tercero, se propuso un conjunto deseable de propiedades de grafo las cuales deberían ser soportadas por un lenguaje de consulta ideal para RDF. Cuarto, se demostró que el lenguaje de consulta estándar definido por el W3C, denominado SPARQL, no soporta las consultas de grafo esenciales que se esperaría encontrar en un lenguaje de consulta para bases de datos de grafo. Esto fue conseguido al estudiar y determinar el poder expresivo de SPARQL. Finalmente, y debido a los resultados anteriores, nos concentramos en la aplicación de los modelos de bases de datos de grafo a la visualización de datos, esto al definir formalmente y caracterizar un modelo de visualización de grafo para RDF.

Main goal of this diploma thesis is characterization of open data, standards and analyzation of adoption and utilization of open principles in the public sector of the Czech Republic. And comparison with European Union and chosen countries. Identifies technology and tools for linked data, used for deployment of highest rating of data openness. Defines geographical data, its standards and INSPIRE directive for spatial information in Europe. The goal of practical part of thesis is to analyze adoption of open principles for geographical data between Czech institutions. Focusing on what data are available, if open principles are applied and on what circumstances are data available. Foreign countries are also covered for the comparison.

The aim of this diploma thesis is a detail description of current possibilities and ways of storing open data. It focuses on tools and database systems used for storing linked open data as well as on the selection of such systems for subsequent analysis and comparison. The practical part of the thesis then focuses on the comparison of selected systems based on a selected use case. This thesis introduces the fundamental terms and concepts concerning linked open data. Besides that, various approaches and formats for storing linked open data (namely file ori-ented approaches and database approaches) are analyzed. . The thesis also focuses on the RDF format and database systems. Ten triplestore database solutions (solutions for storing data in the RDF format) are introduced and described briefly. Out of these, three are cho-sen for a detailed analysis by which they are compared with one another and with a rela-tional database system. The core of the detail analysis lies in performance benchmarks. Ex-isting performance oriented benchmarks of triplestore systems are described and analyzed. In addition to that, the thesis introduces a newly developed benchmark as a collection of database queries. The benchmark is then used for the performance testing. The following systems have been tested: Apache Jena TDB/Fuseki, OpenLink Virtuoso, Oracle Spatial and Graph a Microsoft SQL Server. The main contribution of this thesis consists in a comprehensive presentation of current possibilities of storing linked open data.

This thesis deals with the issue of open data. The aim is to introduce to reader the currently very popular topic. Linking these data together gives us more advantages and opportuni-ties, however a large number of open data datasets are published in the format that cannot be linked together. Therefore, the author put great emphasis into his work on Linked Data. Emphasis is not placed only on the emergence, current status and future development, but also on the technical aspect. First, readers will be familiar with theoretical concepts, principles of Linked Open Data, expansion of open government data in the Czech Republic and abroad. In the next chapter, the author aimed at the data formats RDF, SPARQL language, etc. In the last section, the author introduce to readers the tools to work with Linked Open Data and design sample application using the Linked Open Data. The benefit of the whole work is a comprehensive view of the Linked Open Data both from a theoretical and from a practical part. The main goal is to provide to readers quality introduction to the issue.