Partout: A Distributed Engine for Efficient RDF Processing

The increasing interest in Semantic Web technologies has led not only to a rapid growth of semantic data on the Web but also to an increasing number of backend applications with already more than a trillion triples in some cases. Confronted with such huge amounts of data and the future growth, existing state-of-the-art systems for storing RDF and processing SPARQL queries are no longer sufficient. In this paper, we introduce Partout, a distributed engine for efficient RDF processing in a cluster of machines. We propose an effective approach for fragmenting RDF data sets based on a query log, allocating the fragments to nodes in a cluster, and finding the optimal configuration. Partout can efficiently handle updates and its query optimizer produces efficient query execution plans for ad-hoc SPARQL queries. Our experiments show the superiority of our approach to state-of-the-art approaches for partitioning and distributed SPARQL query processing

Entity Summarisation with Limited Edge Budget on Undirected and Directed Knowledge Graphs

The paper concerns a novel problem of summarising entities with limited presentation budget on entity-relationship knowledge graphs and propose an efficient algorithm for solving this problem. The algorithm has been implemented in two variants: undirected and directed, together with a visualisation tool. Experimental user evaluation of the algorithm was conducted on real large semantic knowledge graphs extracted from the web. The reported results of experimental user evaluation are promising and encourage to continue the work on improving the algorithm.

Efficient creation and incremental maintenance of the hopi index for complex xml document collections

The HOPI index, a connection index for XML documents based on the concept of a 2–hop cover, provides space – and time–efficient reachability tests along the ancestor, descendant, and link axes to support path expressions with wildcards in XML search engines. This paper presents enhanced algorithms for building HOPI, shows how to augment the index with distance information, and discusses incremental index maintenance. Our experiments show substantial improvements over the existing divide-and-conquer algorithm for index creation, low space overhead for including distance information in the index, and efficient updates

Transaktionen in föderierten Datenbanksystemen unter eingeschränkten Isolation Levels

Atomarität und Isolation von Transaktionen sind Schlüsseleigenschaften fortgeschrittener Anwendungen in föderierten Systemen, die aus verteilten, heterogenen Komponenten bestehen. Während Atomarität von praktisch allen realen Systemen durch das Zweiphasen- Commitprotokoll gewährleistet wird, unterstützt kein System eine explizite föderierte Concurrency Control. In der Literatur wurden zwar zahlreiche Lösungsansätze vorgeschlagen, doch sie haben wenig Einfluss auf Produkte genommen, weil sie die weitverbreiteten Isolation Levels nicht berücksichtigen, die Applikationen Optimierungsmöglichkeiten auf Kosten einer eingeschränkten Kontrolle über die Konsistenz der Daten erlauben. Diese Arbeit vergleicht zunächst existierende Definitionen für Isolation Levels und entwickelt eine neuartige, formale Charakterisierung für Snapshot Isolation, dem Isolation Level des Marktführers Oracle. Anschließend werden Algorithmen zur föderierten Concurrency Control vorgestellt, die beweisbar auch unter lokaler Snapshot Isolation die korrekte Ausführung föderierter Transaktionen gewährleisten, und Isolation Levels für föderierte Transaktionen diskutiert. Die Algorithmen sind in ein prototypisches föderiertes System integriert. Performancemessungen an diesem Prototyp zeigen ihre praktische Einsetzbarkeit.Atomicity and isolation of transactions are key requirements of advanced applications in federated systems consisting of distributed and heterogeneous components. While all existing federated systems support atomicity using the two-phase commit protocol, they lack support for federated concurrency control. Many possible solutions have been proposed in the literature, but they failed to make impact on real systems because they completely ignored the widely used concept of isolation levels, which offer optimization options to applications at the cost of less rigorous control over data consistency. This thesis compares existing definitions for isolation levels and develops a new characterization for Snapshot Isolation, an isolation level provided by Oracle, the market leader in the database field. We present algorithms for federated concurrency control that provably guarantee the correct execution of federated transactions even under local Snapshot Isolation, and discuss isolation levels for federated transactions. The algorithms are integrated into a federated system prototype. Performance measurements with this prototype show the practical viability of the developed methods

Transaktionen in föderierten Datenbanksystemen unter eingeschränkten Isolation Levels

Atomarität und Isolation von Transaktionen sind Schlüsseleigenschaften fortgeschrittener Anwendungen in föderierten Systemen, die aus verteilten, heterogenen Komponenten bestehen. Während Atomarität von praktisch allen realen Systemen durch das Zweiphasen- Commitprotokoll gewährleistet wird, unterstützt kein System eine explizite föderierte Concurrency Control. In der Literatur wurden zwar zahlreiche Lösungsansätze vorgeschlagen, doch sie haben wenig Einfluss auf Produkte genommen, weil sie die weitverbreiteten Isolation Levels nicht berücksichtigen, die Applikationen Optimierungsmöglichkeiten auf Kosten einer eingeschränkten Kontrolle über die Konsistenz der Daten erlauben. Diese Arbeit vergleicht zunächst existierende Definitionen für Isolation Levels und entwickelt eine neuartige, formale Charakterisierung für Snapshot Isolation, dem Isolation Level des Marktführers Oracle. Anschließend werden Algorithmen zur föderierten Concurrency Control vorgestellt, die beweisbar auch unter lokaler Snapshot Isolation die korrekte Ausführung föderierter Transaktionen gewährleisten, und Isolation Levels für föderierte Transaktionen diskutiert. Die Algorithmen sind in ein prototypisches föderiertes System integriert. Performancemessungen an diesem Prototyp zeigen ihre praktische Einsetzbarkeit.Atomicity and isolation of transactions are key requirements of advanced applications in federated systems consisting of distributed and heterogeneous components. While all existing federated systems support atomicity using the two-phase commit protocol, they lack support for federated concurrency control. Many possible solutions have been proposed in the literature, but they failed to make impact on real systems because they completely ignored the widely used concept of isolation levels, which offer optimization options to applications at the cost of less rigorous control over data consistency. This thesis compares existing definitions for isolation levels and develops a new characterization for Snapshot Isolation, an isolation level provided by Oracle, the market leader in the database field. We present algorithms for federated concurrency control that provably guarantee the correct execution of federated transactions even under local Snapshot Isolation, and discuss isolation levels for federated transactions. The algorithms are integrated into a federated system prototype. Performance measurements with this prototype show the practical viability of the developed methods

A Mobile Intervention for Self-Efficacious and Goal-Directed Smartphone Use in the General Population: Randomized Controlled Trial

BACKGROUND People spend large parts of their everyday life using their smartphones. Despite various advantages of the smartphone for daily life, problematic forms of smartphone use exist that are related to negative psychological and physiological consequences. To reduce problematic smartphone use, existing interventions are oftentimes app-based and include components that help users to monitor and restrict their smartphone use by setting timers and blockers. These kinds of digital detox interventions, however, fail to exploit psychological resources, such as through promoting self-efficacious and goal-directed smartphone use. OBJECTIVE The aim of this study is to evaluate the theory-based smartphone app "Not Less But Better" that was developed to make people aware of psychological processes while using the smartphone and to support them in using their smartphone in accordance with their goals and values. METHODS In a randomized controlled trial, effects of a 20-day intervention app consisting of five 4-day training modules to foster a goal-directed smartphone use were evaluated. In the active control condition (treatment as usual), participants received a digital detox treatment and planned daily time-outs of at least 1 hour per day. Up to a 3-week follow-up, self-reported problematic smartphone use, objectively measured daily smartphone unlocks, time of smartphone use, self-efficacy, and planning towards goal-directed smartphone use were assessed repeatedly. Linear 2-level models tested intervention effects. Mediation models served to analyze self-efficacy and planning as potential mechanisms of the intervention. RESULTS Out of 232 enrolled participants, 110 (47.4%; 55 participants in each condition) provided data at postintervention and 88 (37.9%; 44 participants in each condition) at 3-week follow-up. Both conditions manifested substantial reductions in problematic smartphone use and in the amount of time spent with the smartphone. The number of daily unlocks did not change over time. Further, modelling changes in self-efficacy as a mediator between the intervention and problematic smartphone use at follow-up fit well to the data and showed an indirect effect (b=-0.09; 95% bias-corrected bootstrap CI -0.26 to -0.01), indicating that self-efficacy was an important intervention mechanism. Another mediation model revealed an indirect effect from changes in planning via smartphone unlocks at postintervention on problematic smartphone use at follow-up (b=-0.029, 95% bias-corrected bootstrap CI -0.078 to -0.003). CONCLUSIONS An innovative, theory-based intervention app on goal-directed smartphone use has been found useful in lowering problematic smartphone use and time spent with the smartphone. However, observed reductions in both outcomes were not superior to the active control condition (ie, digital detox treatment). Nonetheless, the present findings highlight the importance in promoting self-efficacy and planning goal-directed smartphone use to achieve improvements in problematic smartphone use. This scalable intervention app appears suitable for practical use and as an alternative to common digital detox apps. Future studies should address issues of high attrition by adding just-in-time procedures matched to smartphone users' needs

Evaluating the Potential of Explicit Phrases for Retrieval Quality

Abstract. This paper evaluates the potential impact of explicit phrases on retrieval quality through a case study with the TREC Terabyte benchmark. It compares the performance of user-and system-identified phrases with a standard score and a proximity-aware score, and shows that an optimal choice of phrases, including term permutations, can significantly improve query performance