Mitarbeiter Lehrstuhl/Einrichtung der Fakultät für Informatik und Mathematik
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The well-founded semantics has been accepted as the most relevant semantics for logic-based information systems. In this dissertation a framework based on a set of program transformations is presented that generalizes all major computation approaches for the well-founded semantics using a common data structure and provides a common language to describe their evaluation strategy. This rewriting system gives the formal background to analyze and combine different evaluation strategies in a common framework, or to design new algorithms and prove the correctness of its implementations at a high level just by changing the order of program transformations.
This thesis investigates the suitability of state-of-the-art protocols for large-scale and long-term environmental event monitoring using wireless sensor networks based on the application scenario of early forest fire detection. By suitable combination of energy-efficient protocol mechanisms a novel communication protocol, referred to as cross-layer message-merging protocol (XLMMP), is developed. Qualitative and quantitative protocol analyses are carried out to confirm that XLMMP is particularly suitable for this application area. The quantitative analysis is mainly based on finite-source retrial queues with multiple unreliable servers. While this queueing model is widely applicable in various research areas even beyond communication networks, this thesis is the first to determine the distribution of the response time in this model. The model evaluation is mainly carried out using Markovian analysis and the method of phases. The obtained quantitative results show that XLMMP is a feasible basis to design scalable wireless sensor networks that (1) may comprise hundreds of thousands of tiny sensor nodes with reduced node complexity, (2) are suitable to monitor an area of tens of square kilometers, (3) achieve a lifetime of several years. The deduced quantifiable relationships between key network parameters — e.g., node size, node density, size of the monitored area, aspired lifetime, and the maximum end-to-end communication delay — enable application-specific optimization of the protocol.
In this work we present novel query evaluation techniques for data integration systems in different environments, ranging from a central data-warehouse approach, over distributed virtual market places, to peer-to-peer (P2P) systems. Based on a new distributed evaluation technique, the so-called HyperQueries, we present a reference architecture for distributed virtual market places. These HyperQueries enable us to dynamically construct query evaluation plans by referencing sub-plans in the Internet. Furthermore, the process of data integration is structured. Subsequently, we investigate P2P data integration systems without central instances. We introduce so-called Super-Peers which structure a P2P network. Using this Super-Peer based network we "unroll" queries. This allows us to execute even user-defined operators nearby the data sources. Finally, we propose novel, efficient join algorithms for decision support queries in central data-warehouse systems. The proposed order-preserving hashjoins and generalized hashteams are based on early sorting and early partitioning of the inputs and can speed up the query evaluation up to orders of magnitutes.
The thesis proposes a new formal framework for checking the content of web documents along individual reading paths. It is vital for the readability of web documents that their content is consistent and coherent along the possible browsing paths through the document. Manually ensuring the coherence of content along the possibly huge number of different browsing paths in a web document is time-consuming and error-prone. Existing methods for document validation and verification are not sufficiently expressive and efficient. The innovative core idea of this thesis is to combine the temporal logic CTL and description logic ALC for the representation of consistency criteria. The resulting new temporal description logics ALCCTL can - in contrast to existing specification formalisms - compactly represent coherence criteria on documents. Verification of web documents is modelled as a model checking problem of ALCCTL. The decidability and polynomial complexity of the ALCCTL model checking problem is proven and a sound, complete, and optimal model checking algorithm is presented. Case studies on real and realistic web documents demonstrate the performance and adequacy of the proposed methods. Existing methods such as symbolic model checking or XML-based document validation are outperformed in both expressiveness and speed.
Die Agententechnologie hat in den letzten Jahren eine rasante Entwicklung erfahren und mittlerweile großen Einfluss auf verschiedene Bereiche in Wissenschaft und Technik genommen. Insbesondere wurde die agentenbasierte Modellbildung und Simulation als wirkungsvolles Mittel für die Untersuchung realer oder hypothetischer Systeme erkannt. Im Vordergrund stehen hierbei ganz besonders Systeme, die durch menschliches Handeln, Entscheiden und Verhalten beeinflusst werden, oder der Mensch selbst, um seine Eigenschaften und Fähigkeiten aus den Blickwinkeln unterschiedlicher wissenschaftlicher Disziplinen heraus weiterführend zu erforschen. Das primäre Ziel der vorliegenden Arbeit besteht darin, den Entwurf agenten-basierter Simulationsmodelle, in denen menschliches Handeln, Entscheiden und Verhalten von ausschlaggebender Bedeutung sind, auf konzeptioneller Ebene zu unterstützen. Um dieses Ziel zu erreichen, wird das domänen- und theorieunabhängige Referenzmodell PECS vorgestellt, das Strukturierungsprinzipien aus der Informatik mit systemtheoretischen Ansätzen verbindet, um das Wechselspiel vielfältiger Einflussbereiche auf das menschliche Handeln im Rahmen einer integrativen und umfassenden Agentenarchitektur abzubilden. Auf Grundlage des Referenzmodells werden insgesamt vier charakteristische Fallstudien aus der Psychologie, der Sozialpsychologie, der Soziologie und der experimentellen Ökonomie entwickelt, um den Einsatz des Referenzmodells in der Praxis zu demonstrieren.
Aspect-Oriented Programming (AOP) has been promoted as a solution for modularization problems known as the tyranny of the dominant decomposition in literature. However, when analyzing AOP languages it can be doubted that uncontrolled AOP is indeed a silver bullet. The contributions of the work presented in this thesis are twofold. First, we critically analyze AOP language constructs and their effects on program semantics to sensitize programmers and researchers to resulting problems. We further demonstrate that AOP—as available in AspectJ and similar languages—can easily result in less understandable, less evolvable, and thus error prone code—quite opposite to its claims. Second, we examine how tools relying on both static and dynamic program analysis can help to detect problematical usage of aspect-oriented constructs. We propose to use change impact analysis techniques to both automatically determine the impact of aspects and to deal with AOP system evolution. We further introduce an analysis technique to detect potential semantical issues related to undefined advice precedence. The thesis concludes with an overview of available open source AspectJ systems and an assessment of aspect-oriented programming considering both fundamentals of software engineering and the contents of this thesis.
Refactoring is a well known technique to enhance various aspects of an object-oriented program. It has become very popular during recent years, as it allows to overcome deficits present in many programs. Doing refactoring by hand is almost impossible due to the size and complexity of modern software systems. Automated tools provide support for the application of refactorings, but do not give hints, which refactorings to apply and why. The Snelting/Tip analysis is a program analysis, which creates a refactoring proposal for a class hierarchy by analyzing how class members are used inside a program. KABA is an adaption and extension of the Snelting/Tip analysis for Java. It has been implemented and expanded to become a semantic preserving, interactive refactoring system. Case studies of real world programs will show the usefulness of the system and its practical value.
Multimedia retrieval is an essential part of today's world. This situation is observable in industrial domains, e.g., medical imaging, as well as in the private sector, visible by activities in manifold Social Media platforms. This trend led to the creation of a huge environment of multimedia information retrieval services offering multimedia resources for almost any user requests. Indeed, the encompassed data is in general retrievable by (proprietary) APIs and query languages, but unfortunately a unified access is not given due to arising interoperability issues between those services. In this regard, this thesis focuses on two application scenarios, namely a medical retrieval system supporting a radiologist's workflow, as well as an interoperable image retrieval service interconnecting diverse data silos. The scientific contribution of this dissertation is split in three different parts: the first part of this thesis improves the metadata interoperability issue. Here, major contributions to a community-driven, international standardization have been proposed leading to the specification of an API and ontology to enable a unified annotation and retrieval of media resources. The second part issues a metasearch engine especially designed for unified retrieval in distributed and heterogeneous multimedia retrieval environments. This metasearch engine is capable of being operated in a federated as well as autonomous manner inside the aforementioned application scenarios. The remaining third part ensures an efficient retrieval due to the integration of optimization techniques for multimedia retrieval in the overall query execution process of the metasearch engine.
Quantifier elimination (QE) is a powerful tool for problem solving. Once a problem is expressed as a formula, such a method converts it to a simpler, quantifier-free equivalent, thus solving the problem. Particularly many problems live in the domain of real numbers, which makes real QE very interesting. Among the so far implemented methods, QE by cylindrical algebraic decomposition (CAD) is the most important complete method. The aim of this thesis is to develop CAD-based algorithms, which can solve more problems in practice and/or provide more interesting information as output. An algorithm that satisfies these standards would concentrate on generic cases and postpone special and degenerated ones to be treated separately or to be abandoned completely. It would give a solution, which is locally correct for a region the user is interested in. It would give answers, which can provide much valuable information in particular for decision problems. It would combine these methods with more specialized ones, for subcases that allow for. It would exploit degrees of freedom in the algorithms by deciding to proceed in a way that promises to be efficient. It is the focus of this dissertation to treat these challenges. Algorithms described here are implemented in the computer logic system REDLOG and ship with the computer algebra system REDUCE.