<p>Distributed real-time databases can be used to support data sharing</p><p>for applications in wireless ad-hoc networks. In such networks, topology changes frequently and partitions may be unpredictable and last for an unbounded period. In this thesis, the existing database replication protocol PRiDe is extended to handle such long-lasting partitions. The protocol uses optimistic and detached replication to provide predictable response times in unpredictable networks and forward conflict resolution to guarantee progress.</p><p>The extension, pPRiDe, combines update and state transfer strategies. Update transfer for intra-partition communication can reduce bandwidth usage and ease conflict resolution. State transfer for inter partition conflicts removes dependency on a common state between partitions prior to the merge to apply update messages on. This makes the resource usage independent of the life span of partitions. This independence comes at the cost of global data stability guarantees and pPRiDe can thus only provide per partition guarantees. The protocol supports application specific conflict resolution routines for both</p><p>state and update conflicts. A basic simulator for mobile ad-hoc networks has been developed to validate that pPRiDe provides eventual consistency.</p><p>pPRiDe shows that a hybrid approach to change propagation strategy can be beneficial in networks where collaboration by data sharing within long lasting partitions and predictable resource usage is necessary. These types of systems already require the conflict management routines necessary for pPRiDe and can benefit from an existing protocol.</p><p>In addition to pPRiDe and the simulator this thesis provides a flexible object database suitable for future works and an implementation of PRiDe on top of that database.</p>

<p>In distributed database systems, immediate global consistency of replicated data can be achieved by distributed commit protocols that are typically unpredictable. If real-time characteristics are necessary, such unpredictability has to be avoided. In a distributed real-time database, optimistic replication can be used to avoid unpredictable delays by allowing transactions to commit locally. The update of other nodes is performed as soon as possible. If optimistic replication is used, conflicts may occur since data can be changed locally without synchronously informing other nodes. To detect these conflicts, this thesis introduces a conflict detection approach for DeeDS, a distributed, active real-time database that supports a dynamic node set. A comparison of existing conflict detection approaches is performed, and it is found that the dynamic version vector approach is the best fitting approach. The main reason is that it can handle a dynamic node set with a minimum of additional conflict detection data. To show the realization of the approach in DeeDS, dynamic version vectors have been implemented. Additionally, conflict management in DeeDS is redesigned to allow separation of conflict detection and conflict resolution. This makes the software architecture more flexible and is a first step towards application specific conflict resolution.</p>

There is no doubt that BitTorrent nowadays is one of the most popular peer-to-peer (P2P) applications on the Internet, contributing to a significant portion of the total Internet traffic and being a basis for many other emerging services such as P2P Internet Protocol Television and Video on Demand. On the other hand, Network Address Translation (NAT) devices have become pervasive in almost all networking scenarios. Despite of the effort of NAT traversal, it is still very likely that applications, especially P2P ones, cannot receive incoming connection requests properly if they are behind NAT. Although this phenomenon has been widely observed in measurement work, so far there is no quantitative study in the literature examining the impact of NAT on P2P applications. In this work, we build analytical models to capture the performance of BitTorrent-like P2P systems in a steady state, in the presence of homogeneous and heterogeneous NAT peers. We also propose biased optimistic unchoke strategies, in order to improve the overall system performance and fairness metrics considerably. The analytical models have been validated by simulation results, which also reveal some interesting facts about the coexistence of NAT and public peers in P2P systems.

The thesis Risk Denial and Neglect: Studies in Risk Perception examines societal and individual attention to risks and focuses especially on the issue of neglect. Why do some risks get more attention than other risks and how is this difference in attention related to experts’ roles in society? What can explain people’s tendency to perceive risks as more pertinent to other people? These are some of the issues that are discussed in the thesis. The topics are of interest for, e.g., risk policies, risk management, and for designing campaigns aimed at minimizing risk-related behaviors. The dissertation is written within the field of economic psychology. The research questions are addressed in four separate papers based on three empirical studies. The Papers I and II focus on societal attention to risks. They address the issues of what risks are neglected and overemphasized in society and how the identification of risk is related to experts’ domain of expertise. Papers III and IV narrow down the discussion to individual processes of risk denial – why people tend to believe that risks are more pertinent to other people. The results show that experts in the present study tended to rate risks within their own domain as lower than other risks. They were more prone to act as promoters than protectors. In addition, the robust tendency of optimistic bias was shown to exist also for technological risks (related to the use of computers) and economic risks. Most people seem to hang on to their beliefs that risks are other people’s concerns – it simply won’t happen to them. The results of the present thesis suggest that the relevance of prior experience and the commonplaceness of the risk sources is an area that merits further investigation with respect to risk denial.

The nature of creative mathematical thinking undertaken by students in classroom settings was studied through analysis of the autonomy and spontaneity associated with these processes. The theoretical lens developed enabled simultaneous analysis of cognitive, social, and affective elements of the creative process, and student responses to successes and failures during their exploratory activity (resilience or optimism). Collective case study was employed, with each case progressively informing the analysis of subsequent cases. The classrooms of teachers who were seen by their school communities to display 'good teaching practice' were selected for study. It was anticipated that such classrooms would provide more opportunity to study creative thinking than classrooms chosen at random. During the research period, each student participated individually in post-lesson interviews that were stimulated by lesson video material. To generate data to study student thinking, and the social and personal influences upon it, students were asked to identify parts of the lesson that were important to them, and discuss what was happening, and what they were thinking and feeling. Through this process, students who explored mathematical complexities to generate new mathematical knowledge were identified. (For complete abstract open document)

Esta tese propõe um novo método para a avaliação de desempenho de protocolos de sincronização otimistas adotados em programas de simulação distribuída. A utilização do método proposto é atrativa principalmente por permitir um usuário de simulação distribuída otimista optar pelo protocolo que mais se adapta às suas necessidades, sem ter que implementá-lo. O método de avaliação tem o modelo de um processo lógico Time Warp básico como núcleo, levando em consideração as características tanto da aplicação como da plataforma de hardware/software utilizada (o ambiente ALPHA/Sim é utilizado para a modelagem e simulação através de Redes de Petri). As características mais relevantes do protocolo Time Warp básico e suas variações são identificadas, a partir de uma nova taxonomia hierárquica proposta nesta tese, aliada a uma revisão bibliográfica ampla e detalhada. Além disso, domínios de plugins foram identificados, permitindo uma extensão (ou modificação) rápida e direta do modelo do Time Warp básico proposto. Esta tese também contribui apresentando resultados da avaliação de desempenho do protocolo Time Warp básico e do protocolo probabilístico PDSP. / This thesis proposes a novel method for the performance evaluation of optimistic synchronisation protocols adopted in distributed simulation programs. The use of the proposed method is attractive mainly for allowing an optimistic distributed simulation user to choose the protocol that best fits his needs, without having to implement it. The evaluation method has the model for a basic Time Warp logical process as its kernel, that takes into consideration the features of both the application and the hardware/software platform adopted (the ALPHA/Sim environment is used for modelling and simulation purposes through Petri Nets). The more relevant features of the basic Time Warp protocol and its variants are identified by using a new hierarchical taxonomy proposed in this thesis together with a wide and detailed literature survey. Furthermore, plugin domains could also been identified, allowing a quick and straightforward extension (or modification) of the basic Time Warp model proposed. This thesis also contributes by presenting performance evaluation results for both the basic Time Warp protocol and the PDSP probabilistic protocol.

In distributed database systems, immediate global consistency of replicated data can be achieved by distributed commit protocols that are typically unpredictable. If real-time characteristics are necessary, such unpredictability has to be avoided. In a distributed real-time database, optimistic replication can be used to avoid unpredictable delays by allowing transactions to commit locally. The update of other nodes is performed as soon as possible. If optimistic replication is used, conflicts may occur since data can be changed locally without synchronously informing other nodes. To detect these conflicts, this thesis introduces a conflict detection approach for DeeDS, a distributed, active real-time database that supports a dynamic node set. A comparison of existing conflict detection approaches is performed, and it is found that the dynamic version vector approach is the best fitting approach. The main reason is that it can handle a dynamic node set with a minimum of additional conflict detection data. To show the realization of the approach in DeeDS, dynamic version vectors have been implemented. Additionally, conflict management in DeeDS is redesigned to allow separation of conflict detection and conflict resolution. This makes the software architecture more flexible and is a first step towards application specific conflict resolution.

Distributed real-time databases can be used to support data sharing for applications in wireless ad-hoc networks. In such networks, topology changes frequently and partitions may be unpredictable and last for an unbounded period. In this thesis, the existing database replication protocol PRiDe is extended to handle such long-lasting partitions. The protocol uses optimistic and detached replication to provide predictable response times in unpredictable networks and forward conflict resolution to guarantee progress. The extension, pPRiDe, combines update and state transfer strategies. Update transfer for intra-partition communication can reduce bandwidth usage and ease conflict resolution. State transfer for inter partition conflicts removes dependency on a common state between partitions prior to the merge to apply update messages on. This makes the resource usage independent of the life span of partitions. This independence comes at the cost of global data stability guarantees and pPRiDe can thus only provide per partition guarantees. The protocol supports application specific conflict resolution routines for both state and update conflicts. A basic simulator for mobile ad-hoc networks has been developed to validate that pPRiDe provides eventual consistency. pPRiDe shows that a hybrid approach to change propagation strategy can be beneficial in networks where collaboration by data sharing within long lasting partitions and predictable resource usage is necessary. These types of systems already require the conflict management routines necessary for pPRiDe and can benefit from an existing protocol. In addition to pPRiDe and the simulator this thesis provides a flexible object database suitable for future works and an implementation of PRiDe on top of that database.

With the advent of agile programming, lightweight software processes are being favoured over the highly formalised approaches of the 80s and 90s, where the emphasis is on "people, not processes". Likewise, access control may benefit from a less prescriptive approach and an increasing reliance on users to behave ethically. These ideals correlate with optimistic access controls. However, such controls alone may not be adequate as they are retrospective rather proactive. Optimistic access controls may benefit from the stricter enforcement offered by usage control. The latter enables finer-grained control over the usage of digital objects than do traditional access control policies and models, as trust management concerns are also taken into consideration. This thesis investigates the possibility of enhancing optimistic access controls with usage control to ensure that users conduct themselves in a trustworthy manner. Since this kind of approach towards access control has limited applicability, the present study investigates contextualising this approach within a mixed-initiative access control framework. A mixed-initiative access control framework involves combining a minimum of two access control models where the request to information is mediated by a mixture of access policy enforcement agents. In order for this type of integration to be successful, a software development approach was considered that allows for the seamless augmentation of traditional access control with optimistic access control enhanced with usage control, namely the aspect-oriented approach. The aspect-oriented paradigm can facilitate the implementation of additional security features to legacy systems without modifying existing code. This study therefore evaluates the aspect-oriented approach in terms of implementing security concerns. It is evidently difficult to implement access control and in dynamic environments preconfigured access control policies may often change dramatically, depending on the context. In unpredicted circumstances, users who are denied access could often have prevented a catastrophe had they been allowed access. The costs of implementing and maintaining complex preconfigured access control policies sometimes far outweigh the benefits. Optimistic controls are retrospective and allow users to exceed their normal privileges. However, if a user accesses information unethically, the consequences could be disastrous. Therefore it is proposed that optimistic access control be enhanced with some form of usage control, which may prevent the user from engaging in risky behaviour. An initiative towards including security in the earlier phases of the software life cycle is gaining momentum, as it is much easier to design with security from the onset than to use the penetrate-and-patch approach. Unfortunately, incorporating security into software development takes time and developers tend to focus more on the features of the software application. The aspect-oriented paradigm can facilitate the implementation of additional security features in legacy systems without modifying existing code. The current study evaluates the aspect-oriented approach towards enhancing optimistic access control with usage control. The efficacy of the aspect-oriented paradigm has been well established within several areas of software security, as aspect-orientation facilitates the abstraction of these security-related tasks so as to reduce code complexity. / Thesis (PHD)--University of Pretoria, 2010. / Computer Science / unrestricted

The most important factor that affects the decision making process in finance is the risk which is usually measured by variance (total risk) or systematic risk (beta). Since investors' sentiment (whether she is an optimist or pessimist) plays a very important role in the choice of beta measure, any decision made for the same asset within the same time horizon will be different for different individuals. In other words, there will neither be homogeneity of beliefs nor the rational expectation prevalent in the market due to behavioral traits. This dissertation consists of three essays. In the first essay, Investor Sentiment and Intrinsic Stock Prices, a new technical trading strategy is developed using a firm specific individual sentiment measure. This behavioral based trading strategy forecasts a range within which a stock price moves in a particular period and can be used for stock trading. Results show that sample firms trade within a range and show signals as to when to buy or sell. The second essay, Managerial Sentiment and the Value of the Firm, examines the effect of managerial sentiment on the project selection process using net present value criterion and also effect of managerial sentiment on the value of firm. Findings show that high sentiment and low sentiment managers obtain different values for the same firm before and after the acceptance of a project. The last essay, Investor Sentiment and Optimal Portfolio Selection, analyzes how the investor sentiment affects the nature and composition of the optimal portfolio as well as the performance measures. Results suggest that the choice of the investor sentiment completely changes the portfolio composition, i.e., the high sentiment investor will have a completely different choice of assets in the portfolio in comparison with the low sentiment investor. The results indicate the practical application of behavioral model based technical indicators for stock trading. Additional insights developed include the valuation of firms with a behavioral component and the importance of distinguishing portfolio performance based on sentiment factors.