In today's database management systems (DBMS), concurrency control is one of the main issues that draw a lot of attention. Concurrency control protocols prevent changes to the database made by one user to interfere with those made by another user. During last couple of decades, many new concurrency control mechanisms were introduced into the study of database management systems. Researchers have designed new concurrency control algorithms and examined their performances in comparison with well known concurrency control mechanisms, which are widely used in today's database management systems. The results reported to date, rather than being definitive, have tended to be quite contradictory [1]. The main cause of such findings is use of different assumptions and implications when defining a simulation model for database management systems. Different coding schemes and logical programmatic flows play another important role in obtaining questionable results. In this paper, rather than proposing yet another concurrency control algorithm, I will implement a standardized simulation model within windows application that can then be used by any researcher to test performance of his concurrency control protocol. I will implement Optimistic Concurrency control protocol to validate functionality of my application and compare it with two phase locking protocol.

With the continuing growth of the Internet and network protocols, there is a need for Protocol Development Environments. Simulation environments like ns and OPNET require protocol code to be rewritten in a discrete event model. Direct Code Execution Environments (DCEE) solve the Verification and Validation problems by supporting the execution of unmodified protocol code in a controlled environment. Open Network Emulator (ONE) is a system supporting Direct Code Execution in a parallel environment - allowing unmodified protocol code to run on top of a parallel simulation layer, capable of simulating complex network topologies. Traditional approaches to the problem of Parallel Discrete Event Simulation (PDES) broadly fall into two categories. Conservative approaches allow processing of events only after it has been asserted that the event handling would not result in a causality error. Optimistic approaches allow for causality errors and support means of restoring state — i.e., rollback. All standard approaches to the problem of PDES are either flawed by their assumption of existing event patterns in the system or cannot be applied to ONE due to their restricted analysis on simplified models like queues and Petri-nets. The Adaptive Time Window algorithm is a bounded optimistic parallel simulation algorithm with the capability to change the degree of optimism with changes in the degree of causality in the network. The optimism at any instant is bounded by the amount of virtual time called the time window. The algorithm assumes efficient rollback capabilities supported by the â Weaves' framework. The algorithm is reactive and responds to changes in the degree of causality in the system by adjusting the length of its time window. With sufficient history gathered the algorithm adjusts to the increasing causality in the system with a small time window (conservative approach) and increases to a higher value (optimistic approach) during idle periods. The problem of splitting the entire simulation run into time windows of arbitrary length, whereby the total number of rollbacks in the system is minimal, is NP-complete. The Adaptive Time Window algorithm is compared against offline greedy approaches to the NP-complete problem called Oracle Computations. The total number of rollbacks in the system and the total execution time for the Adaptive Time Window algorithm were comparable to the ones for Oracle Computations. / Master of Science

The typical theoretical work on dividend policy suggests five possible imperfections that management should consider. They are taxes, asymmetric information, an incomplete contract, institutional constraints and transaction costs. Different from the typical framework, this dissertation is to study the optimal dividend poly with heterogeneous beliefs among investors. The first model in this study has analyzed investment/dividend policy with heterogeneous beliefs-the full information model in a frictionless economy with divergent types of shareholders. A high dividend policy is optimal with limited endowment for the optimistic investors as the stocks are sold not only to type-o investors, but also to at least one type-p investor holding some shares. A low dividend policy is appropriate with cash dividend D= X0-ao+1 is optimal as the shares are sold only to the type-o investors. Heterogeneous beliefs of investors change dividend policy given the same information even under full information. Following the Miller and Rock (1985) theory, the second model in this dissertation has analyzed heterogeneous beliefs among investors-the two period model in leading to changing a firm¡¦s optimal dividend policy. A firm¡¦s optimal dividend policy is changed not only by the ratio of the pessimistic to optimistic investors, but also heterogeneous beliefs. An increase in the ratio of pessimistic to optimistic investors will result in a higher dividend. On the other hand, as the beliefs of both optimistic and pessimistic investors increase, i.e. a new biotechnology is innovated, a relative low dividend policy is appropriate. Based on the previous analysis, the results show that optimal dividend policy with heterogeneous beliefs among investors in a firm¡¦s earnings exists under heterogeneous beliefs framework. A firm¡¦s optimal dividend policy is different from that of the MM dividend invariance theorem. It is not because of taxes, asymmetric information, incomplete contracts, institutional constraints and traction costs, but heterogeneous beliefs of investors.

The increasing power and decreasing cost of computers has resulted in them being applied in an ever widening area. In the world of Computer Aided Design it is now practicable to involve the machine in the earlier stages where a design is still speculative, as well as in the later stages where the computer's calculating ability becomes paramount. Research on database systems has not followed this trend, concentrating instead on commercial applications, with the result that there are very few systems targeted at the early stages of the design process. In this thesis we consider the design and implementation of the file manager for such a system, first of all from the point of view of a single designer working on an entire design, and then from the point of view of a team of designers, each working on a separate aspect of a design. We consider the functionality required of the type of system we are proposing, defining the terminology of experiments to describe it. Having ascertained our requirements we survey current database technology in order to determine to what extent it meets our requirements. We consider traditional concurrency control methods and conclude that they are incompatible with our requirements. We consider current data models and conclude that, with the exception of the persistent programming model, they are not appropriate in the context required, while the implementation of the persistent programming model provides transactions on data structures but not experiments. The implementation of experiments is considered. We examine a number of potential methods, deciding on differential files as the one most likely both to meet our requirements and to have the lowest overheads. Measurements conducted on both a preliminary and a full-scale implementation confirm that this is the case. There are, nevertheless, further gains in convenience and performance to be obtained by exploiting the capabilities of the hardware to the full; we discuss these in relation to virtual memory systems, with particular reference to the VAX/VMS environment. Turning to the case where several designers are each working on a (nearly) distinct part of a design, we consider how to detect conflicts between experiments. Basing our approach on optimistic concurrency control methods, we show how read and write sets may be used to determine those areas of the database where conflicts might arise. As an aside, we show how the methods we propose can be used in an alternative approach to optimistic concurrency control, giving a reduction in system overheads for certain applications. We consider implementation techniques, concluding that a differential files approach has significant advantages in maintaining write sets, while a two-level bitmap may be used to maintain read sets efficiently.

Background: Individuals experiencing negative and potentially traumatic life events are at increased risk for suicidal thoughts and behaviors; however, suicidal outcomes are not inevitable. Individuals who attribute negative life events to external, transient, and specific factors, rather than internal, stable, and global self-characteristics, may experience fewer deleterious outcomes, including suicidal behavior. Aims: This study examines the moderating effect of explanatory style on the relationship between negative life experiences and suicide ideation in a college student sample. Methods: A total of 138 participants (73% female) were recruited from a rural. Eastern college and completed a self-report psychosocial assessment. Results: Optimistic explanatory style mitigates the influence of negative and potentially traumatic life events on thoughts of suicide, above and beyond the effects of hopelessness and depression. Conclusions: Beliefs about the origin, pervasiveness, and potential recurrence of a negative life event may affect psychological outcomes. Optimistic explanatory style was associated with reduced suicide ideation, whereas pessimistic explanatory style was associated with increased thoughts of suicide. Optimistic reframing of negative life events for clients may have treatment implications for the prevention of suicidal activity.

There is a tradeoff between the availability and consistency properties of any distributed replication system. Optimistic replication favors high availability over strong consistency so that the replication system can support disconnected replicas as well as high network latency between replicas. Optimistic replication improves the availability of these systems by allowing data updates to be committed at their originating replicas first before they are asynchronously replicated out and committed later at the rest of the replicas. This leads the whole system to suffer from a relaxed data consistency. This is due to the lack of any locking mechanism to synchronize access to the replicated data resources in order to mutually exclude one another. When consistency is relaxed, there is a potential of reading from stale data as well as introducing data conflicts due to the concurrent data updates that might have been introduced at different replicas. These issues could be ameliorated if the optimistic replication system is aggressively propagating the data updates at times of good network connectivity between replicas. However, aggressive propagation for data updates does not scale well in write intensive environments and leads to communication overhead in order to keep all replicas in sync. In pursuance of a solution to mitigate the relaxed consistency drawback, a new technique has been developed that improves the consistency of optimistic replication systems without sacrificing its availability and with minimal communication overhead. This new methodology is based on applying the concurrency control technique of leasing in an optimistic way. The optimistic lease technique is built on top of a replication framework that prioritizes metadata replication over data replication. The framework treats the lease requests as replication metadata updates and replicates them aggressively in order to optimistically acquire leases on replicated data resources. The technique is demonstrating a best effort semi-locking semantics that improves the overall system consistency while avoiding any locking issues that could arise in optimistic replication systems.

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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 this thesis a replication protocol - PRiDe - is presented, which supports optimistic replication in distributed real-time databases with deterministic detection and forward resolution of transaction conflicts. The protocol is designed to emphasize node autonomy, allowing individual applications to proceed without being affected by distributed operation. For conflict management, PRiDe groups distributed operations into generations of logically concurrent and potentially conflicting operations. Conflicts between operations in a generation can be resolved with no need for coordination among nodes, and it is shown that nodes eventually converge to mutually consistent states. A generic framework for conflict resolution is presented that allows semantics-based conflict resolution policies and application-specific compensation procedures to be plugged in by the database designer and application developer. It is explained how transaction semantics are supported by the protocol, and how applications can tolerate exposure to temporary database inconsistencies. Transactions can detect inconsistent reads and compensate for inconsistencies through callbacks to application-specific compensation procedures. A tool - VADer - has been constructed, which allows database designers and application programmers to quickly construct prototype applications, conflict resolution policies and compensation procedures. VADer can be used to simulate application and database behavior, and supports run-time visualization of relationships between concurrent transactions. Thus, VADer assists the application programmer in conquering the complexity inherent in optimistic replication and forward conflict resolution.

Aims of this thesis is to discuss and try to found out the requested competencies for leaders during crisis