7 research outputs found
Precedence-agreement concurrency control algorithm for distributed database systems
No full textMost existing concurrency control algorithms cause deadlocks or transaction restarts which may be inefficient in a distributed environment. In this paper we develop two precedence-agreement concurrency control algorithms, namely, PA1 and PA2, which are free from deadlocks and transaction restarts. PA1 resolves conflicts through a distributed negotiation process. PA2 is a two-phase concurrency control algorithm derived from PA1. It is shown to be superior to two-phase locking and timestamp-ordering when system load is light or medium.link_to_subscribed_fulltex
Distributed Database Systems in High Speed Wide-Area Networks
No full textIn a distributed database system (DDBS), the data is partitioned into smaller databases and distributed over multiple sites in a computer network. Associated with a DDBS are functions like query processing and concurrency control. Traditionally in DDBS research, the computer network has been considered a performance bottleneck and a lot of research effort has been directed towards the design of database operations that minimize the data transmission cost. With the development of high speed networks, the network transmission costs go down and new algorithms that efficiently utilize the huge bandwidth available are required. In this paper, we first identify the issues involved in developing this distributed application in a high speed environment. Then we demonstrate the inadequacy of existing database protocols in utilizing the Gigabit-per-second network. And finally, we develop a new concurrency control protocol that performs better than traditional DDBS in a high speed network. Both a..
Distributed query optimization by one-shot fixed-precision semi-join execution
No full textA novel semijoin execution strategy is proposed which allows parallelism and processes multiple semijoins simultaneously. In practice most of the parameters needed for query optimization, such as relation cardinality and selectivity, are of fixed-precision. Imposing this fixed-precision constraint, an efficient distributed query processing algorithm is developed. For situations where the fixed-precision constraint does not apply, a method to truncate the parameters and use the same algorithm to find near-optimal solutions is proposed. By analyzing the truncation errors, a quantitative comparison between the near-optimal solutions and the optimal ones is provided.link_to_subscribed_fulltex
Using Ground-penetrating Radar to Promote the Investigating Efficiency in Mud Pumping Disaster of Railways
Full text linkCurrently about 85% of the railway structure is constructed traditionally in Taiwan, which means the foundation of railways is composed by in-situ soil materials and covered by ballast, sleepers and tracks. The rail is continued by fishplates and then bolted. While train passes here, the deflection caused by repeated loads. The repeatedly force transfers through ballast to the saturated foundation, may create vacuum to draw phenomenon, called pump effect or mud pumping. It could lead to serious train derailment capsized.
Present mud pumping detection method has to be performed during the non-operating time at night by visual. However, this approach may have omissions and shortcomings perspective concerns, and slow to find disasters during the rainy season. Using non-destructive testing techniques (ground-penetrating radar) to inspect the quality of rail bed is widely in foreign. However, detecting the distribution of mud beneath rail is an attractive subject here. By the usage of mention technology in this research, this technology is expected to be promoted.
This study has been agreed by the official administration, Taiwan Railway Bureau, to be carried out in Nanwan branch in Hsinchu by using ground-penetrating radar. Comparing with the visual inspection results, the mud pumping can be verified. Where most serious mud pumping phenomenon observed was open to prove the function. The study is expected to launch the road bed structure further rehabilitation plans and preventative maintenance engineering. Furthermore, cost due to misjudgment is expected to be saved and traffic safety improved
Depression: Can we predict who will relapse?
No full textThis thesis addresses risk factors and proposed mechanisms to explain relapse to depression. Volume 1 comprises three parts: Part 1 is a literature review consisting of meta-reviews of systematic and non-systematic reviews of studies reporting on risk factors for relapse to depression, and a systematic-review of neuroimaging and experimental studies investigating risk factors for relapse and potential mechanisms of action of these risk factors. The reviews found that only residual symptoms of depression at the end of treatment and childhood maltreatment were sufficiently evidenced as predictors of relapse and neither have great clinical utility. A number of psychological and neuropsychological factors were suggested to play a role in conferring risk for relapse. Considering the inter-relationships between these factors the reviews were used to propose a conceptual framework which may be used to help guide future research into relapse to depression in adults. Part 2 is an empirical paper in which data were analysed from service users of a primary care mental health service to identify risk factors for relapse and for the presence of residual symptoms, and survival analysis methods were used to determine when relapses occur most often and what factors impact survival. In addition, a prospective cohort study was formed to investigate the relationship between cognitive control and depressive symptoms. The findings confirmed that cognitive control can be used to predict residual symptoms of depression post-treatment and therefore potentially to predict relapse. Part 3 is a critical appraisal focussing on the theoretical reasons as to why studying relapse in a manner as used in the prospective study is so important and discusses the logistical difficulties conducting such research in the current context of NHS services and of the D.Clin.Psy research project. Methodological decisions made that impacted upon the research process are discussed and reflective conclusions are offered
