摘要
随着信息技术的发展,Internet已经逐渐演变成了一个开放的分布式计算环境。特别是随着Web服务技术的出现和推广,Web服务已经成为公认的实现服务的主流技术选择。在面向服务的计算中,越来越多的企业和组织希望通过动态组合不同服务方提供的服务构建分布式应用系统。其中一个非常重要的问题在于如何动态、高效的预测服务组合的服务质量(如可靠性),进而指导服务发现和组合过程。目前,国内外许多研究机构围绕服务组合的可靠性预测展开研究工作,并取得了很多有价值的研究成果。
本文针对当前服务组合可靠性预测研究中存在的不足,深入研究了基于体系结构的可靠性预测模型和Web服务组合的技术特点,提出用组合服务流程网模型作为分析和预测Web服务组合的可靠性模型。利用该模型,研究了基于BPEL业务流程的组合服务可靠性预测过程、在服务组合中应用软件容错技术增强系统可靠性、以及组合服务的可靠性优化等问题。本文的主要工作主要包括:
1.全面分析了Web服务的技术特点和基于体系结构的可靠性预测的研究现状,并对典型的可靠性预测模型和方法进行了总结。
2.针对Web服务技术和服务组合的特点,本文在随机Petri网的基础上,提出了用组合服务流程网模型描述服务组合的服务-使用剖面。组合服务流程网模型适合于描述服务组合这类并发软件系统。
3.以组合服务流程网模型为基础,研究了基于BPEL业务流程的服务组合可靠性预测方法。该方法首先将BPEL业务流程转换成对应的组合服务流程网模型,结合构件服务的可靠性信息,预测组合服务的可靠性信息。并且,在此基础上研究了组合服务可靠性的敏感性分析方法,用于确定构件服务可靠性和使用剖面对组合服务整体可靠性的影响程度,帮助设计人员选择和确定关键的构件服务和场景。
4.基于冗余的软件容错技术是提高软件系统可靠性的有效途径。在面向服务的计算中,由于Web服务具有的冗余特性,使得软件容错技术更易于应用在组合服务中。本文提出了在基于BPEL业务流程的组合服务中实现软件容错的方法,并对其可靠性和性能进行了定量评估。
5.在服务组合中应用软件容错技术提高组合服务的可靠性的同时,也会带来组合服务的成本增加。本文研究了在满足成本约束的前提下,具备容错策略选择的服务组合可靠性优化问题,并提出了一种基于Pareto最优解的可靠性优化算法。
6.设计和实现了一个组合服务的可靠性预测工具,能够将BPEL流程表示的组合服务自动转换为组合服务流程网模型,预测组合服务的可靠性。
综上所述,本文的工作针对Web服务组合可靠性预测研究中存在的几个关键问题进行了深入的研究,对于Web服务技术和服务组合的广泛应用具有一定的理论和应用价值。
With the progressing of IT technology, the Internet has becoming an open distributed computing infrastructure. Especially, along the emergence and popularization of Web Services, it has become the mainstream technology for service provision. In service-oriented computing, more and more corporations and organizations expect to develop complex distributed applications by the dynamic composition of Internet accessible services from independent providers. A key issue in it is the dynamic and efficient prediction of composite service's overall quality (for example, reliability) to appropriately drive the selection and assembly of services. Many research institutions home and abroad have conducted quite a few researches on reliability prediction of service composition, and have made some valuable achievements.
Aiming at the deficiency of research on reliability prediction of service composition, this dissertation firstly deeply studies the architecture-based reliability prediction models and characteristic of Web Services technology, then propose an approach to analyse the predict the reliability of composite Web Service. Using the reliability prediction model, some key questions are investigated including reliability prediction of BPEL process based service composition, using service composition to implement software fault tolerance, and reliability optimization of service composition. The contributions of this dissertation are as follows:
1. The characteristic of Web Services technology and the status of research on reliability prediction are analysed. Typical models and approaches of reliability prediction are evaluated.
2. Aiming at the characteristic of Web Services and service composition, this dissertation proposes the composite service process net model to describe the service-usage profile of service composition based on the stochastic Petri net. The composite service process net model can be used to describe this kind of concurrent software system.
3. An approach to the reliability prediction of BPEL process based service composition is proposed based on the composite service process net model. Firstly, the BPEL process is transformed to the composite service process net model, then the reliability of service composition is calculated combing with the reliabilities of component services. This dissertation also studies the sensitivity analysis approach of service composition reliability. It is used to investigate the impacts of component service's reliability and the usage profile on the reliability of composite service, and find out the key component service and scenario.
4. Software fault tolerance is an efficient way to improving the reliability of system. In service-oriented computing, software fault tolerance can easily be implemented by service composition by leveraging the redundancy intrinsic to the web service. In this dissertation, we propose an approach to implement software fault tolerance by service composition, and evaluate their reliability and performance measure.
5. Using software fault tolerance in service composition will increase their cost. In this dissertation, we study the service composition's reliability optimization problem with a choice of fault tolerance strategies. A solution method based on pareto optimum solution is proposed.
6. A reliability prediction tool of composite service was designed and implementioned, which can translate the BPEL process into composite service process net model, and calculate the reliability of composite service.
In summary, some key questions about reliability prediction in service-oriented computing are studied in this dissertation. We believe that our contribution make a nice groundwork for the popularization of web service technology and service composition both in theory and practice.
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