Research On Key Technologies Of QoS-aware Dynamic Web Service Composition

In the open and dynamic Internet environment, service composition can realize dynamic resource aggregation and flexible application integration, and has been the major development trend of Service-Oriented Computing. But for the shortage of QoS provision, service composition can't meet the user's satisfaction, though it can meet the base business requirements. Especially for the dynamic nature of business objects and autonomy of service itself, the key problem is how to deal with the non-functional requirements in practice on the premise of provision of functional requirements and meet the demand of the dynamic adaptability during the business execution process.Current research in QoS-based dynamic service composition still faces many new application challenges in some important respects such as the selection of component service, service composition compatibility and interruption recovery during the dynamic executution, etc. The service capability provision of minimal disruption to the consumers is very important feature especially for high-availability and reliability, critical mission and hard real-time systems. Thereby in this dissertation the research work is focused on key technologies such as QoS-based service selection during dynamic service composition, compatibility checking and failure recovery. The contributions of this dissertation are as follows:(1) A service selection model with multiple QoS constraints. The service selection is the first and critical step during service compositon. The non-functional properties of service especially the quality of service are the key aspects in the service-oriented computing paradigm. It is important for business process to construct a complex one with multi-dimensional QoS assurance. But there exist some problems, such as evaluation for QoS properties of service is not full-scale and the criteria is not clear, the weight for each QoS metric doesn't consider both subjective sensations and objective facts. In this dissertation we propose a service selection model to provide multi-dimensional QoS support in service selection and dynamic replacement. We define an evaluation sub-model of QoS, and propose a set of utility functions (Service Candidate Utility Function and Edge Utility Function) as the evaluation standards, which aggregate all QoS metrics after normalizing their values. Then we define an evaluation sub-model of QoS weights and use Subjective-Objective Weight Mode (SOWM) to set the weight of each QoS metric by means of decision mode. Also we propose a heuristic algorithm on the MMKP using the rule-library. Finally the experiments show that the SOWM is fair and rational enough and the algorithm is helpful to finish the selection quickly with the benefit of rule-library reducing search space at a certain extent. The research in this part provides a uniform computing framework of QoS for the key technology research in dynamic service composition.(2) A compatibility model with judgement rules for dynamic web service composition. It is very important to the correct execution of composite service with the reasonableness provision of execution plan in service selection stage, and it's helpful to avoid the runtime failure and reduce the execution cost. The current research on compatibility model is not sufficient and short of manipulation. In practical it is not always possible to find services that are fully compatible with each other. Composers fail to ensure a perfect match in the composition of Web Services. In this dissertation a compatibility model is proposed to ascertain that Web Services can be safely combined, hence avoiding unexpected failures at runtime. Compatibility is checked through a set of rules organized into three levels: syntactic, semantic and non-functional properties levels. The concept of compatibility degree is introduced to cater for partial and total compatibility with the help of compatibility threshold, and a concrete relax function is proposed to embody the compatibility degree of Web Services as numeric value. Finally a simulation is conducted to test the impact of compatibility threshold on the success ratio of service composition, and the result proves the necessity for compatibility checking in the selection stage. This model evaluates the degree of compatibility between services by the judgement rules and computation functions. It has a more strong expression ability and calculability compared with the past research.(3) Efficient dynamic replacement path algorithms based on the execution context-oriented environment. During the execution of Web Service Composition, it is familiar that the running process is interrupted for the uncontrollability of component services and execution environment. A failure recovery mechanism is needed to ensure that execution path can be replaced quickly and efficiently. In this dissertation, we present a context-sensitive adaptive algorithm (WSCCDR) that can use global and local replacement methods respectively, based on the actual execution context. And in the case that most of the business process has been executed, we propose the WSCPR algorithm to find replacement path locally by virtue of the old executed component services to shorten the total execution time. With respect to multi-stage critical sub-paths in the business execution path, another algorithm WSC_KUPB is described to enumerate the k maximum utility simple loopless paths in a digraph, which can find a replacement path for every critical path at one time. The test's result shows the algorithms perform very well that run time increases with the approximate linear feature under different test cases, and high failure recovery success ratio can be achieved on a moderately changing overlay network by our algorithms. Our algorithms have no restriction on the topology of the overlay network that current related algorithms require. And our work suggests a new way to develop efficient dynamic replacement algorithms for QoS-aware service composition. (4) Design and implementation of QoS-aware service composition prototype. Based on the above studies of the key technologies and the Web Services supporting platform named StarWebService developed by National University of Defense Technology, we propose a prototype StarWSCX for Qos-aware dynamic service composition. The architecture of dynamic service composition supporting platform is presented and the concrete components are discussed in details. The key issues such as QoS-based service selection and replacement, the methods and stategies for compatibility checking, SLA and failure recovery mechanism during the execution process are emphasised.