Research On Key Techinologies Of QoS Assurance For Web Service Composition

Resource sharing and application integration across organizational boundaries in open network have been recognized as comprehensive requirements in many domains, such as commerce, scientific research, military area. In recent years, the concept of service has been proposed as a mean of abstracting and wrapping diverse resources in open environment, which makes service composition to be the natural way for dynamic resource aggregation and flexible application integration. Especially, along with the prosperity and acceptance of Web services as the mainstream technology for service provision, dynamic service composition based on Web services has been regarded as one of the core technologies for the service-oriented computing paradigm. Due to the diversity and complexity of applications, as well as the complexity of the user's dynamic requirements, Web Services need to have the ability of dynamic service discovery, selection and binding. However, dynamic and unpredictable environment makes the QoS (Quality of Service) assurance of Web Service composition has become crucial and significant challenges.Many research institutions home and abroad have conducted quite a few researches on QoS assurance of service composition, and have made some valuable achievements. However, some key technologies related to QoS assurance of service composition are not well developed. For example, the service reputation measurement, service selection, fault diagnosis, and exception handling. New issues keep emerging.To address the requirements of resource sharing and application integration in open computing environment, some key issues of QoS assurance for service composition have been discussed in this dissertation:Web service reputation measurement based on QoS similarity, Web service selection with uncertain QoS information, fault diagnosis for Web service composition based on Bayesian network, and exception handling for Web service composition using meta-process. The contributions of this dissertation are as follows:1. Reputation is the basic for verity judgement of advertised QoS information and Web service selection. Currently, the research on reputation measurement of web service is mainly based on users'subjective score. Consequently, some characteristics of web service are not considered deliberately. Therefore, a novel Web service reputation measurement model based on the similarity of advertised and factual QoS is proposed in this dissertation. Upper and lower QoS ontology are designed to describe the semantic of QoS and the hierarchical QoS item system. The proposed QoS similarity computing algorithm can support the hierarchical QoS item similarity computing. Subsequently, a statistics-theory-based Web service reputation measurement method is put forward and it uses the QoS similarity to compute Web service reputation. Evolution trend and fluctuation of QoS are considered in reputation measurement with forgetting factor and fluctuating factor. Experiments show that the model is flexible and practical.2. How to dynamically select web services which can best meet the QoS requirements of consumers is very critical to the success of Web services in business domains. Most of current researches use precise QoS information for Web service selection but the uncertainty of QoS information in Web service selection is not well considered. After analyzing the uncertainty of OoS information in Web service selection, the specification method of uncertain QoS advertisement value, QoS requirement value and weight constraints are designed. Then a Web service selection model is built using possibility degree method for interval numbers ranking and TOPSIS (Technique for Order Preference by Similarity to Ideal Solution) method for MADM (Multiple Attribute Decision Making) problems. The experimental results show that the model can effectively select Web service which can meet the requirements of consumers when QoS information is uncertain. Finally, a QoS aggregation method based on aggregation tree for structure Web service composition and a global service selection algorithm is proposed.3. To identify the component services that cause the problem of Web service composition, a fault diagnosis model based on Bayesian network to identify the most likely problematic services in a Web service composition process is proposed. Firstly, the Bayesian network topology construction method and the parameters configuration method are specified in detail. Then a fault diagnosis algorithm based on the Bayesian network is proposed and the algorithm is analyzed. For reducing the collection cost of evidences, we propose an evidence channel selection method based on k-median algorithm and the concept of information entropy and conditional entropy. The experimental simulation show that the model can rule out the root cause of problems in the Web service composition process effectively and efficiently. Compared with the existing research, the uncertainty in fault diagnosis process of service composition and evidence channel selection method are considered in the model.4. For modular, extensible and adaptable exception handling of web service composition, a model using meta-process is proposed. The detail semantic of the model components and their relationship are specified too. The meta-process is a kind of upper control process consists of five meta-activities and the rule in the model is based on event, activity state and case data of base process. When an exceptional event occurs, corresponding meta-process is invoked as controlling and coordinating mechanism for base processes but not only a activity is invoked if some rules are met. Examples analysis show that the proposed model not only can be used for failure recovery, but also can be used for more complex situation such as process modification.5. Based on aforementioned research, a Web service composition reference framework with QoS assurance is proposed. The framework exhibits our understanding for the QoS assurance technology system of composite service and it give a reference computing framework for development of Web service composition support platform and application based on Web service composition.To sum up, we present well-evaluated solutions in this dissertation for some key issues of QoS assurance for service composition. We believe that our contributions make a nice groundwork for future research and engineering on service composition both in theory and practice.