Pattern-based Policies And Aspects For Supervision Of Composite Web Services

Web Service is the most promising technology to build next generation distributed computing.A single service can only perform limited functions,and web services composition is the primary approach to build service based applications, which creates a new value-added service from existing individual services.Different from the traditional software system,which had a pre-defined,static, monolithic and centralized architecture,in SOA architecture,service-based software systems are often composed dynamically according to user requirements,and may involve continuously as the environment in which they are embedded evolves.This implies the need to constantly monitor the running behavior of the composite web services.Moreover,monitoring the execution of such application is critical to ensure the correctness of system,enforce business policies and meet reliability goals. Although there are many works in the area of runtime monitoring and management of service-based system,existing supervision framework still has some shortcomings, from design-time specification to runtime management.For example,most monitoring specifications are based on some formal logic,such as LTL and EC, which make them very difficult to understand and grasp.Also,these works don't support to specify range explicitly,so it's not easy to express the interest on a part of system behavior while ignoring the rest of it.There are still some problems on how to supervise the composite web services at runtime.For example,existing aspect-oriented extension to WS-BPEL can't observe the history of the process execution,and also most works ignore the QoS management of the partner services.To solve these problems,the thesis discussed several key technologies in the management of composite web services and gave our solution.According to the principle of policy based management,pattern-based supervision policy for composite web services is designed to specify the monitoring requirement and the corresponding management action.At runtime,stateful aspect extension to WS-BPEL is proposed to supervise the running process.To manage the underlying partner services,we focus on QoS management and present a dynamic QoS assessment model to discover the performance bottleneck and support QoS-driven substitution of partner services.Pattern-based supervision policy specification.It mainly consists of three parts: pattern,range and action,where pattern specifies the content of the monitoring, range specifies the boundary of the monitoring and action specifies the concrete management action.The proposed supervision policy is based on a set of fixed patterns and can express the monitoring requirements more intuitively compared with some formal logic,such as LTL,EC,etc.Also,the introduction of range is very important to runtime monitoring,which can reduce the performance overhead of the process execution.Moreover,it can control the faults within defined boundaries by triggering suitable fault handling action timely.Stateful aspect extension to WS-BPEL.Aspect-oriented programming offers a new set of language features to increase modularity and the separation of concerns, which makes it very suitable to runtime monitoring and management.However,in most aspect extension to WS-BPEL,the observation that an aspect can make is confined to the current process state:it's not possible to directly observe the history of the process execution.We present a stateful aspect extension to WS-BPEL to supervise the running process.Specifically,we extend the ordinary pointcut and present a history-based pointcut to express all kinds of temporal relationship of events and use automata to record the process execution history.The characteristic of stateful aspect based runtime monitoring is that it only focuses on events of interest and reduce the performance overhead.QoS-driven Web Service substitution.We propose a dynamic QoS assessment approach for web services to discover the bottleneck of the process execution. Through building Bayes network based QoS assessment model for candidate services,this approach can detect the QoS degradation timely and select/substitute the best suitable service for the client.Also,it supports to update the corresponding QoS assessment model dynamically.The salient feature of this model is that it can correctly predict the quality of service in various combinations of users' QoS requirements,especially to the services with different service levels.To support QoS-driven web services substitution at runtime,AOP based technology is adopted to extend the WS-BPEL engine at the service invocation layer,which reduces the coupling of the modules and improves the scalability of the execution engine. Experimental results show that the proposed QoS assessment model is effective.