Research On Multi-Strategy Runtime Environment Adaptation For Composite Services

Web service composition can provide a new value-added service by integrating a group of Web services with different functions. Features of Web services, such as loosely coupling, dynamic discovery and dynamic binding, lay the foundation for flexible adapting composite services to changes in their runtime environments. However, manual adjustments of composite services are no longer able to satisfy the real-time requirements as the growth of the number of available services and the continuous changing functions and performances. As a result, that how to make composite services able to adjust themselves automatically according to targets on functionalities, performances and dependabilities for achieving higher level satisfaction has already been a critical problem in current service computing studies.As existing runtime environment adaptation mechanisms usually depend on single adaptation strategy, which could lead adaptations with higher cost but fewer or even no gains, this paper proposes a new adaptation mechanism supporting multiple strategies. First, on analyzing information involved in composite service execution, a context model is extracted and defined. This model establishes the relationship between the execution plan and states both of the execution and current candidate services, based on which a comprehensive adaptation process including reactive as well as active adaptation strategies is given. Further, in order to guage the change impact precisely enough to get better-dericted adaptation strategies, the concept of usable services set is brought forth based on local dominance and corresponding algorithms for measuring the impact degree and hierarchical adaptation are introduced. As a result, a layered adaptation strategy, from local to globle optimization, is completed. Finally, based on the principle of reflection, this paper provides a reflective framework for runtime environment adaptation, together with the definition of corresponding meta-models, meta-data and meta-protocles. A prototype system for adaptive Web services composition is implemented by extending the Apache open source service bus for adaptation. Moreover, tests to scenarios of service invocation failures, actual delivered QoS variations and new services appearances show the effectivity of the proposed mechanism and approach.Comparing with most current runtime environment adaptation approaches for composite service, with the composite services execution context model and candidate services organization model based on usable services sets, the approach in this paper combines reactive adaptations together with active adaptations, local optimizations together with gloable optimizations, and is able to select proper adaptation strategy to specific characters of environment variation. As a result, the flexibility and timeliness are both improved.