Research On Compatibility Analysis And Adaptation In Web Services Composition

In recent years, Service-Oriented Computing(SOC) and Service-Oriented Architecture(SOA), as the new computation paradigm and software architecture, have efficiently solved the application integration problem in a distributed, dynamic and heterogeneous environment. With the development of Web services technology, it has become one of the most essential ways to realize SOA and spurred SOA to come into application from concept phases due to the characteristics of good encapsulation, loosely coupled, self-described in nature. Since single Web services provide limited functionalities, they are often composed to form value-added, larger-grained Web services in order to meet the needs of practical applications. Hence, how to analyze and guarantee interoperability among Web services at different levels to achieve correct service compositions has become one of the most challenging issues currently.The main benefit Web service technologies bring is to provide standardization which reduces heterogeneity and solves interoperability issues at primitive levels among Web services. However, when composing independent Web sevices, interoperability at high levels is still a challenge problem. Since services to be composed are developed by different third-parties independently, they are different in the representation of services on the level of interface specifications, interaction protocol, functional semantics, etc. Thus, a fundamental problem of the interoperability of independent services is associated with compatibility analysis which consists in checking if a number of services can interoperate correctly before composition. Due to the common case that services are incompatible to interoperate and can not be directly composed, the typical approach to solve the problem is service adaptation which provides effective ways to resolve incompatibilities without modifying interal codes of Web services so that interoperability among Web services can be guaranteed.This thesis conducts the research on compatibility analysis and adaptation in Web services composition and mainly concentrates on the most important issues: modeling of Web services interface, protocol and semantics, compatibility determination and compatibility degree computation for Web services, automatic construction of adaptors for the design phase, and reconfiguration analysis of adaptors for the running phase. The major work and contributions of the thesis are as follows:(1) To solve the problem that exsiting service modeling approaches lack formal description support for interface, protocol and semantics of services, the method for unified modeling interface, protocol and semantics of services is proposed. By extending finite automata model with interface, protocol and semantic information, IPSFA(interface, protocol and semantics extended finite automata) model is proposed and utilized to formally describe services. Then, the formal semantics of the composition of IPSFA models is defined and how to translate service description languages based on XML into the corresponding IPSFA models is discused. This proposed model can describe not only service information at interface, protocol and semantic level at the same time but also the constraint relationship among them, and thus is an appropriate theoretical model which lays the groundwork for research on compatibility analysis and adaptation of services.(2) To solve the problem that exsiting compatibility analysis approaches often only focus on compatibility between services at protocol level and lack quantitative analysis, the method for service compatibility determination and compatibility degree computation supporting interface, protocol and semantics is proposed. By analyzing the existing service compatibility standards, three kinds of compatibility between services are formally defined and the algorithm for compatibility determination is given based on IPSFA models. In view of the uncertainty in the process of service actual execution, compatibility degree between services is computed, by introducing the concept of execution path and analyzing the dual relationship between two execution paths at interface, protocol and semantic levels. The proposed method can be utilized to identify the interactions that lead to incompatibilities between services and provide the feedback to developers for service re-adjustment or re-design. Besides, the quantitative computation results of the proposed method can be used as the important criteria for service selection.(3) Aiming at the problem that exsiting approaches for design-time service adaptation can not effectively support the aspects such as adaptation at different levels, automatic adaptor construction and BPEL codes generation for the adaptor, an approach to automatic construction of adaptors in the design phase is proposed. Firstly, semantic information of services and semantic containment relation between ontology concepts are utilized to generate the message mapping between services. Then, based on the generated message mapping, the algorithm for adaptor model construction is presented by using on-the-fly reduction techniques in order to improve the efficiency of automatic adaptor construction. Finally, the reduced adaptor model is mapped into the BPEL codes for execution. The constructed adaptor according to the proposed method can not only resolve incompatibilites at interface and protocol levels among multiple services to enable them to be composed together, but also guarantee semantic consistency of the adaptor-based service composition.(4) Aiming at the problem that exsiting approaches for run-time service adaptation can not support reconfiguration analysis of adaptors, an approach to reconfiguration analysis of adaptors in the running phase is proposed. By introducing the notions of backward trace compliance and forward trace compliance, the validity of an adaptor instance under the definition of a new adaptor version is determined with respect to its history executions and future executions. Then, the algorithms for reconfiguration analysis of adaptors are presented and can be used for determining the migratability of an adaptor instance and the corresponding target state of reconfiguration. The approach equips adaptors with reconfiguration capabilities in order to cope with service evolution at run-time.(5) Based on the above methods and theories, a prototype tool for compatibility analysis and adaptation in Web services composition is designed and partially implemented. The system architecture, the functional modules and the execution flow of the tool are introduced respectively. With this tool, compatibility determination and compatibility degree computation in the service selection phase, automatic construction of adaptors in the service composition design phase and reconfiguration analysis of adaptors in the service composition running phase can be conducted.