Chinese Journal of Computers   Full Text
Title	Research on Exception Handling of Composite Services Based on Compensation Business Process Graph
Authors	SHANG Zong-Min CUI Li-Zhen WANG Hai-Yang SHI Yu-Liang
Address	(School of Computer Science and Technology, Shandong University, Jinan 250101)
Year	2008
Issue	No.8(1478—1490)
Abstract &   Background	Abstract To enforce consistency and reliability of composite Web services at run-time, this paper proposes a novel approach based on compensation business process graph to addressing the problem of exception handling in composite services. According to compensation dependencies among tasks in business processes, the automatic generation of compensation business process graph is proposed. Compensation task, compensation goal and application model of service transfer are discussed in detail, and the relevant supporting infrastructure and technique are introduced. Based on this, algorithms for automatic generation of compensation dependency and compensation business process graph, as well as algorithm for exception handling during execution of composite services, are proposed. When exceptions are handled with forward- or backward-recovery methods during execution of composite services, the approach can guarantees compensation achieving, and also gives consideration to the problem that there are no compensation services or the cost to achieve compensation is too expensive. The mechanism described here improves self-adjustability and stability of composite services in the course of deployment and execution. Simulations prove that the approach can efficiently guarantee the consistency and reliability of composite services at run-time. Keywords Web services; business process; exception handling; compensation business process graph Background This work is supported by the National Natural Science Foundation of China under grant No.60673130, and the Natural Science Foundation of Shandong under grant No.Y2007G38. An important object of these projects is to design a smart business process application model with business process management, by gluing together opaque and autonomous Web services, possibly supplied by business partners and third party service providers, into loosely-coupled virtual applications that can span organizational boundaries and connect large-scale business processes. A key challenge facing service-oriented applications is the consistency problem: ensuring that the autonomous services making up these distributed applications always finish in consistent states despite failures and other such exceptional events. This problem can be solved through the use of transaction technology but this solution depends on assumptions of trust and timeliness that no longer apply in the new loosely-coupled and long-running services-based world. Exception handling is the effective mechanism adopted in most of these applications for relaxing the traditional rigid ACID properties in a controlled manner. Exception handling resolves inconsistency by backward or forward error recovery methods. However, if the execution of a process has passed the point of no return, process abortion would violate the atomicity property and lead the process to inconsistent states. In addition, service recovery needs to compensate the effects of committed tasks, some of which, however, may be non-compensable or the cost to compensate is too expensive to be accepted by users. Therefore, an effective compensation mechanism as well as recovery methods is the key to guarantee the consistency and reliability of composite services. The work of this paper is conducted under the background and extends the research mentioned above. An exception handling approach based on compensation business process graph is introduced to enforce consistency and reliability of composite Web services at run-time. Exceptions can be handled automatically with forward- or backward-recovery methods during the execution of composite services. Those tasks that need to be compensated will be compensated by compensation services, which are searched automatically based on compensation business process graph. In addition, a transfer services application model is proposed to solve the problem that there are no compensation services or the cost to achieve compensation is too expensive.