Selective Querying for Adapting Hierarchical Web Service Compositions

Approaches for composing Web services assume that the parameters used to model the environment remain static and accurate throughout the composition’s execution. The Web service compositions (WSC) are built using a pre-defined, fixed model of the environment at design time, and executed. This fundamental assumption is unrealistic as environments are subject to change during execution. For example, a product may go out of stock affecting the availability, the network bandwidth may fluctuate affecting the WS response time, or the cost of invoking a travel agent’s service may increase. Many WSC techniques do not adapt compositions to such changes, leading to suboptimal results. Dynamism manifests in WSC environments in a variety of ways. For example, changes range from the operational level (such as a newly introduced task) to the organizational level (such as new company policies) as mentioned in [van der Aalst and Jablonski 2000; Han and Bussler 1998]. Indeed, these surveys classify a variety of changes in different ways. Solutions have been presented to address some of these changes, ranging from exception handling techniques defined in [Borgida and Murata 1999] to instituting protocol adaptations in [Desai et al. 2006]. However, less attention has been paid to data volatility that exists during execution. As a concrete example, consider a mortgage loan acquisition process in which two title insurance agencies compete for orders from a large mortgage broker. The sequence in which the broker utilizes the services of the two insurers would depend on the probability with which the insurers usually satisfy the requests and the cost of using them. If the preferred insurer’s rate of request satisfaction drops suddenly (due to say, a financial crisis), a cost-conscious broker should replace it with another insurer to remain optimal. Important non-functional service parameters such as cost, availability, or the rate of request satisfaction in the above example, often change during the life-cycle of a WSC. WSCs must be aware of the changing