Selective Querying for Adapting Web Service Compositions Using the Value of Changed Information

Web service composition (WSC) techniques assume that the parameters used to model the environment remain static and accurate throughout the composition´s execution. However, WSCs often operate in environments where the parameters of its component services are volatile. To remain optimal, WSCs must adapt to these changes. Adaptation requires up-to-date knowledge about the revised parameters of each of the services. One way of obtaining this knowledge is to query services for their revised parameters. Querying services for their parameters is time-consuming and expensive. We must therefore carefully manage how queries are conducted. Specifically, an adaptive WSC must know when to query for revised information, and from which service(s) to obtain information. We present a method to selectively query services using the value of changed information (VOC). VOC measures the value of the change that revised information may potentially introduce to the composition. We reduce the complexity of computing the VOC, first by anticipating values of the service parameters that do not change the WSC, and second by using parameter expiration times obtained from predefined service-level agreements. Using two scenarios, we illustrate our approach and demonstrate the computational savings theoretically and experimentally.