A model for the aggregation of QoS in WS compositions involving redundant services

A composition arranges available services resulting in a defined flow of tasks. A discovery process identifies the suitable candidate services for a composition. A subsequent selection process chooses the optimal candidate for each task. The selection process can consider different quality-of-service (QoS) categories of the individual services to optimise the quality of the whole composition. The result is a quality-optimised assignment of candidates to each task. In this work, we discuss the case how already identified candidates, which a selection process originally has separated out, can improve a composition w.r.t. particular QoS categories. To realise this improvement, redundant arrangements involve the alternative candidates in order to supplement the originally assigned service. We formed these redundant arrangements on the basis of our previously introduced compositions patterns. The contribution of this work is a computational model that allows the aggregation of the QoS for the composition when these arrangements are applied. This work presents extensions and refinements to our previous work [jaeger etal 05] where the basic concepts of this computational model were already discussed.