Optimal compensation for hierarchical Web services compositions under restricted visibility

Over the years, the notion of transactions has become synonymous with providing fault-tolerance, reliability and robustness to database systems. The idea then is to extend the same transactional guarantees to new and evolving paradigms, such as Web services. To achieve this, we first need to adapt the transactional mechanisms to the distinguishing characteristics of Web services, mainly composability, long-running nature, and privacy and security concerns. Composability refers to the ability to form new composite services by combining the functionalities of existing services. The existing services may themselves be composite, and this leads to a hierarchical composition. Due to their long-running nature, compensation based mechanisms are usually preferred to provide transactional guarantees for Web services. Compensation consists of semantically undoing the execution effects until failure, and as such requires access (visibility) over the execution details of the services in the composition. However, such visibility may not always be feasible in a compositional context where component services are provided by different providers across organizational boundaries, with very strong privacy and security constraints. Another aspect missing from current Web services composition models is the fact that, in a hierarchical composition, multiple compensation options may be possible at different levels at different costs. Thus, for a hierarchical composition, we show how to find an optimal compensation option under restricted visibility in this paper.