Towards Improving the Availability of Service Compositions

The proliferation of open Internet-scale service-oriented platforms based on standards, such as WSDL, SOAP and BPEL, enables the composition of independent Web services into new value-added services. Such service compositions define the information flows between autonomous and potentially heterogeneous services across the boundaries of independent provider organisations. The availability of individual services in such digital ecosystems is likely to be variable due to fluctuating usage load and resource limitations imposed by a service provider's infrastructure. This problem becomes more acute as the number of services in a composition increases. This paper presents a mediation model for improving the availability of composed services. The mediation model masks failures in a service composition by transparently selecting (and executing) an alternative composition at runtime. Service consumers use a common interface to a set of functionally equivalent service compositions while a selection mechanism identifies the most suitable (alternative) service composition. An evaluation of our implementation of the proposed mediation model demonstrates that the consumer perceived availability of value-added services can be improved significantly.