Monotonicity in Service Orchestrations

Web Service orchestrations are compositions of different Web Services to form a new service. The services called during the orchestration guarantee a given Quality of Service (QoS) to the orchestrator, usually in the form of contracts. These contracts can then be used by the orchestrator to deduce the contract it can offer to its own clients, by performing contract composition. An implicit monotonicity assumption in contract based QoS management is: “the better the component services perform, the better the orchestration’s performance will be”. In some orchestrations, however, monotonicity can be violated, i.e., the performance of the orchestration improves when the performance of a component service degrades. This is highly undesirable since it can render the process of contract composition inconsistent. In this paper we formally define monotonicity for orchestrations modelled by Colored Occurrence Nets (CO-nets) and we characterize the classes of monotonic orchestrations. Contracts can be formulated as hard, possibly nondeterministic, guarantees, or alternatively as probabilistic guarantees. Our work covers both cases. We show that few orchestrations are indeed monotonic, mostly because of complex interactions between control, data, and timing. We also provide user guidelines to get rid of non-monotonicity when designing orchestrations.