  Web Service orchestrations are compositions of different Web Services to form
a new service. The services called during the orchestration guarantee a given
performance to the orchestrater, usually in the form of contracts. These
contracts can be used by the orchestrater to deduce the contract it can offer
to its own clients, by performing contract composition. An implicit assumption
in contract based QoS management is: "the better the component services
perform, the better the orchestration's performance will be". Thus, contract
based QoS management for Web services orchestrations implicitly assumes
monotony. In some orchestrations, however, monotony 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 define monotony for
orchestrations modelled by Colored Occurrence Nets (CO-nets) and we
characterize the classes of monotonic orchestrations. We show that few
orchestrations are indeed monotonic, mostly since latency can be traded for
quality of data. We also propose a sound refinement of monotony, called
conditional monotony, which forbids this kind of cheating and show that
conditional monotony is widely satisfied by orchestrations. This finding leads
to reconsidering the way SLAs should be formulated.
