Negotiation Strategies for Probabilistic Contracts in Web Services Orchestrations

Service Level Agreements (SLAs) have been proposed in the context of web services to maintain acceptable quality of service (QoS) performance. This is specially crucial for composite service orchestrations that invoke many atomic services to render functionality. A consequence of SLA management entails efficient negotiation protocols among orchestrations and invoked services. In composite services where data and QoS (modeled in a probabilistic setting) interact, it is difficult to select an atomic service for negotiation, in order to improve end-to-end QoS performance. A superior improvement in one negotiated domain (eg. latency) might mean deterioration in another domain (eg. cost); improvement in one of the invoked services may be annulled by another due to the control flow specified in the orchestration. In this paper, we propose a integer programming formulation based on first order stochastic dominance as a strategy for re-negotiation over multiple services. A consequence of this is better end-to-end performance of the orchestration compared to random selection of services for re-negotiation. We also demonstrate this optimal strategy can be applied to negotiation protocols specified in languages such as Orc. Such strategies are necessary for composite services where QoS contributions from individual atomic services vary significantly.