Monitoring Probabilistic SLAs in Web Service Orchestrations

Web services are software applications that are published over the Web, and can be searched and invoked by other programs. New Web services can be formed by composing elementary services, such composite services are called Web service orchestrations. Quality of Service (QoS) issues for Web service orchestrations deeply differ from corresponding QoS issues in network management. In an open world of Web services, service level agreements (SLAs) play an important role. They are contracts defining the obligations and rights between the provider of a Web service and a client with respect to the services' function and quality. In a previous work we have advocated using soft contracts of probabilistic nature, for the QoS part of contracts. Soft contracts have no hard bounds on QoS parameters, but rather probability distributions for them. An essential component of SLA management is the continuous monitoring of the performance of called Web services, to check for violation of the agreed SLA. In this paper we propose a statistical technique for QoS contract run time monitoring. Our technique is compatible with the use of soft probabilistic contracts.