SLAs

In the DSCC project we are also addressing the specification and satisfaction of Service Level Agreements (SLAs) for service-centric systems. An SLA is an agreement between the provider of a service and a customer that defines the set of Quality of Service (QoS) guarantees and the obligations of the parties. In a QoS-aware service-centric architecture as depicted in the figure below, we envisage three kinds of QoS specification documents: QoS Capability specification, QoS Requirements specification and SLA specification.

A service provider publishes the set of QoS capabilities that is able to offer in the service registry. A service client specifies the desired QoS requirements for the service and accesses to the service registry to discover and select the service provider that best meet these requirements based on the advertised capabilities. A negotiation then starts between the client and the provider in order to obtain an SLA that satisfies both parties. During the service execution, the SLA will be the document of reference to monitor and assure that the QoS levels are guaranteed. Thus, apart from the required Service Level Objectives (SLOs), an SLA must include information about the measurement process, third measurement or monitoring parties, and the actions to take when an SLO is not fulfilled such as sending notifications to the parties and establishing penalty/reward policies. The desired requirements for an SLA language and the appropriateness of the WSLA language to SLA specification is under conideration.

Our SLA language refers to the concepts of the QoS ontology that has been defined in the project, so the provider and the client have a means of sharing definitions of the terms and concepts used in the SLA. We are also using an aggregated QoS inference engine for services composition, in order to find a composite workflow structure and instantiation that satisfy a given SLA specification.