Service and contract composition: A model and a tool

Cloud computing services are seeing great success in IT scenarios. Dynamic reservation and allocation of network, storage and computational resources, the hiding of visibility of internal IT components, as well as the pay-per-use paradigm are nowadays more and more widespread ways to provide and consume services. Their complexity, however, is clearly visible in composing services to satisfy users´ requests, as well as in performance monitoring and service level comparisons. Effective ways are then needed to model both contracts and Service Level Agreements (SLAs) referring to the services that have to be provided to customers. However, this is hindered by a lack of expressivity in current SLA specifications and a consequent inadequacy in tools for managing SLAs and contract composition aspects. Therefore, we present a possible extension of WSLA, a widely known SLA description language, for modeling contracts and SLAs suitable to support contract owners during service composition and monitoring phases. An ad-hoc developed tool based on tree-graphs is examined to assess the feasibility of the proposed model and to simplify SLA and contract composition.