Agree or Change! Making Services Evolve

Service-centric systems are developed by composition and configuration of services, i.e., "software components that can be used but that are not owned". Services are most often not under control of consumers. They are indeed made available by third parties, they are autonomously developed and managed, and can in principle change without notification. Service Level Agreements (SLAs) define the exact conditions under which services are provided and consumed. From the point of view of the consumer, SLAs make explicitly available all the technical information required for using a service: interfaces and data structures, interaction protocols, security-related information, quality of services parameters and so on. They are therefore key elements for the development of software-centric systems. In this article we discuss one of the most interesting open challenges in the area of services and of SLAs: how to enable the usage of SLAs to support service adaptation and evolution. We discuss in particular two key roles that SLAs can play to this purpose, namely "SLAs for monitoring" and "SLAs for adaptation".