Cloud environments are being increasingly used for deploying and executing business processes and particularly service-based business processes (SBPs). One of the expected facilities of Cloud environments is elasticity at different levels. In this work, we argue the need of supporting elasticity at the service level. We propose a formal model for describing elasticity mechanisms and their strategies for SBPs. Our contribution consists in composing SBP models with models of controllers that describe the behavior of services and container replicas. In addition, we provide means for specifying both reactive and predictive elasticity strategies