Future smart computing environments will heavily rely on the collaboration of services, which are dynamically interconnected to Smart Service Systems in order to develop their full potential. Such computing environments are coined by a high degree of distribution and decentralization rendering a centralized control of service composition and system adaptation infeasible. Self-organizing Software Systems tackle this issue to automatically compose service systems at run time. Due to missing holistic system specifications, they constitute a mainly linear service chain, which contradicts the collaborative nature of Smart Service Systems. Thus, we proposed a Two-Phase Development Methodology, which includes a role-based collaboration specification, for engineering Smart Service Systems. In this paper, we present RoleDiSCo, a middleware for coordinated on-demand composition of Smart Service Systems in decentralized environments in order to bridge the gap between design and run time.