This paper presents a software architecture that ensures system dependability through external coordination. Under this architecture, a distributed system is decomposed into two types of entities, i.e., computational components and coordinators. The system?s computational functionalities are carried out by computational components, whereas coordinators are to coordinate these computational components to respect the system?s non-functional requirements, such as reliability constraints. Coordination in the system is done implicitly through intercepting, rerouting and suppressing communications between components. Additionally, roles are introduced into the system to bridge the connection between components and coordinators. Roles abstract the non-functional behaviors shared by computational components. Thus role-based coordination is more scalable than individual component-based coordination. Additionally, the dynamicity of components is concealed from the coordinators by the roles. A proof of concept implementation is built on top of the Actor Architecture, a middleware for distributed systems based on the Actor model [1]. The results show that not only such separation is feasible, but also that the architecture allows us to better control the dependability of the system. Finally, the application developed based on the proposed model is more modularized and consequently more reusable.