Fortress is a support system for designing and implementing fault-tolerant distributed real-time systems that use commercial of the shelf (COTS) components. The main problem we address in Fortress is that services can-
not always provide their standard properties due the possibility of missed deadlines, dropped messages and process crashes. Fortress allows clients to detect when a service cannot provide its standard semantics anymore due to un-
masked failures. A service is fail-aware if it maintains an indicator that allows its clients to determine if the service provides it standard semantics or some predefined exception semantics. Fortress provides fail-aware clock synchro-
nization, membership and atomic broadcast services. Indicators allow a fail-safe application to switch the system to a safe state in case not all failures can be masked.