Fortress is a support system for designing and imple-
menting 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 possi-
bility 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 excep-
tion semantics. Fortress provides fail-aware clock synchro-
nization, membership and atomic broadcast services. Indi-
cators allow a fail-safe application to switch the system to
a safe state in case not all failures can be masked.