We address the problem of the impossibdity of implement-
ing synchronous fault-tolerant service specifications in asyn-
chronous distributed systems. We introduce a method for
weakening a synchronous service specification so that it be-
comes implementable in “timed” asynchronous systems, that
is, asynchronous systems in which processes have access to
local hardware clocks. The method (1) adds to a service
interface an exception indicator so that a client knows at
any time if a server is currently providing its standard “syn-
chronous” semantics or some other specified exceptional se-
mantics, (2) the standard behavior provided when the excep-
tion indicator does not signal an exception is “similar” to the
original synchronous service behavior, and (3) a server has
to provide its standard semantics whenever the underlying
communication and process services exhibit “synchronous
behavior”. To illustrate our method, we show how the spec-
ification of a synchronous datagram service and an inter-
nal clock synchronization service can be transformed into a
~aii-aware service specification. Further illustrations of the
usefulness of fail-aware services are provided by describing a
fail-safe railway crossing service and a fail-aware weak group
membership service.