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Fault-Tolerant Sequencer: Specification and an Implementation∗

The synchronization among thin, independent and concurrent processes in an open distributed system is a fundamental issue in current architectures (e.g. middlewares, three-tier architectures etc.). “Independent process” means no message has to be exchanged among the processes to synchronize themselves and “open” means that the number of processes that require to synchronize changes along the time. In this paper we present the specification of a sequencer service that allows independent processes to get a sequence number that can be used to label successive operations (e.g. to allow a set of independent and concurrent processes to get a total order on these labelled operations). Moreover, we propose an implementation of the sequencer service in the timed asynchronous model along with its correctness proof.