Leader-Based Consensus

It is now well recognized that consensus is a fundamental problem one has to solve to implement reliable applications on top of unreliable asynchronous distributed systems prone to failures. It has been shown that this problem cannot be solved if the underlying asynchronous system does not satisfy additional assumptions. This paper presents a new consensus protocol based on a leader oracle (denoted Ω in the litterature). Although this protocol uses asynchronous rounds, it is not based on the rotating coordinator paradigm. As a consequence, it does not suffer from drawbacks inherent to ♢S-based consensus protocols that explicity use this paradigm. As Ω and ♢S are equivalent, the proposed protocol does not require assumptions stronger or weaker than the ones abstracted in ♢S. Hence, it also requires f < n/2 (where n is the number of processes and f an upper bound on the number of processes that may crash). From a design point of view, the proposed protocol is surprisingly simple. From an efficiency point of view, it allows the processes to agree in a single round when the oracle provides the processes with the same leader (a common case in practice). It is also shown that the time and message costs of the protocol can be reduced when f < n/3. Moreover, when, in addition to the leader oracle, the system is equipped with a random oracle, the proposed protocol can be extended to provide a hybrid consensus protocol at no additional message cost.