Primary Partition "Virtually-Synchronous Communication" harder than Consensus

The paper considers the problem of implementing \Virtu-ally Synchronous Communication" in the primary partition of an asyn-chronous system. Virtually Synchronous Communication was rst introduced by the Isis system as a powerful mechanism for building fault-tolerant processes that mask failures by replication: it can be understood as a rule for ordering message deliveries (reliable multicasts) with respect to view changes, deened by a membership service. Primary partition Virtually Synchronous Communication, noted PP-VSC, is the problem of implementing Virtually Synchronous Communication in the case of totally ordered views. The paper formally deenes the problem, and shows that surprisingly this problem is harder than consensus: (1) consensus is solvable whenever the PP-VSC problem is solvable, however (2) there are environments where consensus is solvable, but not PP-VSC. The paper also deenes an environment in which PP-VSC can be solved. The practical consequences of the result are discussed.