Brief Announcement: Consensus and Efficient Passive Replication
Passive replication is a popular practical approach to fault tolerance . Using the Paxos consensus protocol  to implement it is seeing a growing popularity lately, but requires taking care of peculiar constraints. State updates must be applied using the same sequence of generation: if a primary is in state A and executes an operation making it transition to state B, the resulting state update δ AB must be applied to the state A. Applying it to a different state C ≠ A is not safe because it might lead to an incorrect state, which is inconsistent with the history observed by replicas and clients. Paxos does not necessarily preserve the dependency between A and the delivery of δ AB , as observed in .
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