In my last post I made the case for the defence of the Paxos Algorithm. In this post we will delve into a tough problem which Paxos solves; state replication across a cluster of replicas using atomic broadcast. It turns out that maintaining primary ordering on secondary servers is straightforward to achieve using Paxo without affecting correctness or efficiency. This may be a surprising result to any reader who has looked at ZAB and Raft. You don’t need an external leader election service as used in the Spinnaker or PaxStore papers. You send the highest committed, and highest proposed, log stream sequence counter during leader failover, and introduce optional, ignorable, retransmission requests. The techniques described in this article are used by the Trex Paxos library.