State Machine Replication
Requests must be ordered in a consistent manner by all servers.
Usually one server manages the ordering process based on information from the other participants, then informs everybody about what was decided.
If the leader dies, a new leader must be selected to ensure progress.
Benign faults: Paxos [Lam98,Lam01]: must contact f+1 out of 2f+1 servers and uses 2 rounds to allow consistent progress.
Byzantine faults: BFT [CL99]: must contact 2f+1 out of 3f+1 servers and uses 3 rounds to allow consistent progress.