Primary-backup

Some assumptions:

Passive Replication

  1. Client sends request to the primary server using the at most once semantics.
  2. Primary server handles all request by causal order.
    1. If there is a duplicated request, return saved response.
  3. Executes request and saves reply.
  4. Primary sends to the secondary servers the new state, reply and request id. Secondary servers should ACK.
  5. Primary replies to client.

TODO