There are five nodes, using master-slave replication. When we start A is the master.
The nodes are capable of synchronous replication, when a client writes,
it gets as relpy the number or replicas that received the write. A client
can consider a write accepted only when "3" or more is returned, otherwise
the result is not determined (false negatives are possbile).
Every node has a serial number, called the replication offset. It is always
incremented as the replication stream is processed by a replica. Replicas
are capable of telling an external entity, called "the controller", what