I read the ZAB paper before, and never realized this question, but find out today that I can't answer why, so I'm bringing it up here.
according to the paper B. Reed and F. P. Junqueira. A simple totally ordered broadcast protocol. In LADIS ’08: Proceedings of the 2nd Workshop on Large-Scale Distributed Systems and Middleware, pages 1–6, New York, NY, USA, 2008. ACM. the leader broadcasts a write to all replicas, and then waits for a quorum to reply, before sending out the COMMIT. why is the quorum necessary (i.e. why can't the leader just wait for one reply and start sending the COMMIT?)?? now that I think about it, it seems that waiting for just one reply is enough, because the connection from leader to replicas are FIFO, as long as the replicas do not die, they will eventually get the writes, even though the writes arrive at them after the leader starts the COMMIT. the only reason I can think of for using a quorum is to tolerate more failures: if the only replied replica dies, and leader dies, then we lose that latest write. by requiring f ACKs, you can tolerate f-1 failures. but then you don't really need 2f+1 nodes in the ZK cluster, just f+1 is enough. Thanks a lot Yang
