Hi, I have question about how zookeeper *remembers* a commit operation.
According to http://hadoop.apache.org/zookeeper/docs/r3.2.2/zookeeperInternals.html#sc_summary <quote> The leader will issue a COMMIT to all followers as soon as a quorum of followers have ACKed a message. Since messages are ACKed in order, COMMITs will be sent by the leader as received by the followers in order. COMMITs are processed in order. Followers deliver a proposals message when that proposal is committed. </quote> My question is will leader wait for COMMIT to be processed by quorum of followers before consider COMMIT to be success? From the documentation it seems that leader handles COMMIT asynchronously and don't expect confirmation from followers. In the extreme case, what happens if leader issue a COMMIT to all followers and crash immediately before the COMMIT message can go out of the network. How the system remembers the COMMIT ever happens? Actually this is related to the leader election process: <quote> ZooKeeper messaging doesn't care about the exact method of electing a leader has long as the following holds: - The leader has seen the highest zxid of all the followers. - A quorum of servers have committed to following the leader. Of these two requirements only the first, the highest zxid amoung the followers needs to hold for correct operation. </quote> Is there a liveness issue try to find "The leader has seen the highest zxid of all the followers"? What if some of the followers (which happens to holding the highest zxid) cannot be contacted(FLP impossible result?) It will be more striaghtforward if COMMIT requires confirmation from a quorum of the followers. But I guess things get optimized according to Zab's FIFO nature...just want to hear some clarification about it. Thanks alot!