Hi Imesha Sudasingha, For example, we have A,B,C,D,E five servers, that formed quorum and assume A is the Leader. Again assume network partition happened between A,B(minority region) and C,D,E. As we know, 3 is the majority factor to form quorum. Since Leader A is in the minority region, the entire quorum will get shutdown and all the servers will automatically moves to leader election phase. Now, A & B will do sending notifications each other and will never succeed to form quorum due to <3 factor. On the other side, C,D and E will participate each other and elect one of them as Leader.
>>>>>But will the partition with a minority will also continue to serve read >>>>>requests until a write request comes to the leader in the minority >>>>>partition? After the partition, all the servers in the minority region will get shutdown and moves to leader election phase. All the client sessions connected to these servers will be disconnected and will receive "KeeperState.Disconnected" event to their watchers, if any registered. But ZooKeeper supports read-only server mode. In this mode, client can connect to the read-only server even when the server might be partitioned from the quorum. Reference:- Please read the section "Read Only Mode Server" in the apache document link, https://zookeeper.apache.org/doc/r3.4.9/zookeeperAdmin.html >>>>>In the above scenario, what will happen to the "watches" that have >>>>>already been registered to a node in the minority partition? Since the quorum is re-forming, all the client session watchers will receive "KeeperState.Disconnected" event. These client session will start sending connection request to all quorum servers(A,B,C,D,E) to re-establish the connection. In my above example, C,D,E re-forms quorum successfully and client sessions will reconnect to one of these servers in the quorum(am assuming zkclient has the C,D,E server host address configured). Clients automatically reset watches during successful session reconnect. There is a feature to disable this watch resetting, please read the configuration "zookeeper.disableAutoWatchReset" section in the following apache doc link to understand more on this, https://zookeeper.apache.org/doc/trunk/zookeeperProgrammers.html Regards, Rakesh On Wed, Oct 5, 2016 at 6:39 PM, Imesha Sudasingha <imesha...@cse.mrt.ac.lk> wrote: > Hi all, > > When I was going through zookeeper, I noticed the following scenarios, > > - Suppose we have a 5 member quorum operating as normal. In a network > partitioning where the leader and a member node goes into one partition > and > other 3 nodes get into the other network partition. I known then the > side > with the majority will elect a new leader and carry on to serve > requests. > But will the partition with a minority will also continue to serve read > requests until a write request comes to the leader in the minority > partition? How does zookeeper handle this occasion? > - In the above scenario, what will happen to the "watches" that have > already been registered to a node in the minority partition? > > Can you clarify the above scenarios? > Thanks in advance! > > -- > *Imesha Sudasingha* > Undergraduate of Department of Computer Science and Engineering, > University of Moratuwa. > +94717086160 > View in Linkedin <https://lk.linkedin.com/in/imeshasudasingha> >
