Hey Mayuresh, thanks for giving another thorough thought on this matter! The session timeout is controlled on the member level (or more specifically member.id level), which has nothing to do with the static member mapping. Putting aside the low possibility of having a duplicate member id assignment throughout the restart, if this indeed happens when we switch a member c1 to dynamic member, the static membership map will only contain [gc1 -> mc1] mapping. This is because c2~c4 will be evicted during rebalance as their corresponding mc2~mc4 are already timeout through heartbeatpugatory. Next time the dynamic member restarts while group is in stable, the static map will be cleaned up and rebalance will be triggered as expected (we are already in dynamic mode now).
I hope this answers your question, thanks! Boyang ________________________________ From: Mayuresh Gharat <gharatmayures...@gmail.com> Sent: Saturday, December 22, 2018 2:21 AM To: dev@kafka.apache.org Subject: Re: [DISCUSS] KIP-345: Reduce multiple consumer rebalances by specifying member id Hi Boyang, Regarding "However, we shall still attempt to remove the member static info if the given `member.id` points to an existing `group.instance.id` upon LeaveGroupRequest, because I could think of the possibility that in long term we could want to add static membership leave group logic for more fine-grained use cases." > I think, there is some confusion here. I am probably not putting it > right. > I agree, If a static member sends LeaveGroupRequest, it should be removed > from the group. > Now getting back to downgrade of static membership to Dynamic membership, > with the example described earlier (copying it again for ease of reading) > : > >> 1. Lets say we have 4 consumers : c1, c2, c3, c4 in the static group. >> 2. The group.instance.id for each of there are as follows : >> - c1 -> gc1, c2 -> gc2, c3 -> gc3, c4 -> gc4 >> 3. The mapping on the GroupCordinator would be : >> - gc1 -> mc1, gc2 -> mc2, gc3 -> mc3, gc4 -> mc4, where mc1, mc2, >> mc3, mc4 are the randomly generated memberIds for c1, c2, c3, c4 >> respectively, by the GroupCoordinator. >> 4. Now we do a restart to move the group to dynamic membership. >> 5. We bounce c1 first and it rejoins with UNKNOWN_MEMBERID (since we >> don't persist the previously assigned memberId mc1 anywhere on the c1). >> > - We agree that there is no way to recognize that c1 was a part of the > group, *earlier*. If yes, the statement : "The dynamic member rejoins > the group without `group.instance.id`. It will be accepted since it is a > known member." is not necessarily true, right? > > - Now I *agree* with "However, we shall still attempt to remove the > member static info if the given `member.id` points to an existing ` > group.instance.id` upon LeaveGroupRequest, because I could think of the > possibility that in long term we could want to add static membership leave > group logic for more fine-grained use cases." > But that would only happen if the GroupCoordinator allocates the same > member.id (mc1) to the consumer c1, when it rejoins the group in step 5 > above as a dynamic member, which is very rare as it is randomly generated, > but possible. > > - This raises another question, if the GroupCoordinator assigns a > member.id (mc1~) to consumer c1 after step 5. It will join the group and > rebalance and the group will become stable, eventually. Now the > GroupCoordinator still maintains a mapping of "group.instance.id -> > member.id" (c1 -> gc1, c2 -> gc2, c3 -> gc3, c4 -> gc4) internally and > after some time, it realizes that it has not received heartbeat from the > consumer with "group.instance.id" = gc1. In that case, it will trigger > another rebalance assuming that a static member has left the group (when > actually it (c1) has not left the group but moved to dynamic membership). > This can result in multiple rebalances as the same will happen for c2, c3, > c4. > Thoughts ??? One thing, I can think of right now is to run : removeMemberFromGroup(String groupId, list<String> groupInstanceIdsToRemove, RemoveMemberFromGroupOptions options) with groupInstanceIdsToRemove = <gc1, gc2, gc3, gc4> once we have bounced all the members in the group. This assumes that we will be able to complete the bounces before the GroupCoordinator realizes that it has not received a heartbeat for any of <gc1, gc2, gc3, gc4>. This is tricky and error prone. Will have to think more on this. Thanks, Mayuresh
