Hey all,
there is a minor change to the stream side logic for static
membership<https://cwiki.apache.org/confluence/display/KAFKA/KIP-345%3A+Introduce+static+membership+protocol+to+reduce+consumer+rebalances>.
Originally we chose to piggy-back on user to supply a unique `client.id`
config that we could use to construct per thread level consumer
`group.instance.id`. This approach has several drawbacks:
1. We already have functionalities relying on `client.id`, and it is not
always the case where user wants to configure it differently for individual
instances. For example, currently user could throttle requests under same
client.id, which is a solid use case where the `client.id` should duplicate.
2. Existing stream users may unconsciously trigger static membership if they
already set `client.id` in their Stream apps. This includes unexpected fatal
errors due to `group.instance.id` fencing we are going to introduce.
In conclusion, it is not good practice to overload existing config that users
rely on unless there is no side effect. To make more fault tolerant upgrade
path, we decide to let stream users choose to set `group.instance.id` if they
want to enable static membership.
Thank you Guozhang and Matthias for the great discussions and enhancements for
the KIP!
Best,
Boyang
________________________________
From: Boyang Chen <bche...@outlook.com>
Sent: Saturday, March 9, 2019 2:28 PM
To: dev@kafka.apache.org
Subject: Re: [DISCUSS] KIP-345: Reduce multiple consumer rebalances by
specifying member id
Hi Mike,
Yes that's the plan!
________________________________
From: Mike Freyberger <mike.freyber...@xandr.com>
Sent: Saturday, March 9, 2019 10:04 AM
To: dev@kafka.apache.org
Subject: Re: [DISCUSS] KIP-345: Reduce multiple consumer rebalances by
specifying member id
Hi Boyang,
Is this work targeted for Kafka 2.3? I am eager to use this new feature.
Thanks,
Mike Freyberger
On 12/21/18, 1:21 PM, "Mayuresh Gharat" <gharatmayures...@gmail.com> wrote:
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
