Thanks for the replies, James and Jason. Let me try to summarize your concerns.
I think James' question is primarily the severity of user using this config wrongly. The impact would be that the same member id being used by multiple or even all of the consumers. The assignment protocol couldn't distinguish any of the overlapping consumers, thus assigning the exact same partitions multiple times to different consumers. I would say the processed result would be including a lot of duplicates and unnecessary heavy load on the client side, The correctness will depend on the user logic, however I'm pessimistic. Although the impact is very high, the symptom is not hard to triage, because user could visualize consumer identity overlapping fairly easily by exported consumer metrics. On the user standpoint, they would be fully aware of the potential erratic status before enabling "member.id" configuration IMO. Let me know your thoughts James! Next is Jason's suggestion. Jason shared a higher viewpoint and pointed out the problem that we need to solve is to maintain "a strong bias towards being able to reuse previous state". The proposed approach is to separate the notion of consumer membership and consumer identity. The original idea of this KIP was on the Stream application, so I understand that the identities of multiple consumers belong to one instance, where each Stream thread will be using one dedicated main consumer. So in a Stream use case, we could internally generate member id with USER_DEFINED_ID + STREAM_THREAD_ID. In pure consumer use case, this could be a little bit challenging since user could arbitrarily initiate multiple consumers on the same instance which is out of our library control. This could add up the possibility of member id collision. So instead of making developers life easier, introducing member id config could break the existing code logic and take long time to understand and fix. Although I still assume this is an advanced config, user may use member id config even before they fully understand the problem, and use the same set of initialization logic cross multiple consumers on the same instance. I hope I have explained my understanding of the pros and cons of this KIP better. Remember the core argument of this KIP: If the broker recognize this consumer as an existing member, it shouldn't trigger rebalance. If we build our discussion on top of this argument, the client management of group membership could be tricky at first, but considering our original motivation to leader-follower rebalance model, I feel that having broker to create membership info and let client maintain would be less appealing and fragile. Having client generate membership data could build up source-of-truth model and streamline the current architecture. We need also consider flexibility introduced by this KIP for cloud users to coordinate consumer/stream instances more freely. Honestly, I'm interested in Jason's registration id proposal and open to more voices, but I feel it would be more complex than the current KIP for user to understand. Hope this makes sense, Jason. Thank you again for the feedback! Best, Boyang ________________________________ From: Jason Gustafson <ja...@confluent.io> Sent: Saturday, July 28, 2018 6:50 AM To: dev Subject: Re: [DISCUSS] KIP-345: Reduce multiple consumer rebalances by specifying member id Hey Boyang, Thanks for the KIP. I think my main question is in the same vein as James'. The problem is that the coordinator needs to be able to identify which instance of a particular memberId is the active one. For EOS, each transactionalId gets an epoch. When a new producer is started, it bumps the epoch which allows the transaction coordinator to fence off any zombie instances which may try to continue doing work with the old epoch. It seems like we need a similar protection for consumer members. Suppose for example that we distinguish between a registration id which is provided by the user and a member id which is assigned uniquely by the coordinator. In the JoinGroup request, both the registration id and the member id are provided. When a consumer is first started, it doesn't know the memberId, so it it provides only the registration id. The coordinator can then assign a new memberId and invalidate the previous one that was associated with the registration id. This would then fence off the previous instance which was still trying to use the member id. Taking a little bit of a step back, I think the main observation in this KIP is that applications with heavy local state need to have a strong bias toward being able to reuse that state. It is a bit like Kafka itself in the sense that a replica is not moved just because the broker is shutdown as the cost of moving the log is extremely high. I'm wondering if we need to think about streams applications in a similar way. Should there be a static notion of the members of the group so that streams can make rebalancing decisions more easily without depending so heavily on transient membership? I feel the hacks we've put in place in some cases to avoid rebalances are a bit brittle. Delaying group joining for example is an example of this. If you knew ahead of time who the stable members of the group were, then this would not be needed. Anyway, just a thought. Thanks, Jason On Fri, Jul 27, 2018 at 1:58 PM, James Cheng <wushuja...@gmail.com> wrote: > When you say that it will "break", what does this breakage look like? Will > the consumer-group be non-functional? Will just those instances be > non-functional? Or will the group be functional, but the rebalancing be > non-optimal and require more round-trips/data-transfer? (similar to the > current algorithm) > > I'm trying to assess the potential for user-error and the impact of > user-error. > > -James > > > On Jul 27, 2018, at 11:25 AM, Boyang Chen <bche...@outlook.com> wrote: > > > > Hey James, > > > > > > the algorithm is relying on client side to provide unique consumer > member id. It will break unless we enforce some sort of validation (host + > port) on the server side. To simplify the first version, we do not plan to > enforce validation. A good comparison would be the EOS producer which is in > charge of generating unique transaction id sequence. IMO for broker logic, > the tolerance of client side error is not unlimited. > > > > > > Thank you! > > > > > > ________________________________ > > From: James Cheng <wushuja...@gmail.com> > > Sent: Saturday, July 28, 2018 1:26 AM > > To: dev@kafka.apache.org > > Subject: Re: [DISCUSS] KIP-345: Reduce multiple consumer rebalances by > specifying member id > > > > > >> On Jul 26, 2018, at 11:09 PM, Guozhang Wang <wangg...@gmail.com> wrote: > >> > >> Hi Boyang, > >> > >> Thanks for the proposed KIP. I made a pass over the wiki and here are > some > >> comments / questions: > >> > >> 1. In order to preserve broker compatibility, we need to make sure the > >> broker version discovery logic can be integrated with this new logic. > I.e. > >> if a newer versioned consumer is talking to an older versioned broker > who > >> does not recognize V4, the client needs to downgrade its > JoinGroupRequest > >> version to V3 and not setting the member-id specifically. You can take a > >> look at the ApiVersionsRequest and see how to work with it. > >> > >> 2. There may exist some manners to validate that two different clients > do > >> not send with the same member id, for example if we pass along the > >> host:port information from KafkaApis to the GroupCoordinator interface. > But > >> I think this is overly complicated the logic and may not worthwhile than > >> relying on users to specify unique member ids. > > > > Boyang, > > > > Thanks for the KIP! How will the algorithm behave if multiple consumers > provide the same member id? > > > > -James > > > >> 3. Minor: you would need to bumping up the version of JoinGroupResponse > to > >> V4 as well. > >> > >> 4. Minor: in the wiki page, you need to specify the actual string value > for > >> `MEMBER_ID`, for example "member.id". > >> > >> 5. When this additional config it specified by users, we should consider > >> setting the default of internal `LEAVE_GROUP_ON_CLOSE_CONFIG` to false, > >> since otherwise its effectiveness would be less. > >> > >> > >> Guozhang > >> > >> > >> > >>> On Thu, Jul 26, 2018 at 9:20 PM, Boyang Chen <bche...@outlook.com> > wrote: > >>> > >>> Hey friends, > >>> > >>> > >>> I would like to open a discussion thread on KIP-345: > >>> > >>> https://cwiki.apache.org/confluence/display/KAFKA/KIP-345%3A > >>> +Reduce+multiple+consumer+rebalances+by+specifying+member+id > >>> > >>> > >>> This KIP is trying to resolve multiple rebalances by maintaining the > >>> consumer member id across rebalance generations. I have verified the > theory > >>> on our internal Stream application, and it could reduce rebalance time > to a > >>> few seconds when service is rolling restart. > >>> > >>> > >>> Let me know your thoughts, thank you! > >>> > >>> > >>> Best, > >>> > >>> Boyang > >>> > >> > >> > >> > >> -- > >> -- Guozhang > >
