Hey Jason, Yes, that is what I meant by > Given those constraints, I think that we can simply mark the group as `PreparingRebalance` with a rebalanceTimeout of the server setting ` group.max.session.timeout.ms`. That's a bit long by default (5 minutes) but I can't seem to come up with a better alternative So either the timeout or all members calling joinGroup, yes
On Tue, Dec 11, 2018 at 8:14 PM Boyang Chen <bche...@outlook.com> wrote: > Hey Jason, > > I think this is the correct understanding. One more question is whether > you feel > we should enforce group size cap statically or on runtime? > > Boyang > ________________________________ > From: Jason Gustafson <ja...@confluent.io> > Sent: Tuesday, December 11, 2018 3:24 AM > To: dev > Subject: Re: [Discuss] KIP-389: Enforce group.max.size to cap member > metadata growth > > Hey Stanislav, > > Just to clarify, I think what you're suggesting is something like this in > order to gracefully shrink the group: > > 1. Transition the group to PREPARING_REBALANCE. No members are kicked out. > 2. Continue to allow offset commits and heartbeats for all current members. > 3. Allow the first n members that send JoinGroup to stay in the group, but > wait for the JoinGroup (or session timeout) from all active members before > finishing the rebalance. > > So basically we try to give the current members an opportunity to finish > work, but we prevent some of them from rejoining after the rebalance > completes. It sounds reasonable if I've understood correctly. > > Thanks, > Jason > > > > On Fri, Dec 7, 2018 at 6:47 AM Boyang Chen <bche...@outlook.com> wrote: > > > Yep, LGTM on my side. Thanks Stanislav! > > ________________________________ > > From: Stanislav Kozlovski <stanis...@confluent.io> > > Sent: Friday, December 7, 2018 8:51 PM > > To: dev@kafka.apache.org > > Subject: Re: [Discuss] KIP-389: Enforce group.max.size to cap member > > metadata growth > > > > Hi, > > > > We discussed this offline with Boyang and figured that it's best to not > > wait on the Cooperative Rebalancing proposal. Our thinking is that we can > > just force a rebalance from the broker, allowing consumers to commit > > offsets if their rebalanceListener is configured correctly. > > When rebalancing improvements are implemented, we assume that they would > > improve KIP-389's behavior as well as the normal rebalance scenarios > > > > On Wed, Dec 5, 2018 at 12:09 PM Boyang Chen <bche...@outlook.com> wrote: > > > > > Hey Stanislav, > > > > > > thanks for the question! `Trivial rebalance` means "we don't start > > > reassignment right now, but you need to know it's coming soon > > > and you should start preparation". > > > > > > An example KStream use case is that before actually starting to shrink > > the > > > consumer group, we need to > > > 1. partition the consumer group into two subgroups, where one will be > > > offline soon and the other will keep serving; > > > 2. make sure the states associated with near-future offline consumers > are > > > successfully replicated on the serving ones. > > > > > > As I have mentioned shrinking the consumer group is pretty much > > equivalent > > > to group scaling down, so we could think of this > > > as an add-on use case for cluster scaling. So my understanding is that > > the > > > KIP-389 could be sequenced within our cooperative rebalancing< > > > > > > https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Fcwiki.apache.org%2Fconfluence%2Fdisplay%2FKAFKA%2FIncremental%2BCooperative%2BRebalancing%253A%2BSupport%2Band%2BPolicies&data=02%7C01%7C%7Cb603e099d6c744d8fac708d65ed51d03%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C636800666735874264&sdata=BX4DHEX1OMgfVuBOREwSjiITu5aV83Q7NAz77w4avVc%3D&reserved=0 > > > > > > > proposal. > > > > > > Let me know if this makes sense. > > > > > > Best, > > > Boyang > > > ________________________________ > > > From: Stanislav Kozlovski <stanis...@confluent.io> > > > Sent: Wednesday, December 5, 2018 5:52 PM > > > To: dev@kafka.apache.org > > > Subject: Re: [Discuss] KIP-389: Enforce group.max.size to cap member > > > metadata growth > > > > > > Hey Boyang, > > > > > > I think we still need to take care of group shrinkage because even if > > users > > > change the config value we cannot guarantee that all consumer groups > > would > > > have been manually shrunk. > > > > > > Regarding 2., I agree that forcefully triggering a rebalance might be > the > > > most intuitive way to handle the situation. > > > What does a "trivial rebalance" mean? Sorry, I'm not familiar with the > > > term. > > > I was thinking that maybe we could force a rebalance, which would cause > > > consumers to commit their offsets (given their rebalanceListener is > > > configured correctly) and subsequently reject some of the incoming > > > `joinGroup` requests. Does that sound like it would work? > > > > > > On Wed, Dec 5, 2018 at 1:13 AM Boyang Chen <bche...@outlook.com> > wrote: > > > > > > > Hey Stanislav, > > > > > > > > I read the latest KIP and saw that we already changed the default > value > > > to > > > > -1. Do > > > > we still need to take care of the consumer group shrinking when doing > > the > > > > upgrade? > > > > > > > > However this is an interesting topic that worth discussing. Although > > > > rolling > > > > upgrade is fine, `consumer.group.max.size` could always have conflict > > > with > > > > the current > > > > consumer group size which means we need to adhere to one source of > > truth. > > > > > > > > 1.Choose the current group size, which means we never interrupt the > > > > consumer group until > > > > it transits to PREPARE_REBALANCE. And we keep track of how many join > > > group > > > > requests > > > > we have seen so far during PREPARE_REBALANCE. After reaching the > > consumer > > > > cap, > > > > we start to inform over provisioned consumers that you should send > > > > LeaveGroupRequest and > > > > fail yourself. Or with what Mayuresh proposed in KIP-345, we could > mark > > > > extra members > > > > as hot backup and rebalance without them. > > > > > > > > 2.Choose the `consumer.group.max.size`. I feel incremental > rebalancing > > > > (you proposed) could be of help here. > > > > When a new cap is enforced, leader should be notified. If the current > > > > group size is already over limit, leader > > > > shall trigger a trivial rebalance to shuffle some topic partitions > and > > > let > > > > a subset of consumers prepare the ownership > > > > transition. Until they are ready, we trigger a real rebalance to > remove > > > > over-provisioned consumers. It is pretty much > > > > equivalent to `how do we scale down the consumer group without > > > > interrupting the current processing`. > > > > > > > > I personally feel inclined to 2 because we could kill two birds with > > one > > > > stone in a generic way. What do you think? > > > > > > > > Boyang > > > > ________________________________ > > > > From: Stanislav Kozlovski <stanis...@confluent.io> > > > > Sent: Monday, December 3, 2018 8:35 PM > > > > To: dev@kafka.apache.org > > > > Subject: Re: [Discuss] KIP-389: Enforce group.max.size to cap member > > > > metadata growth > > > > > > > > Hi Jason, > > > > > > > > > 2. Do you think we should make this a dynamic config? > > > > I'm not sure. Looking at the config from the perspective of a > > > prescriptive > > > > config, we may get away with not updating it dynamically. > > > > But in my opinion, it always makes sense to have a config be > > dynamically > > > > configurable. As long as we limit it to being a cluster-wide config, > we > > > > should be fine. > > > > > > > > > 1. I think it would be helpful to clarify the details on how the > > > > coordinator will shrink the group. It will need to choose which > members > > > to > > > > remove. Are we going to give current members an opportunity to commit > > > > offsets before kicking them from the group? > > > > > > > > This turns out to be somewhat tricky. I think that we may not be able > > to > > > > guarantee that consumers don't process a message twice. > > > > My initial approach was to do as much as we could to let consumers > > commit > > > > offsets. > > > > > > > > I was thinking that we mark a group to be shrunk, we could keep a map > > of > > > > consumer_id->boolean indicating whether they have committed offsets. > I > > > then > > > > thought we could delay the rebalance until every consumer commits (or > > > some > > > > time passes). > > > > In the meantime, we would block all incoming fetch calls (by either > > > > returning empty records or a retriable error) and we would continue > to > > > > accept offset commits (even twice for a single consumer) > > > > > > > > I see two problems with this approach: > > > > * We have async offset commits, which implies that we can receive > fetch > > > > requests before the offset commit req has been handled. i.e consmer > > sends > > > > fetchReq A, offsetCommit B, fetchReq C - we may receive A,C,B in the > > > > broker. Meaning we could have saved the offsets for B but rebalance > > > before > > > > the offsetCommit for the offsets processed in C come in. > > > > * KIP-392 Allow consumers to fetch from closest replica > > > > < > > > > > > > > > > https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Fcwiki.apache.org%2Fconfluence%2Fdisplay%2FKAFKA%2FKIP-392%253A%2BAllow%2Bconsumers%2Bto%2Bfetch%2Bfrom%2Bclosest%2Breplica&data=02%7C01%7C%7Cb603e099d6c744d8fac708d65ed51d03%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C636800666735874264&sdata=bekXj%2FVdA6flZWQ70%2BSEyHm31%2F2WyWO1EpbvqyjWFJw%3D&reserved=0 > > > > > > > > > would > > > > make it significantly harder to block poll() calls on consumers whose > > > > groups are being shrunk. Even if we implemented a solution, the same > > race > > > > condition noted above seems to apply and probably others > > > > > > > > > > > > Given those constraints, I think that we can simply mark the group as > > > > `PreparingRebalance` with a rebalanceTimeout of the server setting ` > > > > group.max.session.timeout.ms`. That's a bit long by default (5 > > minutes) > > > > but > > > > I can't seem to come up with a better alternative > > > > > > > > I'm interested in hearing your thoughts. > > > > > > > > Thanks, > > > > Stanislav > > > > > > > > On Fri, Nov 30, 2018 at 8:38 AM Jason Gustafson <ja...@confluent.io> > > > > wrote: > > > > > > > > > Hey Stanislav, > > > > > > > > > > What do you think about the use case I mentioned in my previous > reply > > > > about > > > > > > a more resilient self-service Kafka? I believe the benefit there > is > > > > > bigger. > > > > > > > > > > > > > > > I see this config as analogous to the open file limit. Probably > this > > > > limit > > > > > was intended to be prescriptive at some point about what was > deemed a > > > > > reasonable number of open files for an application. But mostly > people > > > > treat > > > > > it as an annoyance which they have to work around. If it happens to > > be > > > > hit, > > > > > usually you just increase it because it is not tied to an actual > > > resource > > > > > constraint. However, occasionally hitting the limit does indicate > an > > > > > application bug such as a leak, so I wouldn't say it is useless. > > > > Similarly, > > > > > the issue in KAFKA-7610 was a consumer leak and having this limit > > would > > > > > have allowed the problem to be detected before it impacted the > > cluster. > > > > To > > > > > me, that's the main benefit. It's possible that it could be used > > > > > prescriptively to prevent poor usage of groups, but like the open > > file > > > > > limit, I suspect administrators will just set it large enough that > > > users > > > > > are unlikely to complain. > > > > > > > > > > Anyway, just a couple additional questions: > > > > > > > > > > 1. I think it would be helpful to clarify the details on how the > > > > > coordinator will shrink the group. It will need to choose which > > members > > > > to > > > > > remove. Are we going to give current members an opportunity to > commit > > > > > offsets before kicking them from the group? > > > > > > > > > > 2. Do you think we should make this a dynamic config? > > > > > > > > > > Thanks, > > > > > Jason > > > > > > > > > > > > > > > > > > > > > > > > > On Wed, Nov 28, 2018 at 2:42 AM Stanislav Kozlovski < > > > > > stanis...@confluent.io> > > > > > wrote: > > > > > > > > > > > Hi Jason, > > > > > > > > > > > > You raise some very valid points. > > > > > > > > > > > > > The benefit of this KIP is probably limited to preventing > > "runaway" > > > > > > consumer groups due to leaks or some other application bug > > > > > > What do you think about the use case I mentioned in my previous > > reply > > > > > about > > > > > > a more resilient self-service Kafka? I believe the benefit there > is > > > > > bigger > > > > > > > > > > > > * Default value > > > > > > You're right, we probably do need to be conservative. Big > consumer > > > > groups > > > > > > are considered an anti-pattern and my goal was to also hint at > this > > > > > through > > > > > > the config's default. Regardless, it is better to not have the > > > > potential > > > > > to > > > > > > break applications with an upgrade. > > > > > > Choosing between the default of something big like 5000 or an > > opt-in > > > > > > option, I think we should go with the *disabled default option* > > > (-1). > > > > > > The only benefit we would get from a big default of 5000 is > default > > > > > > protection against buggy/malicious applications that hit the > > > KAFKA-7610 > > > > > > issue. > > > > > > While this KIP was spawned from that issue, I believe its value > is > > > > > enabling > > > > > > the possibility of protection and helping move towards a more > > > > > self-service > > > > > > Kafka. I also think that a default value of 5000 might be > > misleading > > > to > > > > > > users and lead them to think that big consumer groups (> 250) > are a > > > > good > > > > > > thing. > > > > > > > > > > > > The good news is that KAFKA-7610 should be fully resolved and the > > > > > rebalance > > > > > > protocol should, in general, be more solid after the planned > > > > improvements > > > > > > in KIP-345 and KIP-394. > > > > > > > > > > > > * Handling bigger groups during upgrade > > > > > > I now see that we store the state of consumer groups in the log > and > > > > why a > > > > > > rebalance isn't expected during a rolling upgrade. > > > > > > Since we're going with the default value of the max.size being > > > > disabled, > > > > > I > > > > > > believe we can afford to be more strict here. > > > > > > During state reloading of a new Coordinator with a defined > > > > max.group.size > > > > > > config, I believe we should *force* rebalances for groups that > > exceed > > > > the > > > > > > configured size. Then, only some consumers will be able to join > and > > > the > > > > > max > > > > > > size invariant will be satisfied. > > > > > > > > > > > > I updated the KIP with a migration plan, rejected alternatives > and > > > the > > > > > new > > > > > > default value. > > > > > > > > > > > > Thanks, > > > > > > Stanislav > > > > > > > > > > > > On Tue, Nov 27, 2018 at 5:25 PM Jason Gustafson < > > ja...@confluent.io> > > > > > > wrote: > > > > > > > > > > > > > Hey Stanislav, > > > > > > > > > > > > > > Clients will then find that coordinator > > > > > > > > and send `joinGroup` on it, effectively rebuilding the group, > > > since > > > > > the > > > > > > > > cache of active consumers is not stored outside the > > Coordinator's > > > > > > memory. > > > > > > > > (please do say if that is incorrect) > > > > > > > > > > > > > > > > > > > > > Groups do not typically rebalance after a coordinator change. > You > > > > could > > > > > > > potentially force a rebalance if the group is too big and kick > > out > > > > the > > > > > > > slowest members or something. A more graceful solution is > > probably > > > to > > > > > > just > > > > > > > accept the current size and prevent it from getting bigger. We > > > could > > > > > log > > > > > > a > > > > > > > warning potentially. > > > > > > > > > > > > > > My thinking is that we should abstract away from conserving > > > resources > > > > > and > > > > > > > > focus on giving control to the broker. The issue that spawned > > > this > > > > > KIP > > > > > > > was > > > > > > > > a memory problem but I feel this change is useful in a more > > > general > > > > > > way. > > > > > > > > > > > > > > > > > > > > > So you probably already know why I'm asking about this. For > > > consumer > > > > > > groups > > > > > > > anyway, resource usage would typically be proportional to the > > > number > > > > of > > > > > > > partitions that a group is reading from and not the number of > > > > members. > > > > > > For > > > > > > > example, consider the memory use in the offsets cache. The > > benefit > > > of > > > > > > this > > > > > > > KIP is probably limited to preventing "runaway" consumer groups > > due > > > > to > > > > > > > leaks or some other application bug. That still seems useful > > > though. > > > > > > > > > > > > > > I completely agree with this and I *ask everybody to chime in > > with > > > > > > opinions > > > > > > > > on a sensible default value*. > > > > > > > > > > > > > > > > > > > > > I think we would have to be very conservative. The group > protocol > > > is > > > > > > > generic in some sense, so there may be use cases we don't know > of > > > > where > > > > > > > larger groups are reasonable. Probably we should make this an > > > opt-in > > > > > > > feature so that we do not risk breaking anyone's application > > after > > > an > > > > > > > upgrade. Either that, or use a very high default like 5,000. > > > > > > > > > > > > > > Thanks, > > > > > > > Jason > > > > > > > > > > > > > > On Tue, Nov 27, 2018 at 3:27 AM Stanislav Kozlovski < > > > > > > > stanis...@confluent.io> > > > > > > > wrote: > > > > > > > > > > > > > > > Hey Jason and Boyang, those were important comments > > > > > > > > > > > > > > > > > One suggestion I have is that it would be helpful to put > your > > > > > > reasoning > > > > > > > > on deciding the current default value. For example, in > certain > > > use > > > > > > cases > > > > > > > at > > > > > > > > Pinterest we are very likely to have more consumers than 250 > > when > > > > we > > > > > > > > configure 8 stream instances with 32 threads. > > > > > > > > > For the effectiveness of this KIP, we should encourage > people > > > to > > > > > > > discuss > > > > > > > > their opinions on the default setting and ideally reach a > > > > consensus. > > > > > > > > > > > > > > > > I completely agree with this and I *ask everybody to chime in > > > with > > > > > > > opinions > > > > > > > > on a sensible default value*. > > > > > > > > My thought process was that in the current model rebalances > in > > > > large > > > > > > > groups > > > > > > > > are more costly. I imagine most use cases in most Kafka users > > do > > > > not > > > > > > > > require more than 250 consumers. > > > > > > > > Boyang, you say that you are "likely to have... when we..." - > > do > > > > you > > > > > > have > > > > > > > > systems running with so many consumers in a group or are you > > > > planning > > > > > > > to? I > > > > > > > > guess what I'm asking is whether this has been tested in > > > production > > > > > > with > > > > > > > > the current rebalance model (ignoring KIP-345) > > > > > > > > > > > > > > > > > Can you clarify the compatibility impact here? What > > > > > > > > > will happen to groups that are already larger than the max > > > size? > > > > > > > > This is a very important question. > > > > > > > > From my current understanding, when a coordinator broker gets > > > shut > > > > > > > > down during a cluster rolling upgrade, a replica will take > > > > leadership > > > > > > of > > > > > > > > the `__offset_commits` partition. Clients will then find that > > > > > > coordinator > > > > > > > > and send `joinGroup` on it, effectively rebuilding the group, > > > since > > > > > the > > > > > > > > cache of active consumers is not stored outside the > > Coordinator's > > > > > > memory. > > > > > > > > (please do say if that is incorrect) > > > > > > > > Then, I believe that working as if this is a new group is a > > > > > reasonable > > > > > > > > approach. Namely, fail joinGroups when the max.size is > > exceeded. > > > > > > > > What do you guys think about this? (I'll update the KIP after > > we > > > > > settle > > > > > > > on > > > > > > > > a solution) > > > > > > > > > > > > > > > > > Also, just to be clear, the resource we are trying to > > conserve > > > > > here > > > > > > is > > > > > > > > what? Memory? > > > > > > > > My thinking is that we should abstract away from conserving > > > > resources > > > > > > and > > > > > > > > focus on giving control to the broker. The issue that spawned > > > this > > > > > KIP > > > > > > > was > > > > > > > > a memory problem but I feel this change is useful in a more > > > general > > > > > > way. > > > > > > > It > > > > > > > > limits the control clients have on the cluster and helps > Kafka > > > > > become a > > > > > > > > more self-serving system. Admin/Ops teams can better control > > the > > > > > impact > > > > > > > > application developers can have on a Kafka cluster with this > > > change > > > > > > > > > > > > > > > > Best, > > > > > > > > Stanislav > > > > > > > > > > > > > > > > > > > > > > > > On Mon, Nov 26, 2018 at 8:00 PM Jason Gustafson < > > > > ja...@confluent.io> > > > > > > > > wrote: > > > > > > > > > > > > > > > > > Hi Stanislav, > > > > > > > > > > > > > > > > > > Thanks for the KIP. Can you clarify the compatibility > impact > > > > here? > > > > > > What > > > > > > > > > will happen to groups that are already larger than the max > > > size? > > > > > > Also, > > > > > > > > just > > > > > > > > > to be clear, the resource we are trying to conserve here is > > > what? > > > > > > > Memory? > > > > > > > > > > > > > > > > > > -Jason > > > > > > > > > > > > > > > > > > On Mon, Nov 26, 2018 at 2:44 AM Boyang Chen < > > > bche...@outlook.com > > > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > Thanks Stanislav for the update! One suggestion I have is > > > that > > > > it > > > > > > > would > > > > > > > > > be > > > > > > > > > > helpful to put your > > > > > > > > > > > > > > > > > > > > reasoning on deciding the current default value. For > > example, > > > > in > > > > > > > > certain > > > > > > > > > > use cases at Pinterest we are very likely > > > > > > > > > > > > > > > > > > > > to have more consumers than 250 when we configure 8 > stream > > > > > > instances > > > > > > > > with > > > > > > > > > > 32 threads. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > For the effectiveness of this KIP, we should encourage > > people > > > > to > > > > > > > > discuss > > > > > > > > > > their opinions on the default setting and ideally reach a > > > > > > consensus. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Best, > > > > > > > > > > > > > > > > > > > > Boyang > > > > > > > > > > > > > > > > > > > > ________________________________ > > > > > > > > > > From: Stanislav Kozlovski <stanis...@confluent.io> > > > > > > > > > > Sent: Monday, November 26, 2018 6:14 PM > > > > > > > > > > To: dev@kafka.apache.org > > > > > > > > > > Subject: Re: [Discuss] KIP-389: Enforce group.max.size to > > cap > > > > > > member > > > > > > > > > > metadata growth > > > > > > > > > > > > > > > > > > > > Hey everybody, > > > > > > > > > > > > > > > > > > > > It's been a week since this KIP and not much discussion > has > > > > been > > > > > > > made. > > > > > > > > > > I assume that this is a straight forward change and I > will > > > > open a > > > > > > > > voting > > > > > > > > > > thread in the next couple of days if nobody has anything > to > > > > > > suggest. > > > > > > > > > > > > > > > > > > > > Best, > > > > > > > > > > Stanislav > > > > > > > > > > > > > > > > > > > > On Thu, Nov 22, 2018 at 12:56 PM Stanislav Kozlovski < > > > > > > > > > > stanis...@confluent.io> > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > Greetings everybody, > > > > > > > > > > > > > > > > > > > > > > I have enriched the KIP a bit with a bigger Motivation > > > > section > > > > > > and > > > > > > > > also > > > > > > > > > > > renamed it. > > > > > > > > > > > KIP: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Fcwiki.apache.org%2Fconfluence%2Fdisplay%2FKAFKA%2FKIP-389%253A%2BIntroduce%2Ba%2Bconfigurable%2Bconsumer%2Bgroup%2Bsize%2Blimit&data=02%7C01%7C%7Cb603e099d6c744d8fac708d65ed51d03%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C636800666735874264&sdata=dLVLofL8NnQatVq6WEDukxfIorh7HeQR9TyyUifcAPo%3D&reserved=0 > > > > > > > > > > > > > > > > > > > > > > I'm looking forward to discussions around it. > > > > > > > > > > > > > > > > > > > > > > Best, > > > > > > > > > > > Stanislav > > > > > > > > > > > > > > > > > > > > > > On Tue, Nov 20, 2018 at 1:47 PM Stanislav Kozlovski < > > > > > > > > > > > stanis...@confluent.io> wrote: > > > > > > > > > > > > > > > > > > > > > >> Hey there everybody, > > > > > > > > > > >> > > > > > > > > > > >> Thanks for the introduction Boyang. I appreciate the > > > effort > > > > > you > > > > > > > are > > > > > > > > > > >> putting into improving consumer behavior in Kafka. > > > > > > > > > > >> > > > > > > > > > > >> @Matt > > > > > > > > > > >> I also believe the default value is high. In my > opinion, > > > we > > > > > > should > > > > > > > > aim > > > > > > > > > > to > > > > > > > > > > >> a default cap around 250. This is because in the > current > > > > model > > > > > > any > > > > > > > > > > consumer > > > > > > > > > > >> rebalance is disrupting to every consumer. The bigger > > the > > > > > group, > > > > > > > the > > > > > > > > > > longer > > > > > > > > > > >> this period of disruption. > > > > > > > > > > >> > > > > > > > > > > >> If you have such a large consumer group, chances are > > that > > > > your > > > > > > > > > > >> client-side logic could be structured better and that > > you > > > > are > > > > > > not > > > > > > > > > using > > > > > > > > > > the > > > > > > > > > > >> high number of consumers to achieve high throughput. > > > > > > > > > > >> 250 can still be considered of a high upper bound, I > > > believe > > > > > in > > > > > > > > > practice > > > > > > > > > > >> users should aim to not go over 100 consumers per > > consumer > > > > > > group. > > > > > > > > > > >> > > > > > > > > > > >> In regards to the cap being global/per-broker, I think > > > that > > > > we > > > > > > > > should > > > > > > > > > > >> consider whether we want it to be global or > *per-topic*. > > > For > > > > > the > > > > > > > > time > > > > > > > > > > >> being, I believe that having it per-topic with a > global > > > > > default > > > > > > > > might > > > > > > > > > be > > > > > > > > > > >> the best situation. Having it global only seems a bit > > > > > > restricting > > > > > > > to > > > > > > > > > me > > > > > > > > > > and > > > > > > > > > > >> it never hurts to support more fine-grained > > > configurability > > > > > > (given > > > > > > > > > it's > > > > > > > > > > the > > > > > > > > > > >> same config, not a new one being introduced). > > > > > > > > > > >> > > > > > > > > > > >> On Tue, Nov 20, 2018 at 11:32 AM Boyang Chen < > > > > > > bche...@outlook.com > > > > > > > > > > > > > > > > > > wrote: > > > > > > > > > > >> > > > > > > > > > > >>> Thanks Matt for the suggestion! I'm still open to any > > > > > > suggestion > > > > > > > to > > > > > > > > > > >>> change the default value. Meanwhile I just want to > > point > > > > out > > > > > > that > > > > > > > > > this > > > > > > > > > > >>> value is a just last line of defense, not a real > > scenario > > > > we > > > > > > > would > > > > > > > > > > expect. > > > > > > > > > > >>> > > > > > > > > > > >>> > > > > > > > > > > >>> In the meanwhile, I discussed with Stanislav and he > > would > > > > be > > > > > > > > driving > > > > > > > > > > the > > > > > > > > > > >>> 389 effort from now on. Stanislav proposed the idea > in > > > the > > > > > > first > > > > > > > > > place > > > > > > > > > > and > > > > > > > > > > >>> had already come up a draft design, while I will keep > > > > > focusing > > > > > > on > > > > > > > > > > KIP-345 > > > > > > > > > > >>> effort to ensure solving the edge case described in > the > > > > JIRA< > > > > > > > > > > >>> > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Fissues.apache.org%2Fjira%2Fbrowse%2FKAFKA-7610&data=02%7C01%7C%7Cb603e099d6c744d8fac708d65ed51d03%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C636800666735874264&sdata=F55UaGVkDXaj4q7v7jUvPL50pD74GE90R7OGX%2FV3f%2Fs%3D&reserved=0 > > > > > > > > > > >. > > > > > > > > > > >>> > > > > > > > > > > >>> > > > > > > > > > > >>> Thank you Stanislav for making this happen! > > > > > > > > > > >>> > > > > > > > > > > >>> > > > > > > > > > > >>> Boyang > > > > > > > > > > >>> > > > > > > > > > > >>> ________________________________ > > > > > > > > > > >>> From: Matt Farmer <m...@frmr.me> > > > > > > > > > > >>> Sent: Tuesday, November 20, 2018 10:24 AM > > > > > > > > > > >>> To: dev@kafka.apache.org > > > > > > > > > > >>> Subject: Re: [Discuss] KIP-389: Enforce > group.max.size > > to > > > > cap > > > > > > > > member > > > > > > > > > > >>> metadata growth > > > > > > > > > > >>> > > > > > > > > > > >>> Thanks for the KIP. > > > > > > > > > > >>> > > > > > > > > > > >>> Will this cap be a global cap across the entire > cluster > > > or > > > > > per > > > > > > > > > broker? > > > > > > > > > > >>> > > > > > > > > > > >>> Either way the default value seems a bit high to me, > > but > > > > that > > > > > > > could > > > > > > > > > > just > > > > > > > > > > >>> be > > > > > > > > > > >>> from my own usage patterns. I'd have probably started > > > with > > > > > 500 > > > > > > or > > > > > > > > 1k > > > > > > > > > > but > > > > > > > > > > >>> could be easily convinced that's wrong. > > > > > > > > > > >>> > > > > > > > > > > >>> Thanks, > > > > > > > > > > >>> Matt > > > > > > > > > > >>> > > > > > > > > > > >>> On Mon, Nov 19, 2018 at 8:51 PM Boyang Chen < > > > > > > bche...@outlook.com > > > > > > > > > > > > > > > > > > wrote: > > > > > > > > > > >>> > > > > > > > > > > >>> > Hey folks, > > > > > > > > > > >>> > > > > > > > > > > > >>> > > > > > > > > > > > >>> > I would like to start a discussion on KIP-389: > > > > > > > > > > >>> > > > > > > > > > > > >>> > > > > > > > > > > > >>> > > > > > > > > > > > >>> > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Fcwiki.apache.org%2Fconfluence%2Fdisplay%2FKAFKA%2FKIP-389%253A%2BEnforce%2Bgroup.max.size%2Bto%2Bcap%2Bmember%2Bmetadata%2Bgrowth&data=02%7C01%7C%7Cb603e099d6c744d8fac708d65ed51d03%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C636800666735874264&sdata=n%2FHp2DM4k48Q9hayOlc8q5VlcBKFtVWnLDOAzm%2FZ25Y%3D&reserved=0 > > > > > > > > > > >>> > > > > > > > > > > > >>> > > > > > > > > > > > >>> > This is a pretty simple change to cap the consumer > > > group > > > > > size > > > > > > > for > > > > > > > > > > >>> broker > > > > > > > > > > >>> > stability. Give me your valuable feedback when you > > got > > > > > time. > > > > > > > > > > >>> > > > > > > > > > > > >>> > > > > > > > > > > > >>> > Thank you! > > > > > > > > > > >>> > > > > > > > > > > > >>> > > > > > > > > > > >> > > > > > > > > > > >> > > > > > > > > > > >> -- > > > > > > > > > > >> Best, > > > > > > > > > > >> Stanislav > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > -- > > > > > > > > > > > Best, > > > > > > > > > > > Stanislav > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > -- > > > > > > > > > > Best, > > > > > > > > > > Stanislav > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > -- > > > > > > > > Best, > > > > > > > > Stanislav > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > -- > > > > > > Best, > > > > > > Stanislav > > > > > > > > > > > > > > > > > > > > > > > -- > > > > Best, > > > > Stanislav > > > > > > > > > > > > > -- > > > Best, > > > Stanislav > > > > > > > > > -- > > Best, > > Stanislav > > > -- Best, Stanislav
