> > So if you're just subscribing to one or a couple of topics, why not just > compute the hash by filtering out everything but the topics you are > subscribed to? The problem there is if you ever add/remove subscriptions > and want to support rolling upgrades. If the group was subscribed to topic > A, but later changes require subscribing to A + B, then to achieve a > seamless rolling upgrade would require one (old) consumer to be subscribing > to A and one (new) consumer to be subscribing to A+B. If we computed > metadata hashes based on filtered metadata, those two would disagree and we > could not perform assignment while the upgrade was in progress.
When I suggested including just the metadata hash, I didn't mean for all topics in the cluster but just the topics the consumer wants to subscribe to. I understand that it temporarily halts consumption while the topic subscription changes but the question is whether we want to support consumer groups with members that have different subscription preferences. This came up during various discussions and the general consensus was that we couldn't come up with compelling reasons to allow that. Upgrades are one but then the question is how many upgrades would involve letting some instances subscribe to a different set of topics for a long period of time as against a simple rolling upgrade. I'm questioning the value of allowing the flexibility of variation in consumer group topic subscriptions while trading off simplicity of the consumer metadata format. The latter is important given the variety of non-java consumer clients that need to exist and evolve with this change. Becket, If # of partitions or topic metadata changes, whether the coordinator handles it or the consumer, there will be rebalance attempts until the changes subside. Thanks, Neha On Fri, Aug 14, 2015 at 12:57 PM, Ewen Cheslack-Postava <e...@confluent.io> wrote: > On Fri, Aug 14, 2015 at 10:59 AM, Jiangjie Qin <j...@linkedin.com.invalid> > wrote: > > > Neha and Ewen, > > > > About the metadata change frequency. I guess it really depends on how > > frequent the metadata change might occur. If we run Kafka as a service, I > > can see that happens from time to time. As I can imagine people will > create > > some topic, test and maybe delete the topic in some automated test. If > so, > > the proposed protocol might be a little bit vulnerable. > > > > More specifically the scenario I am thinking is: > > 1. Consumer 0 periodically refresh metadata and detected a metadata > change. > > So it sends a JoinGroupRequest with metadata_hash_0. > > 2. Consumer 1 was notified by controller to start a rebalance, so it > > refreshes its metadata and send a JoingGroupRequest with metadata_hash_1, > > which is different from metadata hash 0. > > 3. Rebalance failed and both consumer refresh there metadata again from > > different brokers. > > 4. Depending on the metadata change frequency(or some admin operation > like > > partition reassigment), they may or may not have the same metadata > > returned, so the restart from 3 again. > > > > I agree that step 4 might not be a big concern if consumers updates > > metadata at almost the same time, but I'm a little bit worried whether > that > > assumption really stands because we do not have control over how frequent > > the metadata can change. > > > > > Is this really that different from what would happen if the coordinator > distributed the metadata to consumers? In that case you would trivially > have everyone in a consistent state, but those metadata changes would still > cause churn and require JoinGroup rounds, during which processing is > stalled for the nodes that are waiting on other members to re-join the > group. > > -Ewen > > > > Thanks, > > > > Jiangjie (Becket) Qin > > > > On Fri, Aug 14, 2015 at 2:03 AM, Ewen Cheslack-Postava < > e...@confluent.io> > > wrote: > > > > > On Thu, Aug 13, 2015 at 11:07 PM, Neha Narkhede <n...@confluent.io> > > wrote: > > > > > > > Becket, > > > > > > > > As you say, the metadata hash addresses the concern you originally > > raised > > > > about large topic subscriptions. Can you please list other problems > you > > > are > > > > raising more clearly? It is more helpful to know problems that the > > > proposal > > > > does not address or addresses poorly. > > > > > > > > Regarding other things you said - > > > > > > > > it is required that each > > > > > consumer refresh their metadata before sending a JoinGroupRequest > > > > > > > > > > > > > This is required for wildcard topic subscriptions anyway. So this > > > proposal > > > > does not introduce a regression. We had agreed earlier that it does > not > > > > make sense for the server to deserialize regular expressions sent by > > the > > > > consumer. > > > > > > > > > > I don't think consumers need to do a metadata refresh before sending a > > > JoinGroupRequest. Metadata changes that affect assignment are rare -- > it > > > requires changing the number of partitions in a topic. But you might > > send a > > > JoinGroupRequest simply because a new member is trying to join the > group. > > > That case is presumably much more common. > > > > > > I think it's actually a good idea to have the first JoinGroup cycle > fail > > in > > > some cases, and has little impact. Lets say the metadata does change > > > because partitions are added. Then we might fail in the first round, > but > > > then all members detect that issue *immediately*, refresh their > metadata, > > > and submit a new join group request. This second cycle does not > require a > > > full heartbeat cycle. It happens much more quickly because everyone > > > detected the inconsistency based on the first JoinGroupResponse. The > > > inconsistency should be resolved very quickly (barring other failures > > like > > > a member leaving mid-rebalance) > > > > > > > > > > > > > > > > > > > > > the metadata might still be inconsistent if there is a topic or > > > partition > > > > > change because the > > > > > UpdateMetadataRequest from controller might be handled at different > > > time. > > > > > > > > > > > > > Topic metadata does not change frequently and even if it did, a > couple > > > > rebalance attempts will be needed whether the coordinator drives the > > > > assignments or the consumer. Because guess how the coordinator knows > > > about > > > > the topic metadata changes -- indirectly through either a zk callback > > or > > > > UpdateMetadataRequest, so it is completely possible the coordinator > > sees > > > > the topic metadata changes in batches, not all at once. > > > > > > > > > > > On Thu, Aug 13, 2015 at 10:50 PM, Neha Narkhede <n...@confluent.io> > > > wrote: > > > > > > > > > Ewen/Jason, > > > > > > > > > > The metadata hash is a clever approach and certainly addresses the > > > > problem > > > > > of large metadata for consumers like mirror maker. Few comments - > > > > > > > > > > > > > > > 1. In the interest of simplifying the format of the consumer's > > > > > metadata - Why not just always include only the topic names in > the > > > > metadata > > > > > followed by the metadata hash? If the metadata hash check > > succeeds, > > > > each > > > > > consumer uses the # of partitions it had fetched. If it fails, a > > > > rebalance > > > > > happens and the metadata is not used anyway. > > > > > > > > > > Doing this requires that every consumer always fetch the full metadata. > > The > > > most common use case is consumers that just want to consume one or a > > couple > > > of topics, in which case grabbing all metadata for the entire cluster > is > > > wasteful. If I subscribe only to topic A, why make all consumers grab > > > metadata for the entire topic (and need to rebalance every time it > > > changes!). Including the # of partitions for each topic lets you avoid > > > having to grab the global set of metadata. > > > > > > So if you're just subscribing to one or a couple of topics, why not > just > > > compute the hash by filtering out everything but the topics you are > > > subscribed to? The problem there is if you ever add/remove > subscriptions > > > and want to support rolling upgrades. If the group was subscribed to > > topic > > > A, but later changes require subscribing to A + B, then to achieve a > > > seamless rolling upgrade would require one (old) consumer to be > > subscribing > > > to A and one (new) consumer to be subscribing to A+B. If we computed > > > metadata hashes based on filtered metadata, those two would disagree > and > > we > > > could not perform assignment while the upgrade was in progress. > > > > > > The solution is to differentiate between the cases when a very small > > amount > > > of the metadata is needed (one or a couple of topic subscriptions; > > > communicate and share this via metadata in the JoinGroup protocol) vs > > when > > > *all* the metadata is needed (regex subscription; verify agreement via > > > hash). > > > > > > > > > > > > > > > > 2. Do you need a topic list and topic regex to be separate? A > > single > > > > > topic or list of topics can be expressed as a regex. > > > > > > > > > > See above note about collecting all metadata when you really only need > it > > > for 1 or 2 topics. There's probably some debate to be had about whether > > > this cost would be too high -- every consumer would need to request the > > > metadata for all topics, and they'd need to request that all every time > > > they might be out of date. > > > > > Are we going to allow consumers in the same group to subscribe to > different > > topic set? If we do, we need to let them refresh metadata for all the > > topics a group is consuming from. If we don't then in the protocol we > only > > need a subscription set hash. > > > > > > > > > > > > > 3. Let's include a version explicitly at the beginning of the > > > > > ProtocolMetadata. The version dictates how to deserialize the > > > > > ProtocolMetadata blob and is consistent with the rest of Kafka. > > > > > > > > > > If I'm understanding correctly, in JoinGroupRequest I would change > > > > > > GroupProtocols => [Protocol ProtocolMetadata] > > > > > > to > > > > > > GroupProtocols => [Protocol ProtocolVersion ProtocolMetadata] > > > > > > We had been talking about just baking the version into the Protocol > > field, > > > but making it separate seems perfectly reasonable to me. Jason, any > issue > > > with splitting the version out into a separate field like this? > > > > > > > > > > > > That can simplify the metadata format to the following: > > > > > > > > > > GroupType => "consumer" > > > > >> > > > > >> Protocol => AssignmentStrategy > > > > >> AssignmentStrategy => String > > > > >> > > > > >> ProtocolMetadata => Version Subscription > AssignmentStrategyMetadata > > > > > > > > > > Version => String > > > > > > > > > > Subscription => TopicRegex MetadataHash > > > > >> TopicRegex => String > > > > >> MetadataHash => String > > > > >> AssignmentStrategyMetadata => bytes > > > > > > > > > > > > > > > > > > On Thu, Aug 13, 2015 at 6:28 PM, Jiangjie Qin > > > <j...@linkedin.com.invalid > > > > > > > > > > wrote: > > > > > > > > > >> Ewen and Jason, > > > > >> > > > > >> Thanks for the reply. Sorry I missed the metadata hash. Yes, that > > is a > > > > >> clever approach and would solve my concern about the data passing > > > > around. > > > > >> I > > > > >> can see both pros and cons from doing this, though. The advantage > is > > > we > > > > >> don't need the topic metadata in JoinGroupResponse anymore. The > > > downside > > > > >> is > > > > >> that now rebalance have extra dependency on the consensus of > > metadata > > > of > > > > >> all consumers, which is obtained separately. So it is required > that > > > each > > > > >> consumer refresh their metadata before sending a JoinGroupRequest, > > > > >> otherwise in some cases (e.g. wildcard consumers) will almost > > > certainly > > > > >> fail for the first rebalance attempt. Even if we do that, since > the > > > > >> consumers are getting metadata from different brokers, the > metadata > > > > might > > > > >> still be inconsistent if there is a topic or partition change > > because > > > > the > > > > >> UpdateMetadataRequest from controller might be handled at > different > > > > time. > > > > >> Just want to make sure we think through the cases so the protocol > > does > > > > not > > > > >> cause us unexpected issues. > > > > >> > > > > >> About the number of consumers, I think with the current liveliness > > > > >> definition, we can tolerate churns by bumping up the session > > timeout. > > > > Also > > > > >> I guess we will see an increasing number of consumers for new > > > consumer, > > > > >> because every the old consumer thread will probably become a > > consumer. > > > > >> > > > > >> It is a valid concern for consumers that have large subscription > > set. > > > > This > > > > >> might not be avoided though for client side assignment approach. > One > > > > >> solution is having topic names associate with a topic ID. And only > > use > > > > >> topic ID in JoinGroupRequest and JoinGroupResponse, There is a > > > > discussion > > > > >> thread about this to solve the topic renaming case but this is a > > > > >> completely > > > > >> different discussion. > > > > >> > > > > >> Thanks, > > > > >> > > > > >> Jiangjie (Becket) Qin > > > > >> > > > > >> > > > > >> > > > > >> On Thu, Aug 13, 2015 at 2:14 PM, Jason Gustafson < > > ja...@confluent.io> > > > > >> wrote: > > > > >> > > > > >> > Thanks Jiangjie, that information helps. I agree the protocol > must > > > > >> consider > > > > >> > scalability. My point was that the synchronization barrier in > the > > > > >> current > > > > >> > protocol already effectively limits the number of consumers > since > > it > > > > >> > provides no way to gracefully handle churn. It wouldn't be worth > > > > >> worrying > > > > >> > about scaling up to 100,000 members, for example, because > there's > > no > > > > way > > > > >> > the group would be stable. So we just need to set some clear > > > > >> expectations > > > > >> > on the size we can scale to, and that can help inform the > > discussion > > > > on > > > > >> the > > > > >> > size of messages in this protocol. > > > > >> > > > > > >> > Ewen and I were discussing this morning along similar lines to > > what > > > > >> you're > > > > >> > suggesting. However, even if the coordinator decides on the > > metadata > > > > for > > > > >> > the group, each member still needs to communicate its > > subscriptions > > > to > > > > >> the > > > > >> > rest of the group. This is nice for the regex case since the > regex > > > is > > > > >> > probably small, but if the members have a large topic list, then > > we > > > > have > > > > >> > the same problem. One thing I was thinking about was whether we > > > really > > > > >> need > > > > >> > to handle different subscriptions for every member. If the > > > coordinator > > > > >> > could guarantee that all members had the same subscription, then > > > there > > > > >> > would be no need for the coordinator to return the subscriptions > > for > > > > >> each > > > > >> > member. However, this would prevent graceful upgrades. We might > be > > > > able > > > > >> to > > > > >> > fix that problem by allowing the consumer to provide two > > > subscriptions > > > > >> to > > > > >> > allowing rolling updates, but that starts to sound pretty nasty. > > > > >> > > > > > >> > -Jason > > > > >> > > > > > >> > On Thu, Aug 13, 2015 at 1:41 PM, Jiangjie Qin > > > > <j...@linkedin.com.invalid > > > > >> > > > > > >> > wrote: > > > > >> > > > > > >> > > Jason, > > > > >> > > > > > > >> > > The protocol has to consider the scalability. The protocol in > > the > > > > wiki > > > > >> > > means the JoinGroupResoponse size would be: > > > > >> > > NumberOfTopics * (AvgTopicNameLength + 4) * > > (NumberOfConsumers)^2 > > > > >> > > > > > > >> > > To give some real number, we have 26-node Mirror Maker > cluster, > > > each > > > > >> > with 4 > > > > >> > > consumers. That is 104 consumers using regex ".*". And most of > > our > > > > >> > clusters > > > > >> > > have around 3000 topics, whose topic name are typically around > > 20 > > > > >> > > characters. > > > > >> > > > > > > >> > > I think the key issue for client side partition assignment > logic > > > is > > > > to > > > > >> > make > > > > >> > > sure 1) all the clients run the same algorithm. 2) all the > > clients > > > > >> make > > > > >> > > decision on the same topic metadata. The second purpose can be > > > done > > > > by > > > > >> > > simply letting coordinator provide the topic metadata and all > > then > > > > >> member > > > > >> > > information as source of truth. Is it necessary to pass topic > > > > >> metadata of > > > > >> > > each consumer around? Can we keep the protocol metadata field > > > > >> completely > > > > >> > > independent of topic metadata? I think In the > JoinGroupResponse, > > > we > > > > >> > should > > > > >> > > have only one copy of topic metadata provided by coordinator > and > > > is > > > > >> > outside > > > > >> > > of protocol metadata. If user decides to put some metadata in > > the > > > > >> > > JoinGroupRequest and let coordinator pass around, they are > > > > responsible > > > > >> > for > > > > >> > > understanding the risk. > > > > >> > > > > > > >> > > Thanks, > > > > >> > > > > > > >> > > Jiangjie (Becket) Qin > > > > >> > > > > > > >> > > > > > > >> > > On Thu, Aug 13, 2015 at 12:41 PM, Jason Gustafson < > > > > ja...@confluent.io > > > > >> > > > > > >> > > wrote: > > > > >> > > > > > > >> > > > Hey Onur and Jiangjie, > > > > >> > > > > > > > >> > > > I've updated that wiki with a proposal to add regex > > > subscriptions > > > > to > > > > >> > the > > > > >> > > > consumer metadata. Can you have a look to see if it > addresses > > > your > > > > >> > > concern? > > > > >> > > > In general, I think we should be a little careful when we > are > > > > >> talking > > > > >> > > about > > > > >> > > > the scalability of the protocol. Regardless of whether > > > assignment > > > > is > > > > >> > done > > > > >> > > > on the server or the client, the protocol assumes a > relatively > > > > >> stable > > > > >> > > > configuration. When the number of consumers increases > beyond a > > > > >> certain > > > > >> > > > limit, then membership churn becomes a major concern. > > Similarly > > > > >> there > > > > >> > is > > > > >> > > a > > > > >> > > > notion of metadata churn when topics are added, deleted, or > > > > >> resized. If > > > > >> > > > either membership or metadata changes, then the protocol > > forces > > > > all > > > > >> > > > consumers to stop consumption and rejoin the group. If this > > > > happens > > > > >> > often > > > > >> > > > enough, then it can severely impact the ability of the > > consumer > > > to > > > > >> make > > > > >> > > > progress. The point is that the protocol may already be > > unsuited > > > > to > > > > >> > cases > > > > >> > > > where there are either a large number of consumers or > topics. > > I > > > > >> wonder > > > > >> > if > > > > >> > > > you guys can share your thoughts about your scaling > > > expectations? > > > > >> > > > > > > > >> > > > -Jason > > > > >> > > > > > > > >> > > > > > > > >> > > > > > > > >> > > > > > > > >> > > > On Wed, Aug 12, 2015 at 12:28 PM, Jason Gustafson < > > > > >> ja...@confluent.io> > > > > >> > > > wrote: > > > > >> > > > > > > > >> > > > > Hey Jiangjie, > > > > >> > > > > > > > > >> > > > > That's a great point. In the worst case (the mirror maker > > > case I > > > > >> > > guess), > > > > >> > > > > the join group response can be massive. This would be > > > especially > > > > >> > deadly > > > > >> > > > > when there is a lot of churn in the group (e.g. in a > rolling > > > > >> > upgrade). > > > > >> > > > The > > > > >> > > > > current protocol is not great for this case either, but > it's > > > > >> > > > significantly > > > > >> > > > > better. Here are a couple ways to deal with the size: > > > > >> > > > > > > > > >> > > > > 1. First, we could have the coordinator compress the > > > responses. > > > > >> This > > > > >> > > > would > > > > >> > > > > probably be pretty effective if applied across the > metadata > > > from > > > > >> all > > > > >> > > > > members. > > > > >> > > > > > > > > >> > > > > 2. I think the regex case is the main problem. Is that > > right? > > > We > > > > >> > could > > > > >> > > > > extend the metadata to allow the consumer to embed its > regex > > > > >> > > subscription > > > > >> > > > > in the metadata directly (note this might be a good idea > > > > >> regardless > > > > >> > of > > > > >> > > > the > > > > >> > > > > rest of this proposal). To support regex on the consumer, > we > > > > must > > > > >> > fetch > > > > >> > > > > metadata for all topics. Rather than having all regex > > > > subscribers > > > > >> > embed > > > > >> > > > all > > > > >> > > > > of this metadata in their join group requests, they could > > > > instead > > > > >> > > embed a > > > > >> > > > > hash of it. Then after the join group responses are > > received, > > > > they > > > > >> > just > > > > >> > > > > need to check that the hashes are the same. If there is a > > > > mismatch > > > > >> > > (which > > > > >> > > > > should only occur when topics are created, deleted, or > > > resized), > > > > >> then > > > > >> > > the > > > > >> > > > > group members must refetch the metadata and rejoin the > > group. > > > > >> This is > > > > >> > > > also > > > > >> > > > > how the current protocol behaves when there is a change in > > the > > > > >> topic > > > > >> > > > > metadata affecting the group--someone (either the > > coordinator > > > or > > > > >> the > > > > >> > > > > consumer) detects the change and forces the group to > > > rebalance. > > > > >> > > > > > > > > >> > > > > What do you think? > > > > >> > > > > > > > > >> > > > > (Also I think adding groupId/generationId to fetch and > > produce > > > > >> > requests > > > > >> > > > > seems like an interesting line of thought.) > > > > >> > > > > > > > > >> > > > > -Jason > > > > >> > > > > > > > > >> > > > > > > > > >> > > > > > > > > >> > > > > On Wed, Aug 12, 2015 at 10:57 AM, Jiangjie Qin > > > > >> > > <j...@linkedin.com.invalid > > > > >> > > > > > > > > >> > > > > wrote: > > > > >> > > > > > > > > >> > > > >> Hey Ewen, > > > > >> > > > >> > > > > >> > > > >> Onur and I discussed this a little bit more. And we are > > still > > > > >> > worrying > > > > >> > > > >> about passing all the metadata of all consumers around. > > > > >> > > > >> > > > > >> > > > >> Let's say I have a cluster has 10,000 topics, the average > > > topic > > > > >> name > > > > >> > > > >> length > > > > >> > > > >> is 10 bytes. In this case, the opaque metadata will have > > 10 * > > > > >> > 10,000 = > > > > >> > > > >> 100KB for topic name, for each topic, there is a 4-byte > > > integer > > > > >> of > > > > >> > > > number > > > > >> > > > >> of partitions, that's another 40KB. So one global topic > > > > metadata > > > > >> > will > > > > >> > > > have > > > > >> > > > >> 140KB data. If I have 100 consumers who are using > wildcard > > to > > > > >> > consume > > > > >> > > > from > > > > >> > > > >> all the topics. That means the protocol metadata end up > in > > > the > > > > >> > > > >> JoinGroupResponse will be 140KB * 100 = 14MB. And the > > > > >> > > JoinGroupResponse > > > > >> > > > >> will need to be sent to 100 different consumers, that > means > > > > 14MB > > > > >> * > > > > >> > > 100 = > > > > >> > > > >> 1.4GB need to be sent by the consumer coordinator for one > > > > >> rebalance. > > > > >> > > How > > > > >> > > > >> would that work? > > > > >> > > > >> > > > > >> > > > >> Also, having two consumers (old owner and new owner) > > > consuming > > > > >> from > > > > >> > > the > > > > >> > > > >> same partition might also be a problem. e.g. people are > > > > updating > > > > >> > > > database. > > > > >> > > > >> One thing might worth doing is to add GroupId and > > Generation > > > ID > > > > >> to > > > > >> > > > >> ProducerRequest and FetchRequest as well. This will also > > help > > > > >> with > > > > >> > the > > > > >> > > > >> single producer use case. However, this is probably > > > orthogonal > > > > to > > > > >> > this > > > > >> > > > >> thread given the current new consumer also has this > problem > > > > and I > > > > >> > > > believe > > > > >> > > > >> we need to fix it. > > > > >> > > > >> > > > > >> > > > >> Thanks, > > > > >> > > > >> > > > > >> > > > >> Jiangjie (Becket) Qin > > > > >> > > > >> > > > > >> > > > >> On Tue, Aug 11, 2015 at 11:43 PM, Ewen Cheslack-Postava < > > > > >> > > > >> e...@confluent.io> > > > > >> > > > >> wrote: > > > > >> > > > >> > > > > >> > > > >> > On Tue, Aug 11, 2015 at 11:29 PM, Jiangjie Qin > > > > >> > > > >> <j...@linkedin.com.invalid> > > > > >> > > > >> > wrote: > > > > >> > > > >> > > > > > >> > > > >> > > Ewen, > > > > >> > > > >> > > > > > > >> > > > >> > > Thanks for the explanation. > > > > >> > > > >> > > > > > > >> > > > >> > > For (1), I am more concerned about the failure case > > > instead > > > > >> of > > > > >> > > > normal > > > > >> > > > >> > case. > > > > >> > > > >> > > What if a consumer somehow was kick out of a group > but > > is > > > > >> still > > > > >> > > > >> consuming > > > > >> > > > >> > > and committing offsets? Does that mean the new owner > > and > > > > old > > > > >> > owner > > > > >> > > > >> might > > > > >> > > > >> > > potentially consuming from and committing offsets for > > the > > > > >> same > > > > >> > > > >> partition? > > > > >> > > > >> > > In the old consumer, this won't happen because the > new > > > > >> consumer > > > > >> > > will > > > > >> > > > >> not > > > > >> > > > >> > be > > > > >> > > > >> > > able to start consumption unless the previous owner > has > > > > >> released > > > > >> > > its > > > > >> > > > >> > > ownership. Basically, without the ownership > guarantee, > > I > > > > >> don't > > > > >> > see > > > > >> > > > how > > > > >> > > > >> > the > > > > >> > > > >> > > communication among consumers themselves alone can > > solve > > > > the > > > > >> > > problem > > > > >> > > > >> > here. > > > > >> > > > >> > > > > > > >> > > > >> > > > > > >> > > > >> > The generation ID check still applies to offset > commits. > > If > > > > >> one of > > > > >> > > the > > > > >> > > > >> > consumers is kicked out and misbehaving, it can > obviously > > > > still > > > > >> > > fetch > > > > >> > > > >> and > > > > >> > > > >> > process messages, but offset commits will not work > since > > it > > > > >> will > > > > >> > not > > > > >> > > > >> have > > > > >> > > > >> > the current generation ID. > > > > >> > > > >> > > > > > >> > > > >> > > > > > >> > > > >> > > > > > > >> > > > >> > > For (2) and (3), now I understand how metadata are > > used. > > > > But > > > > >> I > > > > >> > > still > > > > >> > > > >> > don't > > > > >> > > > >> > > see why should we let the consumers to pass the topic > > > > >> > information > > > > >> > > > >> across > > > > >> > > > >> > > instead of letting coordinator give the information. > > The > > > > >> single > > > > >> > > > >> producer > > > > >> > > > >> > > use case does not solve the ownership problem in > > abnormal > > > > >> case > > > > >> > > > either, > > > > >> > > > >> > > which seems to be a little bit vulnerable. > > > > >> > > > >> > > > > > > >> > > > >> > > > > > >> > > > >> > One of the goals here was to generalize group > membership > > so > > > > we > > > > >> > can, > > > > >> > > > for > > > > >> > > > >> > example, use it for balancing Copycat tasks across > > workers. > > > > >> > There's > > > > >> > > no > > > > >> > > > >> > topic subscription info in that case. The metadata for > > > > copycat > > > > >> > > workers > > > > >> > > > >> > would instead need to somehow indicate the current set > of > > > > tasks > > > > >> > that > > > > >> > > > >> need > > > > >> > > > >> > to be assigned to workers. By making the metadata > > > completely > > > > >> > opaque > > > > >> > > to > > > > >> > > > >> the > > > > >> > > > >> > protocol, it becomes more generally useful since it > > focuses > > > > >> > squarely > > > > >> > > > on > > > > >> > > > >> the > > > > >> > > > >> > group membership problem, allowing for that additional > > bit > > > of > > > > >> > > metadata > > > > >> > > > >> so > > > > >> > > > >> > you don't just get a list of members, but also get a > > little > > > > >> bit of > > > > >> > > > info > > > > >> > > > >> > about each of them. > > > > >> > > > >> > > > > > >> > > > >> > A different option that we explored is to use a sort of > > > mixed > > > > >> > model > > > > >> > > -- > > > > >> > > > >> > still bake all the topic subscriptions directly into > the > > > > >> protocol > > > > >> > > but > > > > >> > > > >> also > > > > >> > > > >> > include metadata. That would allow us to maintain the > > > > existing > > > > >> > > > >> > coordinator-driven approach to handling the metadata > and > > > > change > > > > >> > > events > > > > >> > > > >> like > > > > >> > > > >> > the ones Onur pointed out. Then something like the > > Copycat > > > > >> workers > > > > >> > > > would > > > > >> > > > >> > just not fill in any topic subscriptions and it would > be > > > > >> handled > > > > >> > as > > > > >> > > a > > > > >> > > > >> > degenerate case. Based on the way I explained that we > can > > > > >> handle > > > > >> > > those > > > > >> > > > >> > types of events, I personally feel its cleaner and a > > nicer > > > > >> > > > >> generalization > > > > >> > > > >> > to not include the subscriptions in the join group > > > protocol, > > > > >> > making > > > > >> > > it > > > > >> > > > >> part > > > > >> > > > >> > of the metadata instead. > > > > >> > > > >> > > > > > >> > > > >> > For the single producer case, are you saying it doesn't > > > solve > > > > >> > > > ownership > > > > >> > > > >> in > > > > >> > > > >> > the abnormal case because a producer that doesn't know > it > > > has > > > > >> been > > > > >> > > > >> kicked > > > > >> > > > >> > out of the group yet can still produce data even though > > it > > > > >> > shouldn't > > > > >> > > > be > > > > >> > > > >> > able to anymore? I definitely agree that that is a risk > > -- > > > > this > > > > >> > > > >> provides a > > > > >> > > > >> > way to get closer to a true single-writer, but there > are > > > > >> > definitely > > > > >> > > > >> still > > > > >> > > > >> > failure modes that this does not address. > > > > >> > > > >> > > > > > >> > > > >> > -Ewen > > > > >> > > > >> > > > > > >> > > > >> > > > > > >> > > > >> > > > > > > >> > > > >> > > Thanks, > > > > >> > > > >> > > > > > > >> > > > >> > > Jiangjie (Becket) Qin > > > > >> > > > >> > > > > > > >> > > > >> > > > > > > >> > > > >> > > On Tue, Aug 11, 2015 at 11:06 PM, Ewen > > Cheslack-Postava < > > > > >> > > > >> > e...@confluent.io > > > > >> > > > >> > > > > > > > >> > > > >> > > wrote: > > > > >> > > > >> > > > > > > >> > > > >> > > > On Tue, Aug 11, 2015 at 10:15 PM, Jiangjie Qin > > > > >> > > > >> > <j...@linkedin.com.invalid > > > > >> > > > >> > > > > > > > >> > > > >> > > > wrote: > > > > >> > > > >> > > > > > > > >> > > > >> > > > > Hi Jason, > > > > >> > > > >> > > > > > > > > >> > > > >> > > > > Thanks for writing this up. It would be useful to > > > > >> generalize > > > > >> > > the > > > > >> > > > >> > group > > > > >> > > > >> > > > > concept. I have a few questions below. > > > > >> > > > >> > > > > > > > > >> > > > >> > > > > 1. In old consumer actually the partition > > assignment > > > > are > > > > >> > done > > > > >> > > by > > > > >> > > > >> > > > consumers > > > > >> > > > >> > > > > themselves. We used zookeeper to guarantee that a > > > > >> partition > > > > >> > > will > > > > >> > > > >> only > > > > >> > > > >> > > be > > > > >> > > > >> > > > > consumed by one consumer thread who successfully > > > > claimed > > > > >> its > > > > >> > > > >> > ownership. > > > > >> > > > >> > > > > Does the new protocol plan to provide the same > > > > guarantee? > > > > >> > > > >> > > > > > > > > >> > > > >> > > > > > > > >> > > > >> > > > Once you have all the metadata from all the > > consumers, > > > > >> > > assignment > > > > >> > > > >> > should > > > > >> > > > >> > > > just be a simple function mapping that > > Map<ConsumerId, > > > > >> > Metadata> > > > > >> > > > to > > > > >> > > > >> > > > Map<ConsumerId, List<TopicPartition>>. If everyone > is > > > > >> > consistent > > > > >> > > > in > > > > >> > > > >> > > > computing that, you don't need ZK involved at all. > > > > >> > > > >> > > > > > > > >> > > > >> > > > In practice, this shouldn't be that hard to ensure > > for > > > > most > > > > >> > > > >> assignment > > > > >> > > > >> > > > strategies just by having decent unit testing on > > them. > > > > You > > > > >> > just > > > > >> > > > >> have to > > > > >> > > > >> > > do > > > > >> > > > >> > > > things like ensure your assignment strategy sorts > > lists > > > > >> into a > > > > >> > > > >> > consistent > > > > >> > > > >> > > > order. > > > > >> > > > >> > > > > > > > >> > > > >> > > > You do give up the ability to use some techniques > > (e.g. > > > > any > > > > >> > > > >> randomized > > > > >> > > > >> > > > algorithm if you can't distribute the seed w/ the > > > > metadata) > > > > >> > and > > > > >> > > > it's > > > > >> > > > >> > true > > > > >> > > > >> > > > that nothing validates the assignment, but if that > > > > >> assignment > > > > >> > > > >> algorithm > > > > >> > > > >> > > > step is kept simple, small, and well tested, the > risk > > > is > > > > >> very > > > > >> > > > >> minimal. > > > > >> > > > >> > > > > > > > >> > > > >> > > > > > > > >> > > > >> > > > > > > > > >> > > > >> > > > > 2. It looks that both JoinGroupRequest and > > > > >> JoinGroupResponse > > > > >> > > has > > > > >> > > > >> the > > > > >> > > > >> > > > > ProtocolMetadata.AssignmentStrategyMetadata, what > > > would > > > > >> be > > > > >> > the > > > > >> > > > >> > metadata > > > > >> > > > >> > > > be > > > > >> > > > >> > > > > sent and returned by coordinator? How will the > > > > >> coordinator > > > > >> > > > handle > > > > >> > > > >> the > > > > >> > > > >> > > > > metadata? > > > > >> > > > >> > > > > > > > > >> > > > >> > > > > > > > >> > > > >> > > > The coordinator is basically just blindly > > broadcasting > > > > all > > > > >> of > > > > >> > it > > > > >> > > > to > > > > >> > > > >> > group > > > > >> > > > >> > > > members so they have a consistent view. > > > > >> > > > >> > > > > > > > >> > > > >> > > > So from the coordinators perspective, it sees > > something > > > > >> like: > > > > >> > > > >> > > > > > > > >> > > > >> > > > Consumer 1 -> JoinGroupRequest with GroupProtocols > = > > [ > > > > >> > > "consumer" > > > > >> > > > >> > > > <Consumer1 opaque byte[]>] > > > > >> > > > >> > > > Consumer 2 -> JoinGroupRequest with GroupProtocols > = > > [ > > > > >> > > "consumer" > > > > >> > > > >> > > > <Consumer2 opaque byte[]>] > > > > >> > > > >> > > > > > > > >> > > > >> > > > Then, in the responses would look like: > > > > >> > > > >> > > > > > > > >> > > > >> > > > Consumer 1 <- JoinGroupResponse with GroupProtocol > = > > > > >> > "consumer" > > > > >> > > > and > > > > >> > > > >> > > > GroupMembers = [ Consumer 1 <Consumer1 opaque > > byte[]>, > > > > >> > Consumer > > > > >> > > 2 > > > > >> > > > >> > > > <Consumer2 opaque byte[]>] > > > > >> > > > >> > > > Consumer 2 <- JoinGroupResponse with GroupProtocol > = > > > > >> > "consumer" > > > > >> > > > and > > > > >> > > > >> > > > GroupMembers = [ Consumer 1 <Consumer1 opaque > > byte[]>, > > > > >> > Consumer > > > > >> > > 2 > > > > >> > > > >> > > > <Consumer2 opaque byte[]>] > > > > >> > > > >> > > > > > > > >> > > > >> > > > So all the responses include all the metadata for > > every > > > > >> member > > > > >> > > in > > > > >> > > > >> the > > > > >> > > > >> > > > group, and everyone can use that to consistently > > decide > > > > on > > > > >> > > > >> assignment. > > > > >> > > > >> > > The > > > > >> > > > >> > > > broker doesn't care and cannot even understand the > > > > metadata > > > > >> > > since > > > > >> > > > >> the > > > > >> > > > >> > > data > > > > >> > > > >> > > > format for it is dependent on the assignment > strategy > > > > being > > > > >> > > used. > > > > >> > > > >> > > > > > > > >> > > > >> > > > As another example that is *not* a consumer, let's > > say > > > > you > > > > >> > just > > > > >> > > > >> want to > > > > >> > > > >> > > > have a single writer in the group which everyone > will > > > > >> forward > > > > >> > > > >> requests > > > > >> > > > >> > > to. > > > > >> > > > >> > > > To accomplish this, you could use a very dumb > > > assignment > > > > >> > > strategy: > > > > >> > > > >> > there > > > > >> > > > >> > > is > > > > >> > > > >> > > > no metadata (empty byte[]) and all we care about is > > who > > > > is > > > > >> the > > > > >> > > > first > > > > >> > > > >> > > member > > > > >> > > > >> > > > in the group (e.g. when IDs are sorted > > > > lexicographically). > > > > >> > That > > > > >> > > > >> member > > > > >> > > > >> > is > > > > >> > > > >> > > > selected as the writer. In that case, we actually > > just > > > > care > > > > >> > > about > > > > >> > > > >> the > > > > >> > > > >> > > > membership list, there's no additional info about > > each > > > > >> member > > > > >> > > that > > > > >> > > > >> is > > > > >> > > > >> > > > required to determine who is the writer. > > > > >> > > > >> > > > > > > > >> > > > >> > > > > > > > >> > > > >> > > > > 3. Do you mean that the number of partitions in > > > > >> > > > JoinGroupResponse > > > > >> > > > >> > will > > > > >> > > > >> > > be > > > > >> > > > >> > > > > the max partition number of a topic among all the > > > > >> reported > > > > >> > > > >> partition > > > > >> > > > >> > > > number > > > > >> > > > >> > > > > by consumers? Is there any reason not just let > > > > >> Coordinator > > > > >> > to > > > > >> > > > >> return > > > > >> > > > >> > > the > > > > >> > > > >> > > > > number of partitions of a topic in its metadata > > > cache? > > > > >> > > > >> > > > > > > > > >> > > > >> > > > > > > > >> > > > >> > > > Nothing from the embedded protocol is touched by > the > > > > >> broker. > > > > >> > The > > > > >> > > > >> broker > > > > >> > > > >> > > > just collects opaque bytes of metadata, does the > > > > selection > > > > >> of > > > > >> > > the > > > > >> > > > >> > > strategy > > > > >> > > > >> > > > if multiple are supported by some consumers, and > then > > > > >> returns > > > > >> > > that > > > > >> > > > >> > opaque > > > > >> > > > >> > > > metadata for all the members back to every member. > In > > > > that > > > > >> way > > > > >> > > > they > > > > >> > > > >> all > > > > >> > > > >> > > > have a consistent view of the group. For regular > > > > consumers, > > > > >> > that > > > > >> > > > >> view > > > > >> > > > >> > of > > > > >> > > > >> > > > the group includes information about how many > > > partitions > > > > >> each > > > > >> > > > >> consumer > > > > >> > > > >> > > > currently thinks the topics it is subscribed to > has. > > > > These > > > > >> > could > > > > >> > > > be > > > > >> > > > >> > > > inconsistent due to out of date metadata and it > would > > > be > > > > >> up to > > > > >> > > the > > > > >> > > > >> > > > assignment strategy on the *client* to resolve > that. > > As > > > > you > > > > >> > > point > > > > >> > > > >> out, > > > > >> > > > >> > in > > > > >> > > > >> > > > that case they could just take the max value that > any > > > > >> consumer > > > > >> > > > >> reported > > > > >> > > > >> > > > seeing and use that. The consumers that notice that > > > their > > > > >> > > metadata > > > > >> > > > >> had > > > > >> > > > >> > a > > > > >> > > > >> > > > smaller # of partitions should also trigger a > > metadata > > > > >> update > > > > >> > > when > > > > >> > > > >> they > > > > >> > > > >> > > see > > > > >> > > > >> > > > someone else observing a larger # of partitions. > > > > >> > > > >> > > > > > > > >> > > > >> > > > > > > > >> > > > >> > > > > > > > > >> > > > >> > > > > Thanks, > > > > >> > > > >> > > > > > > > > >> > > > >> > > > > Jiangjie (Becket) Qin > > > > >> > > > >> > > > > > > > > >> > > > >> > > > > > > > > >> > > > >> > > > > > > > > >> > > > >> > > > > > > > > >> > > > >> > > > > On Tue, Aug 11, 2015 at 1:19 PM, Jason Gustafson > < > > > > >> > > > >> ja...@confluent.io > > > > >> > > > >> > > > > > > >> > > > >> > > > > wrote: > > > > >> > > > >> > > > > > > > > >> > > > >> > > > > > Hi Kafka Devs, > > > > >> > > > >> > > > > > > > > > >> > > > >> > > > > > One of the nagging issues in the current design > > of > > > > the > > > > >> new > > > > >> > > > >> consumer > > > > >> > > > >> > > has > > > > >> > > > >> > > > > > been the need to support a variety of > assignment > > > > >> > strategies. > > > > >> > > > >> We've > > > > >> > > > >> > > > > > encountered this in particular in the design of > > > > copycat > > > > >> > and > > > > >> > > > the > > > > >> > > > >> > > > > processing > > > > >> > > > >> > > > > > framework (KIP-28). From what I understand, > Samza > > > > also > > > > >> > has a > > > > >> > > > >> number > > > > >> > > > >> > > of > > > > >> > > > >> > > > > use > > > > >> > > > >> > > > > > cases with custom assignment needs. The new > > > consumer > > > > >> > > protocol > > > > >> > > > >> > > supports > > > > >> > > > >> > > > > new > > > > >> > > > >> > > > > > assignment strategies by hooking them into the > > > > broker. > > > > >> For > > > > >> > > > many > > > > >> > > > >> > > > > > environments, this is a major pain and in some > > > > cases, a > > > > >> > > > >> > non-starter. > > > > >> > > > >> > > It > > > > >> > > > >> > > > > > also challenges the validation that the > > coordinator > > > > can > > > > >> > > > provide. > > > > >> > > > >> > For > > > > >> > > > >> > > > > > example, some assignment strategies call for > > > > >> partitions to > > > > >> > > be > > > > >> > > > >> > > assigned > > > > >> > > > >> > > > > > multiple times, which means that the > coordinator > > > can > > > > >> only > > > > >> > > > check > > > > >> > > > >> > that > > > > >> > > > >> > > > > > partitions have been assigned at least once. > > > > >> > > > >> > > > > > > > > > >> > > > >> > > > > > To solve these issues, we'd like to propose > > moving > > > > >> > > assignment > > > > >> > > > to > > > > >> > > > >> > the > > > > >> > > > >> > > > > > client. I've written a wiki which outlines some > > > > >> protocol > > > > >> > > > >> changes to > > > > >> > > > >> > > > > achieve > > > > >> > > > >> > > > > > this: > > > > >> > > > >> > > > > > > > > > >> > > > >> > > > > > > > > > >> > > > >> > > > > > > > > >> > > > >> > > > > > > > >> > > > >> > > > > > > >> > > > >> > > > > > >> > > > >> > > > > >> > > > > > > > >> > > > > > > >> > > > > > >> > > > > > > > > > > https://cwiki.apache.org/confluence/display/KAFKA/Kafka+Client-side+Assignment+Proposal > > > > >> > > > >> > > > > > . > > > > >> > > > >> > > > > > To summarize briefly, instead of the > coordinator > > > > >> assigning > > > > >> > > the > > > > >> > > > >> > > > partitions > > > > >> > > > >> > > > > > itself, all subscriptions are forwarded to each > > > > member > > > > >> of > > > > >> > > the > > > > >> > > > >> group > > > > >> > > > >> > > > which > > > > >> > > > >> > > > > > then decides independently which partitions it > > > should > > > > >> > > consume. > > > > >> > > > >> The > > > > >> > > > >> > > > > protocol > > > > >> > > > >> > > > > > provides a mechanism for the coordinator to > > > validate > > > > >> that > > > > >> > > all > > > > >> > > > >> > > consumers > > > > >> > > > >> > > > > use > > > > >> > > > >> > > > > > the same assignment strategy, but it does not > > > ensure > > > > >> that > > > > >> > > the > > > > >> > > > >> > > resulting > > > > >> > > > >> > > > > > assignment is "correct." This provides a > powerful > > > > >> > capability > > > > >> > > > for > > > > >> > > > >> > > users > > > > >> > > > >> > > > to > > > > >> > > > >> > > > > > control the full data flow on the client side. > > They > > > > >> > control > > > > >> > > > how > > > > >> > > > >> > data > > > > >> > > > >> > > is > > > > >> > > > >> > > > > > written to partitions through the Partitioner > > > > interface > > > > >> > and > > > > >> > > > they > > > > >> > > > >> > > > control > > > > >> > > > >> > > > > > how data is consumed through the assignment > > > strategy, > > > > >> all > > > > >> > > > >> without > > > > >> > > > >> > > > > touching > > > > >> > > > >> > > > > > the server. > > > > >> > > > >> > > > > > > > > > >> > > > >> > > > > > Of course nothing comes for free. In > particular, > > > this > > > > >> > change > > > > >> > > > >> > removes > > > > >> > > > >> > > > the > > > > >> > > > >> > > > > > ability of the coordinator to validate that > > commits > > > > are > > > > >> > made > > > > >> > > > by > > > > >> > > > >> > > > consumers > > > > >> > > > >> > > > > > who were assigned the respective partition. > This > > > > might > > > > >> not > > > > >> > > be > > > > >> > > > >> too > > > > >> > > > >> > bad > > > > >> > > > >> > > > > since > > > > >> > > > >> > > > > > we retain the ability to validate the > generation > > > id, > > > > >> but > > > > >> > it > > > > >> > > > is a > > > > >> > > > >> > > > > potential > > > > >> > > > >> > > > > > concern. We have considered alternative > protocols > > > > which > > > > >> > add > > > > >> > > a > > > > >> > > > >> > second > > > > >> > > > >> > > > > > round-trip to the protocol in order to give the > > > > >> > coordinator > > > > >> > > > the > > > > >> > > > >> > > ability > > > > >> > > > >> > > > > to > > > > >> > > > >> > > > > > confirm the assignment. As mentioned above, the > > > > >> > coordinator > > > > >> > > is > > > > >> > > > >> > > somewhat > > > > >> > > > >> > > > > > limited in what it can actually validate, but > > this > > > > >> would > > > > >> > > > return > > > > >> > > > >> its > > > > >> > > > >> > > > > ability > > > > >> > > > >> > > > > > to validate commits. The tradeoff is that it > > > > increases > > > > >> the > > > > >> > > > >> > protocol's > > > > >> > > > >> > > > > > complexity which means more ways for the > protocol > > > to > > > > >> fail > > > > >> > > and > > > > >> > > > >> > > > > consequently > > > > >> > > > >> > > > > > more edge cases in the code. > > > > >> > > > >> > > > > > > > > > >> > > > >> > > > > > It also misses an opportunity to generalize the > > > group > > > > >> > > > membership > > > > >> > > > >> > > > protocol > > > > >> > > > >> > > > > > for additional use cases. In fact, after you've > > > gone > > > > to > > > > >> > the > > > > >> > > > >> trouble > > > > >> > > > >> > > of > > > > >> > > > >> > > > > > moving assignment to the client, the main thing > > > that > > > > is > > > > >> > left > > > > >> > > > in > > > > >> > > > >> > this > > > > >> > > > >> > > > > > protocol is basically a general group > management > > > > >> > capability. > > > > >> > > > >> This > > > > >> > > > >> > is > > > > >> > > > >> > > > > > exactly what is needed for a few cases that are > > > > >> currently > > > > >> > > > under > > > > >> > > > >> > > > > discussion > > > > >> > > > >> > > > > > (e.g. copycat or single-writer producer). We've > > > taken > > > > >> this > > > > >> > > > >> further > > > > >> > > > >> > > step > > > > >> > > > >> > > > > in > > > > >> > > > >> > > > > > the proposal and attempted to envision what > that > > > > >> general > > > > >> > > > >> protocol > > > > >> > > > >> > > might > > > > >> > > > >> > > > > > look like and how it could be used both by the > > > > consumer > > > > >> > and > > > > >> > > > for > > > > >> > > > >> > some > > > > >> > > > >> > > of > > > > >> > > > >> > > > > > these other cases. > > > > >> > > > >> > > > > > > > > > >> > > > >> > > > > > Anyway, since time is running out on the new > > > > consumer, > > > > >> we > > > > >> > > have > > > > >> > > > >> > > perhaps > > > > >> > > > >> > > > > one > > > > >> > > > >> > > > > > last chance to consider a significant change in > > the > > > > >> > protocol > > > > >> > > > >> like > > > > >> > > > >> > > this, > > > > >> > > > >> > > > > so > > > > >> > > > >> > > > > > have a look at the wiki and share your > thoughts. > > > I've > > > > >> no > > > > >> > > doubt > > > > >> > > > >> that > > > > >> > > > >> > > > some > > > > >> > > > >> > > > > > ideas seem clearer in my mind than they do on > > > paper, > > > > so > > > > >> > ask > > > > >> > > > >> > questions > > > > >> > > > >> > > > if > > > > >> > > > >> > > > > > there is any confusion. > > > > >> > > > >> > > > > > > > > > >> > > > >> > > > > > Thanks! > > > > >> > > > >> > > > > > Jason > > > > >> > > > >> > > > > > > > > > >> > > > >> > > > > > > > > >> > > > >> > > > > > > > >> > > > >> > > > > > > > >> > > > >> > > > > > > > >> > > > >> > > > -- > > > > >> > > > >> > > > Thanks, > > > > >> > > > >> > > > Ewen > > > > >> > > > >> > > > > > > > >> > > > >> > > > > > > >> > > > >> > > > > > >> > > > >> > > > > > >> > > > >> > > > > > >> > > > >> > -- > > > > >> > > > >> > Thanks, > > > > >> > > > >> > Ewen > > > > >> > > > >> > > > > > >> > > > >> > > > > >> > > > > > > > > >> > > > > > > > > >> > > > > > > > >> > > > > > > >> > > > > > >> > > > > > > > > > > > > > > > > > > > > -- > > > > > Thanks, > > > > > Neha > > > > > > > > > > > > > > > > > > > > > -- > > > > Thanks, > > > > Neha > > > > > > > > > > > > > > > > -- > > > Thanks, > > > Ewen > > > > > > > > > -- > Thanks, > Ewen > -- Thanks, Neha
