2. The reason we did not expose generation.id from KafkaConsumer public APIs directly is to abstract this notion from users (since it is an implementation detail of the rebalance protocol itself, e.g. if user calls consumer.assign() they do not need to invoke ConsumerCoordinator and no need to be aware of generation.id at all).
On the other hand, with the current proposal the txn.coordiantor did not know about the latest generation from the source-of-truth group.coordinator; instead, it will only bump up the generation from the producer's InitProducerIdRequest only. The key here is that GroupCoordinator, when handling `InitProducerIdRequest 3. I agree that if we rely on the group coordinator to block on returning offset-fetch-response if read-committed is enabled, then we do not need to store partition assignment on txn coordinator and therefore it's better to still decouple them. For that case we still need to update the KIP wiki page that includes: 3.a. Augment OffsetFetchRequest with the ISOLATION_LEVEL as well. 3.b. Add new error code in OffsetFetchResponse to let client backoff and retry if there are pending txns including the interested partitions. 3.c. Also in the worst case we would let the client be blocked for the txn.timeout period, and for that rationale we may need to consider reducing our default txn.timeout value as well. 4. According to Colin it seems we do not need to create another KIP and we can just complete it as part of KIP-117 / KAFKA-5214; and we need to do some cleanup to have BrokerApiVersion exposed from AdminClient (@Colin please let use know if you have any concerns exposing it). Guozhang On Tue, Jun 25, 2019 at 6:43 PM Jason Gustafson <ja...@confluent.io> wrote: > For reference, we have BrokerApiVersionCommand already as a public > interface. We have a bit of tech debt at the moment because it uses a > custom AdminClient. It would be nice to clean that up. In general, I think > it is reasonable to expose from AdminClient. It can be used by management > tools to inspect running Kafka versions for example. > > -Jason > > On Tue, Jun 25, 2019 at 4:37 PM Boyang Chen <reluctanthero...@gmail.com> > wrote: > > > Thank you for the context Colin. The groupId was indeed a copy-paste > error. > > Our use case here for 447 is (Quoted from Guozhang): > > ''' > > I think if we can do something else to > > avoid this config though, for example we can use the embedded AdminClient > > to send the APIVersion request upon starting up, and based on the > returned > > value decides whether to go to the old code path or the new behavior. > > ''' > > The benefit we get is to avoid adding a new configuration to make a > > decision simply base on broker version. If you have concerns with > exposing > > ApiVersion for client, we could > > try to think of alternative solutions too. > > > > Boyang > > > > > > > > On Tue, Jun 25, 2019 at 4:20 PM Colin McCabe <cmcc...@apache.org> wrote: > > > > > kafka.api.ApiVersion is an internal class, not suitable to exposing > > > through AdminClient. That class is not even accessible without having > > the > > > broker jars on your CLASSPATH. > > > > > > Another question is, what is the groupId parameter doing in the call? > > The > > > API versions are the same no matter what consumer group we use, right? > > > Perhaps this was a copy and paste error? > > > > > > This is not the first time we have discussed having a method in > > > AdminClient to retrieve API version information. In fact, the original > > KIP > > > which created KafkaAdminClient specified an API for fetching version > > > information. It was called apiVersions and it is still there on the > > wiki. > > > See > > > > > > https://cwiki.apache.org/confluence/display/KAFKA/KIP-117%3A+Add+a+public+AdminClient+API+for+Kafka+admin+operations > > > > > > However, this API wasn't ready in time for 0.11.0 so we shipped without > > > it. There was a JIRA to implement it for later versions, > > > https://issues.apache.org/jira/browse/KAFKA-5214 , as well as a PR, > > > https://github.com/apache/kafka/pull/3012 . However, we started to > > > rethink whether this AdminClient function was even necessary. Most of > > the > > > use-cases we could think of seemed like horrible hacks. So it has > never > > > really been implemented (yet?). > > > > > > best, > > > Colin > > > > > > > > > On Tue, Jun 25, 2019, at 15:46, Boyang Chen wrote: > > > > Actually, after a second thought, I think it actually makes sense to > > > > support auto upgrade through admin client to help use get api version > > > > from > > > > broker. > > > > A draft KIP is here: > > > > > > > > > > https://cwiki.apache.org/confluence/display/KAFKA/KIP-483%3A++Add+Broker+Version+API+in+Admin+Client > > > > > > > > Boyang > > > > > > > > On Tue, Jun 25, 2019 at 2:57 PM Boyang Chen < > > reluctanthero...@gmail.com> > > > > wrote: > > > > > > > > > Thank you Guozhang, some of my understandings are inline below. > > > > > > > > > > On Tue, Jun 25, 2019 at 11:05 AM Jason Gustafson < > ja...@confluent.io > > > > > > > > wrote: > > > > > > > > > >> > > > > > >> > I think co-locating does have some merits here, i.e. letting the > > > > >> > ConsumerCoordinator which has the source-of-truth of assignment > to > > > act > > > > >> as > > > > >> > the TxnCoordinator as well; but I agree there's also some cons > of > > > > >> coupling > > > > >> > them together. I'm still a bit inclining towards colocation but > if > > > there > > > > >> > are good rationales not to do so I can be convinced as well. > > > > >> > > > > >> > > > > >> The good rationale is that we have no mechanism to colocate > > > partitions ;). > > > > >> Are you suggesting we store the group and transaction state in the > > > same > > > > >> log? Can you be more concrete about the benefit? > > > > >> > > > > >> -Jason > > > > >> > > > > >> On Tue, Jun 25, 2019 at 10:51 AM Guozhang Wang < > wangg...@gmail.com> > > > > >> wrote: > > > > >> > > > > >> > Hi Boyang, > > > > >> > > > > > >> > 1. One advantage of retry against on-hold is that it will not > > > tie-up a > > > > >> > handler thread (of course the latter could do the same but that > > > involves > > > > >> > using a purgatory which is more complicated), and also it is > less > > > > >> likely to > > > > >> > violate request timeout. So I think there are some rationales to > > > prefer > > > > >> > retries. > > > > >> > > > > > >> > > > > > That sounds fair to me, also we are avoiding usage of another > > > purgatory > > > > > instance. Usually for one back-off > > > > > we are only delaying 50ms during startup which is trivial cost. > This > > > > > behavior shouldn't be changed. > > > > > > > > > > > 2. Regarding "ConsumerRebalanceListener": both > > > ConsumerRebalanceListener > > > > >> > and PartitionAssignors are user-customizable modules, and only > > > > >> difference > > > > >> > is that the former is specified via code and the latter is > > > specified via > > > > >> > config. > > > > >> > > > > > >> > Regarding Jason's proposal of ConsumerAssignment, one thing to > > note > > > > >> though > > > > >> > with KIP-429 the onPartitionAssigned may not be called if the > > > assignment > > > > >> > does not change, whereas onAssignment would always be called at > > the > > > end > > > > >> of > > > > >> > sync-group response. My proposed semantics is that > > > > >> > `RebalanceListener#onPartitionsXXX` are used for notifications > to > > > user, > > > > >> and > > > > >> > hence if there's no changes these will not be called, whereas > > > > >> > `PartitionAssignor` is used for assignor logic, whose callback > > would > > > > >> always > > > > >> > be called no matter if the partitions have changed or not. > > > > >> > > > > >> I think a third option is to gracefully expose generation id as > part > > > of > > > > > consumer API, so that we don't need to > > > > > bother overloading various callbacks. Of course, this builds upon > the > > > > > assumption that topic partitions > > > > > will not be included in new initTransaction API. > > > > > > > > > > > 3. I feel it is a bit awkward to let the TxnCoordinator keeping > > > partition > > > > >> > assignments since it is sort of taking over the job of the > > > > >> > ConsumerCoordinator, and may likely cause a split-brain problem > as > > > two > > > > >> > coordinators keep a copy of this assignment which may be > > different. > > > > >> > > > > > >> > I think co-locating does have some merits here, i.e. letting the > > > > >> > ConsumerCoordinator which has the source-of-truth of assignment > to > > > act > > > > >> as > > > > >> > the TxnCoordinator as well; but I agree there's also some cons > of > > > > >> coupling > > > > >> > them together. I'm still a bit inclining towards colocation but > if > > > there > > > > >> > are good rationales not to do so I can be convinced as well. > > > > >> > > > > > >> > > > > > The purpose of co-location is to let txn coordinator see the group > > > > > assignment. This priority is weakened > > > > > when we already have defense on the consumer offset fetch, so I > guess > > > it's > > > > > not super important anymore. > > > > > > > > > > > > > > >> > 4. I guess I'm preferring the philosophy of "only add configs if > > > > >> there's no > > > > >> > other ways", since more and more configs would make it less and > > less > > > > >> > intuitive out of the box to use. > > > > >> > > > > > >> > I think it's a valid point that checks upon starting up does not > > > cope > > > > >> with > > > > >> > brokers downgrading but even with a config, but it is still hard > > for > > > > >> users > > > > >> > to determine when they can be ensured the broker would never > > > downgrade > > > > >> > anymore and hence can safely switch the config. So my feeling is > > > that > > > > >> this > > > > >> > config would not be helping too much still. If we want to be at > > the > > > > >> safer > > > > >> > side, then I'd suggest we modify the Coordinator -> > NetworkClient > > > > >> hierarchy > > > > >> > to allow the NetworkClient being able to pass the APIVersion > > > metadata to > > > > >> > Coordinator, so that Coordinator can rely on that logic to > change > > > its > > > > >> > behavior dynamically. > > > > >> > > > > > The stream thread init could not be supported by a client > coordinator > > > > > behavior change on the fly, > > > > > we are only losing possibilities after we initialized. (main thread > > > gets > > > > > exit and no thread has global picture anymore) > > > > > If we do want to support auto version detection, admin client > request > > > in > > > > > this sense shall be easier. > > > > > > > > > > > > > > >> > > > > > >> > 5. I do not have a concrete idea about how the impact on Connect > > > would > > > > >> > make, maybe Randall or Konstantine can help here? > > > > >> > > > > > >> > > > > > Sounds good, let's see their thoughts. > > > > > > > > > > > > > > >> > Guozhang > > > > >> > > > > > >> > On Mon, Jun 24, 2019 at 10:26 PM Boyang Chen < > > > > >> reluctanthero...@gmail.com> > > > > >> > wrote: > > > > >> > > > > > >> > > Hey Jason, > > > > >> > > > > > > >> > > thank you for the proposal here. Some of my thoughts below. > > > > >> > > > > > > >> > > On Mon, Jun 24, 2019 at 8:58 PM Jason Gustafson < > > > ja...@confluent.io> > > > > >> > > wrote: > > > > >> > > > > > > >> > > > Hi Boyang, > > > > >> > > > > > > > >> > > > Thanks for picking this up! Still reading through the > updates, > > > but > > > > >> here > > > > >> > > are > > > > >> > > > a couple initial comments on the APIs: > > > > >> > > > > > > > >> > > > 1. The `TxnProducerIdentity` class is a bit awkward. I think > > we > > > are > > > > >> > > trying > > > > >> > > > to encapsulate state from the current group assignment. > Maybe > > > > >> something > > > > >> > > > like `ConsumerAssignment` would be clearer? If we make the > > usage > > > > >> > > consistent > > > > >> > > > across the consumer and producer, then we can avoid exposing > > > > >> internal > > > > >> > > state > > > > >> > > > like the generationId. > > > > >> > > > > > > > >> > > > For example: > > > > >> > > > > > > > >> > > > // Public API > > > > >> > > > interface ConsumerAssignment { > > > > >> > > > Set<TopicPartition> partittions(); > > > > >> > > > } > > > > >> > > > > > > > >> > > > // Not a public API > > > > >> > > > class InternalConsumerAssignment implements > > ConsumerAssignment { > > > > >> > > > Set<TopicPartition> partittions; > > > > >> > > > int generationId; > > > > >> > > > } > > > > >> > > > > > > > >> > > > Then we can change the rebalance listener to something like > > > this: > > > > >> > > > onPartitionsAssigned(ConsumerAssignment assignment) > > > > >> > > > > > > > >> > > > And on the producer: > > > > >> > > > void initTransactions(String groupId, ConsumerAssignment > > > > >> assignment); > > > > >> > > > > > > > >> > > > 2. Another bit of awkwardness is the fact that we have to > pass > > > the > > > > >> > > groupId > > > > >> > > > through both initTransactions() and > > sendOffsetsToTransaction(). > > > We > > > > >> > could > > > > >> > > > consider a config instead. Maybe something like ` > > > > >> > transactional.group.id > > > > >> > > `? > > > > >> > > > Then we could simplify the producer APIs, potentially even > > > > >> deprecating > > > > >> > > the > > > > >> > > > current sendOffsetsToTransaction. In fact, for this new > usage, > > > the ` > > > > >> > > > transational.id` config is not needed. It would be nice if > we > > > don't > > > > >> > have > > > > >> > > > to > > > > >> > > > provide it. > > > > >> > > > > > > > >> > > > I like the idea of combining 1 and 2. We could definitely > pass > > > in a > > > > >> > > group.id config > > > > >> > > so that we could avoid exposing that information in a public > > API. > > > The > > > > >> > > question I have > > > > >> > > is that whether we should name the interface `GroupAssignment` > > > > >> instead, > > > > >> > so > > > > >> > > that Connect later > > > > >> > > could also extend on the same interface, just to echo > Guozhang's > > > point > > > > >> > > here, Also the base interface > > > > >> > > is better to be defined empty for easy extension, or define an > > > > >> abstract > > > > >> > > type called `Resource` to be shareable > > > > >> > > later IMHO. > > > > >> > > > > > > >> > > > > > > >> > > > By the way, I'm a bit confused about discussion above about > > > > >> colocating > > > > >> > > the > > > > >> > > > txn and group coordinators. That is not actually necessary, > is > > > it? > > > > >> > > > > > > > >> > > > Yes, this is not a requirement for this KIP, because it is > > > > >> inherently > > > > >> > > impossible to > > > > >> > > achieve co-locating topic partition of transaction log and > > > consumed > > > > >> > offset > > > > >> > > topics. > > > > >> > > > > > > >> > > > > > > >> > > > Thanks, > > > > >> > > > Jason > > > > >> > > > > > > > >> > > On Mon, Jun 24, 2019 at 10:07 AM Boyang Chen < > > > > >> reluctanthero...@gmail.com > > > > >> > > > > > > >> > > > wrote: > > > > >> > > > > > > > >> > > > > Thank you Ismael for the suggestion. We will attempt to > > > address > > > > >> it by > > > > >> > > > > giving more details to rejected alternative section. > > > > >> > > > > > > > > >> > > > > > > > > >> > > > > Thank you for the comment Guozhang! Answers are inline > > below. > > > > >> > > > > > > > > >> > > > > > > > > >> > > > > > > > > >> > > > > On Sun, Jun 23, 2019 at 6:33 PM Guozhang Wang < > > > wangg...@gmail.com > > > > >> > > > > > >> > > > wrote: > > > > >> > > > > > > > > >> > > > > > Hello Boyang, > > > > >> > > > > > > > > > >> > > > > > Thanks for the KIP, I have some comments below: > > > > >> > > > > > > > > > >> > > > > > 1. "Once transactions are complete, the call will > return." > > > This > > > > >> > seems > > > > >> > > > > > different from the existing behavior, in which we would > > > return a > > > > >> > > > > retriable > > > > >> > > > > > CONCURRENT_TRANSACTIONS and let the client to retry, is > > this > > > > >> > > > intentional? > > > > >> > > > > > > > > > >> > > > > > > > > >> > > > > I don’t think it is intentional, and I will defer this > > > question to > > > > >> > > Jason > > > > >> > > > > when he got time to answer since from what I understood > > retry > > > and > > > > >> on > > > > >> > > hold > > > > >> > > > > seem both valid approaches. > > > > >> > > > > > > > > >> > > > > > > > > >> > > > > > 2. "an overload to onPartitionsAssigned in the > consumer's > > > > >> rebalance > > > > >> > > > > > listener interface": as part of KIP-341 we've already > add > > > this > > > > >> > > > > information > > > > >> > > > > > to the onAssignment callback. Would this be sufficient? > Or > > > more > > > > >> > > > generally > > > > >> > > > > > speaking, which information have to be passed around in > > > > >> rebalance > > > > >> > > > > callback > > > > >> > > > > > while others can be passed around in PartitionAssignor > > > > >> callback? In > > > > >> > > > > Streams > > > > >> > > > > > for example both callbacks are used but most critical > > > > >> information > > > > >> > is > > > > >> > > > > passed > > > > >> > > > > > via onAssignment. > > > > >> > > > > > > > > > >> > > > > > > > > >> > > > > We still need to extend ConsumerRebalanceListener because > > > it’s the > > > > >> > > > > interface we could have public access to. The > #onAssignment > > > call > > > > >> is > > > > >> > > > defined > > > > >> > > > > on PartitionAssignor level which is not easy to work with > > > external > > > > >> > > > > producers. > > > > >> > > > > > > > > >> > > > > > > > > >> > > > > > 3. "We propose to use a separate record type in order to > > > store > > > > >> the > > > > >> > > > group > > > > >> > > > > > assignment.": hmm, I thought with the third typed > > > > >> FindCoordinator, > > > > >> > > the > > > > >> > > > > same > > > > >> > > > > > broker that act as the consumer coordinator would > always > > be > > > > >> > selected > > > > >> > > > as > > > > >> > > > > > the txn coordinator, in which case it can access its > local > > > cache > > > > >> > > > > metadata / > > > > >> > > > > > offset topic to get this information already? We just > need > > > to > > > > >> think > > > > >> > > > about > > > > >> > > > > > how to make these two modules directly exchange > > information > > > > >> without > > > > >> > > > > messing > > > > >> > > > > > up the code hierarchy. > > > > >> > > > > > > > > > >> > > > > > > > > >> > > > > These two coordinators will be on the same broker only > when > > > > >> number of > > > > >> > > > > partitions for transaction state topic and consumer offset > > > topic > > > > >> are > > > > >> > > the > > > > >> > > > > same. This normally holds true, but I'm afraid > > > > >> > > > > we couldn't make this assumption? > > > > >> > > > > > > > > >> > > > > 4. The config of "CONSUMER_GROUP_AWARE_TRANSACTION": it > > seems > > > the > > > > >> > goal > > > > >> > > of > > > > >> > > > > > this config is just to avoid old-versioned broker to not > > be > > > > >> able to > > > > >> > > > > > recognize newer versioned client. I think if we can do > > > something > > > > >> > else > > > > >> > > > to > > > > >> > > > > > avoid this config though, for example we can use the > > > embedded > > > > >> > > > AdminClient > > > > >> > > > > > to send the APIVersion request upon starting up, and > based > > > on > > > > >> the > > > > >> > > > > returned > > > > >> > > > > > value decides whether to go to the old code path or the > > new > > > > >> > behavior. > > > > >> > > > > > Admittedly asking a random broker about APIVersion does > > not > > > > >> > guarantee > > > > >> > > > the > > > > >> > > > > > whole cluster's versions, but what we can do is to first > > 1) > > > find > > > > >> > the > > > > >> > > > > > coordinator (and if the random broker does not even > > > recognize > > > > >> the > > > > >> > new > > > > >> > > > > > discover type, fall back to old path directly), and then > > 2) > > > ask > > > > >> the > > > > >> > > > > > discovered coordinator about its supported APIVersion. > > > > >> > > > > > > > > > >> > > > > > > > > >> > > > > The caveat here is that we have to make sure both the > group > > > > >> > coordinator > > > > >> > > > and > > > > >> > > > > transaction coordinator are on the latest version during > > init > > > > >> stage. > > > > >> > > This > > > > >> > > > > is potentially doable as we only need a consumer group.id > > > > >> > > > > to check that. In the meantime, a hard-coded config is > > still a > > > > >> > > favorable > > > > >> > > > > backup in case the server has downgraded, so you will want > > to > > > use > > > > >> a > > > > >> > new > > > > >> > > > > version client without `consumer group` transactional > > support. > > > > >> > > > > > > > > >> > > > > 5. This is a meta question: have you considered how this > can > > > be > > > > >> > applied > > > > >> > > > to > > > > >> > > > > > Kafka Connect as well? For example, for source > connectors, > > > the > > > > >> > > > assignment > > > > >> > > > > > is not by "partitions", but by some other sort of > > > "resources" > > > > >> based > > > > >> > > on > > > > >> > > > > the > > > > >> > > > > > source systems, how KIP-447 would affect Kafka > Connectors > > > that > > > > >> > > > > implemented > > > > >> > > > > > EOS as well? > > > > >> > > > > > > > > > >> > > > > > > > > >> > > > > No, it's not currently included in the scope. Could you > > point > > > me > > > > >> to a > > > > >> > > > > sample source connector who uses EOS? Could always > > piggy-back > > > into > > > > >> > the > > > > >> > > > > TxnProducerIdentity struct with more information such as > > > tasks. If > > > > >> > > > > this is something to support in near term, an abstract > type > > > called > > > > >> > > > > "Resource" could be provided and let topic partition and > > > connect > > > > >> task > > > > >> > > > > implement it. > > > > >> > > > > > > > > >> > > > > > > > > >> > > > > > > > > > >> > > > > > Guozhang > > > > >> > > > > > > > > > >> > > > > > > > > > >> > > > > > On Sat, Jun 22, 2019 at 8:40 PM Ismael Juma < > > > ism...@juma.me.uk> > > > > >> > > wrote: > > > > >> > > > > > > > > > >> > > > > > > Hi Boyang, > > > > >> > > > > > > > > > > >> > > > > > > Thanks for the KIP. It's good that we listed a number > of > > > > >> rejected > > > > >> > > > > > > alternatives. It would be helpful to have an > explanation > > > of > > > > >> why > > > > >> > > they > > > > >> > > > > were > > > > >> > > > > > > rejected. > > > > >> > > > > > > > > > > >> > > > > > > Ismael > > > > >> > > > > > > > > > > >> > > > > > > On Sat, Jun 22, 2019 at 8:31 PM Boyang Chen < > > > > >> bche...@outlook.com > > > > >> > > > > > > >> > > > > wrote: > > > > >> > > > > > > > > > > >> > > > > > > > Hey all, > > > > >> > > > > > > > > > > > >> > > > > > > > I would like to start a discussion for KIP-447: > > > > >> > > > > > > > > > > > >> > > > > > > > > > > > >> > > > > > > > > > > >> > > > > > > > > > >> > > > > > > > > >> > > > > > > > >> > > > > > > >> > > > > > >> > > > > > > https://cwiki.apache.org/confluence/display/KAFKA/KIP-447%3A+Producer+scalability+for+exactly+once+semantics > > > > >> > > > > > > > > > > > >> > > > > > > > this is a work originated by Jason Gustafson and we > > > would > > > > >> like > > > > >> > to > > > > >> > > > > > proceed > > > > >> > > > > > > > into discussion stage. > > > > >> > > > > > > > > > > > >> > > > > > > > Let me know your thoughts, thanks! > > > > >> > > > > > > > > > > > >> > > > > > > > Boyang > > > > >> > > > > > > > > > > > >> > > > > > > > > > > >> > > > > > > > > > >> > > > > > > > > > >> > > > > > -- > > > > >> > > > > > -- Guozhang > > > > >> > > > > > > > > > >> > > > > > > > > >> > > > > > > > >> > > > > > > >> > > > > > >> > > > > > >> > -- > > > > >> > -- Guozhang > > > > >> > > > > > >> > > > > > > > > > > > > > > > -- -- Guozhang
