I think you're saying that depending on the bug, in the worst case, you may have to downgrade the client. I think that's fair. Note that one advantage of making this a fatal error is that we'll be more likely to hit unexpected edge cases in system tests.
-Jason On Tue, Dec 19, 2017 at 11:26 AM, Dong Lin <lindon...@gmail.com> wrote: > Hey Jason, > > Yeah this may sound a bit confusing. Let me explain my thoughts. > > If there is no bug in the client library, after consumer rebalance or > consumer restart, consume will fetch the previously committed offset and > fetch the committed metadata until the leader epoch in the metadata >= the > leader epoch in the OffsetFetchResponse. Therefore, when consumer commits > offset later, the leader epoch in the OffsetCommitRequest should be larger > than the leader epoch from the previously committed offset. Does this sound > correct? > > Given the above understanding, it seems to suggest that the only > explanation for this exception is that there is bug in the client library. > And due to this specific bug, I am not sure we can avoid this error by > simply restarting consumer. And because this error is non-retriable, user > may be forced to downgrade client library. Did I miss something here? > > Thanks, > Dong > > > On Tue, Dec 19, 2017 at 11:19 AM, Jason Gustafson <ja...@confluent.io> > wrote: > > > Hey Dong, > > > > Thanks for the updates. Just one question: > > > > When application receives > > > this exception, the only choice will be to revert Kafka client library > to > > > an earlier version. > > > > > > Not sure I follow this. Wouldn't we just restart the consumer? That would > > cause it to fetch the previous committed offset and then fetch the > correct > > metadata. > > > > Thanks, > > Jason > > > > On Tue, Dec 19, 2017 at 10:36 AM, Dong Lin <lindon...@gmail.com> wrote: > > > > > Hey Jason, > > > > > > Thanks for the comments. These make sense. I have updated the KIP to > > > include a new error INVALID_LEADER_EPOCH. This will be a non-retriable > > > error which may be thrown from consumer's API. When application > receives > > > this exception, the only choice will be to revert Kafka client library > to > > > an earlier version. > > > > > > Previously I think it may be better to simply log an error because I am > > not > > > sure it is a good idea to force user to downgrade Kafka client library > > when > > > the error itself, e.g. smaller leader epoch, may not be that fatal. One > > the > > > other hand it could be argued that we don't know what else can go wrong > > in > > > the buggy client library and it may be a good reason to force user to > > > downgrade library. > > > > > > Thanks, > > > Dong > > > > > > > > > On Tue, Dec 19, 2017 at 9:06 AM, Jason Gustafson <ja...@confluent.io> > > > wrote: > > > > > > > Hey Dong, > > > > > > > > > > > > > I think it is a good idea to let coordinator do the additional > sanity > > > > check > > > > > to ensure the leader epoch from OffsetCommitRequest never > decreases. > > > This > > > > > can help us detect bug. The next question will be what should we do > > if > > > > > OffsetCommitRequest provides a smaller leader epoch. One possible > > > > solution > > > > > is to return a non-retriable error to consumer which will then be > > > thrown > > > > to > > > > > user application. But I am not sure it is worth doing it given its > > > impact > > > > > on the user. Maybe it will be safer to simply have an error message > > in > > > > the > > > > > server log and allow offset commit to succeed. What do you think? > > > > > > > > > > > > I think the check would only have value if you return an error when > it > > > > fails. It seems primarily useful to detect buggy consumer logic, so a > > > > non-retriable error makes sense to me. Clients which don't implement > > this > > > > capability can use the sentinel value and keep the current behavior. > > > > > > > > It seems that FetchResponse includes leader epoch via the path > > > > > FetchResponse -> MemoryRecords -> MutableRecordBatch -> > > > > DefaultRecordBatch > > > > > -> partitionLeaderEpoch. Could this be an existing case where we > > expose > > > > the > > > > > leader epoch to clients? > > > > > > > > > > > > Right, in this case the client has no direct dependence on the field, > > but > > > > it could still be argued that it is exposed (I had actually > considered > > > > stuffing this field into an opaque blob of bytes in the message > format > > > > which the client wasn't allowed to touch, but it didn't happen in the > > > end). > > > > I'm not opposed to using the leader epoch field here, I was just > > > mentioning > > > > that it does tie clients a bit tighter to something which could be > > > > considered a Kafka internal implementation detail. It makes the > > protocol > > > a > > > > bit less intuitive as well since it is rather difficult to explain > the > > > edge > > > > case it is protecting. That said, we've hit other scenarios where > being > > > > able to detect stale metadata in the client would be helpful, so I > > think > > > it > > > > might be worth the tradeoff. > > > > > > > > -Jason > > > > > > > > On Mon, Dec 18, 2017 at 6:09 PM, Dong Lin <lindon...@gmail.com> > wrote: > > > > > > > > > Hey Jason, > > > > > > > > > > Thanks much for reviewing the KIP. > > > > > > > > > > I think it is a good idea to let coordinator do the additional > sanity > > > > check > > > > > to ensure the leader epoch from OffsetCommitRequest never > decreases. > > > This > > > > > can help us detect bug. The next question will be what should we do > > if > > > > > OffsetCommitRequest provides a smaller leader epoch. One possible > > > > solution > > > > > is to return a non-retriable error to consumer which will then be > > > thrown > > > > to > > > > > user application. But I am not sure it is worth doing it given its > > > impact > > > > > on the user. Maybe it will be safer to simply have an error message > > in > > > > the > > > > > server log and allow offset commit to succeed. What do you think? > > > > > > > > > > It seems that FetchResponse includes leader epoch via the path > > > > > FetchResponse -> MemoryRecords -> MutableRecordBatch -> > > > > DefaultRecordBatch > > > > > -> partitionLeaderEpoch. Could this be an existing case where we > > expose > > > > the > > > > > leader epoch to clients? > > > > > > > > > > Thanks, > > > > > Dong > > > > > > > > > > > > > > > > > > > > On Mon, Dec 18, 2017 at 3:27 PM, Jason Gustafson < > ja...@confluent.io > > > > > > > > wrote: > > > > > > > > > > > Hi Dong, > > > > > > > > > > > > Thanks for the KIP. Good job identifying the problem. One minor > > > > question > > > > > I > > > > > > had is whether the coordinator should enforce that the leader > epoch > > > > > > associated with an offset commit can only go forward for each > > > > partition? > > > > > > Currently it looks like we just depend on the client for this, > but > > > > since > > > > > > we're caching the leader epoch anyway, it seems like a cheap > safety > > > > > > condition. To support old clients, you can always allow the > commit > > if > > > > the > > > > > > leader epoch is unknown. > > > > > > > > > > > > I agree that we shouldn't expose the leader epoch in > > > OffsetAndMetadata > > > > in > > > > > > the consumer API for what it's worth. As you have noted, it is > more > > > of > > > > an > > > > > > implementation detail. By the same argument, it's also a bit > > > > unfortunate > > > > > > that we have to expose it in the request API since that is nearly > > as > > > > > > binding in terms of how it limits future iterations. I could be > > > wrong, > > > > > but > > > > > > this appears to be the first case where clients will depend on > the > > > > > concept > > > > > > of leader epoch. Might not be a big deal considering how deeply > > > > embedded > > > > > > leader epochs already are in the inter-broker RPCs and the > message > > > > format > > > > > > itself, but just wanted to mention the fact that good > encapsulation > > > > > applies > > > > > > to the client request API as well. > > > > > > > > > > > > Thanks, > > > > > > Jason > > > > > > > > > > > > On Mon, Dec 18, 2017 at 1:58 PM, Dong Lin <lindon...@gmail.com> > > > wrote: > > > > > > > > > > > > > Hey Jun, > > > > > > > > > > > > > > Thanks much for your comments. These are very thoughtful ideas. > > > > Please > > > > > > see > > > > > > > my comments below. > > > > > > > > > > > > > > On Thu, Dec 14, 2017 at 6:38 PM, Jun Rao <j...@confluent.io> > > wrote: > > > > > > > > > > > > > > > Hi, Dong, > > > > > > > > > > > > > > > > Thanks for the update. A few more comments below. > > > > > > > > > > > > > > > > 10. It seems that we need to return the leader epoch in the > > fetch > > > > > > > response > > > > > > > > as well When fetching data, we could be fetching data from a > > > leader > > > > > > epoch > > > > > > > > older than what's returned in the metadata response. So, we > > want > > > to > > > > > use > > > > > > > the > > > > > > > > leader epoch associated with the offset being fetched for > > > > committing > > > > > > > > offsets. > > > > > > > > > > > > > > > > > > > > > > It seems that we may have two separate issues here. The first > > issue > > > > is > > > > > > that > > > > > > > consumer uses metadata that is older than the one it uses > before. > > > The > > > > > > > second issue is that consumer uses metadata which is newer than > > the > > > > > > > corresponding leader epoch in the leader broker. We know that > the > > > > > > > OffsetOutOfRangeException described in this KIP can be > prevented > > by > > > > > > > avoiding the first issue. On the other hand, it seems that the > > > > > > > OffsetOffsetOutOfRangeException can still happen even if we > > avoid > > > > the > > > > > > > second issue -- if consumer uses an older version of metadata, > > the > > > > > leader > > > > > > > epoch in its metadata may equal the leader epoch in the broker > > even > > > > if > > > > > > the > > > > > > > leader epoch in the broker is oudated. > > > > > > > > > > > > > > Given this understanding, I am not sure why we need to return > the > > > > > leader > > > > > > > epoch in the fetch response. As long as consumer's metadata is > > not > > > > > going > > > > > > > back in version, I think we are good. Did I miss something > here? > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > 11. Should we now extend OffsetAndMetadata used in the offset > > > > commit > > > > > > api > > > > > > > in > > > > > > > > KafkaConsumer to include leader epoch? Similarly, should we > > > return > > > > > > leader > > > > > > > > epoch in endOffsets(), beginningOffsets() and position()? We > > > > probably > > > > > > > need > > > > > > > > to think about how to make the api backward compatible. > > > > > > > > > > > > > > > > > > > > > > After thinking through this carefully, I think we probably > don't > > > want > > > > > to > > > > > > > extend OffsetAndMetadata to include leader epoch because leader > > > epoch > > > > > is > > > > > > > kind of implementation detail which ideally should be hidden > from > > > > user. > > > > > > The > > > > > > > consumer can include leader epoch in the OffsetCommitRequest > > after > > > > > taking > > > > > > > offset from commitSync(final Map<TopicPartition, > > OffsetAndMetadata> > > > > > > > offsets). Similarly consumer can store leader epoch from > > > > > > > OffsetFetchResponse and only provide offset to user via > > > > > > > consumer.committed(topicPartition). This solution seems to > work > > > well > > > > > and > > > > > > > we > > > > > > > don't have to make changes to consumer's public API. Does this > > > sound > > > > > OK? > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > 12. It seems that we now need to store leader epoch in the > > offset > > > > > > topic. > > > > > > > > Could you include the new schema for the value of the offset > > > topic > > > > > and > > > > > > > add > > > > > > > > upgrade notes? > > > > > > > > > > > > > > > > > > > > > You are right. I have updated the KIP to specify the new schema > > for > > > > the > > > > > > > value of the offset topic. Can you take another look? > > > > > > > > > > > > > > For existing messages in the offset topic, leader_epoch will be > > > > > missing. > > > > > > We > > > > > > > will use leader_epoch = -1 to indicate the missing > leader_epoch. > > > Then > > > > > the > > > > > > > consumer behavior will be the same as it is now because any > > > > > leader_epoch > > > > > > in > > > > > > > the MetadataResponse will be larger than the leader_epoch = -1 > in > > > the > > > > > > > OffetFetchResponse. Thus we don't need specific procedure for > > > > upgrades > > > > > > due > > > > > > > to this change in the offset topic schema. By "upgrade nodes", > do > > > you > > > > > > mean > > > > > > > the sentences we need to include in the upgrade.html in the PR > > > later? > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Jun > > > > > > > > > > > > > > > > > > > > > > > > On Tue, Dec 12, 2017 at 5:19 PM, Dong Lin < > lindon...@gmail.com > > > > > > > > wrote: > > > > > > > > > > > > > > > > > Hey Jun, > > > > > > > > > > > > > > > > > > I see. Sounds good. Yeah it is probably simpler to leave > this > > > to > > > > > > > another > > > > > > > > > KIP in the future. > > > > > > > > > > > > > > > > > > Thanks for all the comments. Since there is no further > > comment > > > in > > > > > the > > > > > > > > > community, I will open the voting thread. > > > > > > > > > > > > > > > > > > Thanks, > > > > > > > > > Dong > > > > > > > > > > > > > > > > > > On Mon, Dec 11, 2017 at 5:37 PM, Jun Rao <j...@confluent.io > > > > > > wrote: > > > > > > > > > > > > > > > > > > > Hi, Dong, > > > > > > > > > > > > > > > > > > > > The case that I am thinking is network partitioning. > > Suppose > > > > one > > > > > > > > deploys > > > > > > > > > a > > > > > > > > > > stretched cluster across multiple AZs in the same region. > > If > > > > the > > > > > > > > machines > > > > > > > > > > in one AZ can't communicate to brokers in other AZs due > to > > a > > > > > > network > > > > > > > > > issue, > > > > > > > > > > the brokers in that AZ won't get any new metadata. > > > > > > > > > > > > > > > > > > > > We can potentially solve this problem by requiring some > > kind > > > of > > > > > > > regular > > > > > > > > > > heartbeats between the controller and the broker. This > may > > > need > > > > > > some > > > > > > > > more > > > > > > > > > > thoughts. So, it's probably fine to leave this to another > > KIP > > > > in > > > > > > the > > > > > > > > > > future. > > > > > > > > > > > > > > > > > > > > Thanks, > > > > > > > > > > > > > > > > > > > > Jun > > > > > > > > > > > > > > > > > > > > On Mon, Dec 11, 2017 at 2:55 PM, Dong Lin < > > > lindon...@gmail.com > > > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > Hey Jun, > > > > > > > > > > > > > > > > > > > > > > Thanks for the comment. I am open to improve this KIP > to > > > > > address > > > > > > > more > > > > > > > > > > > problems. I probably need more help in understanding > what > > > is > > > > > the > > > > > > > > > current > > > > > > > > > > > problem with consumer using outdated metadata and > whether > > > it > > > > is > > > > > > > > easier > > > > > > > > > to > > > > > > > > > > > address it together with this KIP. > > > > > > > > > > > > > > > > > > > > > > I agree that a consumer can potentially talk to old > > leader > > > > for > > > > > a > > > > > > > long > > > > > > > > > > time > > > > > > > > > > > even after this KIP. But after this KIP, the consumer > > > > probably > > > > > > > should > > > > > > > > > not > > > > > > > > > > > get OffetOutofRangeException and therefore will not > cause > > > > > offset > > > > > > > > rewind > > > > > > > > > > > issue. So the only problem is that consumer will not be > > > able > > > > to > > > > > > > fetch > > > > > > > > > > data > > > > > > > > > > > until it has updated metadata. It seems that this > > situation > > > > can > > > > > > > only > > > > > > > > > > happen > > > > > > > > > > > if the broker is too slow in processing > > LeaderAndIsrRequest > > > > > since > > > > > > > > > > otherwise > > > > > > > > > > > the consumer will be forced to update metadata due to > > > > > > > > > > > NotLeaderForPartitionException. So the problem we are > > > having > > > > > > here > > > > > > > is > > > > > > > > > > that > > > > > > > > > > > consumer will not be able to fetch data if some broker > is > > > too > > > > > > slow > > > > > > > in > > > > > > > > > > > processing LeaderAndIsrRequest. > > > > > > > > > > > > > > > > > > > > > > Because Kafka propagates LeaderAndIsrRequest > > asynchronously > > > > to > > > > > > all > > > > > > > > > > brokers > > > > > > > > > > > in the cluster, there will always be a period of time > > when > > > > > > consumer > > > > > > > > can > > > > > > > > > > not > > > > > > > > > > > fetch data for the partition during the leadership > > change. > > > > Thus > > > > > > it > > > > > > > > > seems > > > > > > > > > > > more like a broker-side performance issue instead of > > > > > client-side > > > > > > > > > > > correctness issue. My gut feel is that it is not > causing > > a > > > > > much a > > > > > > > > > problem > > > > > > > > > > > as the problem to be fixed in this KIP. And if we were > to > > > > > address > > > > > > > it, > > > > > > > > > we > > > > > > > > > > > probably need to make change in the broker side, e.g. > > with > > > > > > > > prioritized > > > > > > > > > > > queue for controller-related requests, which may be > kind > > of > > > > > > > > orthogonal > > > > > > > > > to > > > > > > > > > > > this KIP. I am not very sure it will be easier to > address > > > it > > > > > with > > > > > > > the > > > > > > > > > > > change in this KIP. Do you have any recommendation? > > > > > > > > > > > > > > > > > > > > > > Thanks, > > > > > > > > > > > Dong > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > On Mon, Dec 11, 2017 at 1:51 PM, Jun Rao < > > j...@confluent.io > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > > > Hi, Dong, > > > > > > > > > > > > > > > > > > > > > > > > Thanks for the reply. > > > > > > > > > > > > > > > > > > > > > > > > My suggestion of forcing the metadata refresh from > the > > > > > > controller > > > > > > > > may > > > > > > > > > > not > > > > > > > > > > > > work in general since the cached controller could be > > > > outdated > > > > > > > too. > > > > > > > > > The > > > > > > > > > > > > general problem is that if a consumer's metadata is > > > > outdated, > > > > > > it > > > > > > > > may > > > > > > > > > > get > > > > > > > > > > > > stuck with the old leader for a long time. We can > > address > > > > the > > > > > > > issue > > > > > > > > > of > > > > > > > > > > > > detecting outdated metadata in a separate KIP in the > > > future > > > > > if > > > > > > > you > > > > > > > > > > didn't > > > > > > > > > > > > intend to address it in this KIP. > > > > > > > > > > > > > > > > > > > > > > > > Thanks, > > > > > > > > > > > > > > > > > > > > > > > > Jun > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > On Sat, Dec 9, 2017 at 10:12 PM, Dong Lin < > > > > > lindon...@gmail.com > > > > > > > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > > > > > Hey Jun, > > > > > > > > > > > > > > > > > > > > > > > > > > Thanks much for your comments. Given that client > > needs > > > to > > > > > > > > > > de-serialize > > > > > > > > > > > > the > > > > > > > > > > > > > metadata anyway, the extra overhead of checking the > > > > > > > per-partition > > > > > > > > > > > version > > > > > > > > > > > > > for every partition should not be a big concern. > Thus > > > it > > > > > > makes > > > > > > > > > sense > > > > > > > > > > to > > > > > > > > > > > > use > > > > > > > > > > > > > leader epoch as the per-partition version instead > of > > > > > > creating a > > > > > > > > > > global > > > > > > > > > > > > > metadata version. I will update the KIP to do that. > > > > > > > > > > > > > > > > > > > > > > > > > > Regarding the detection of outdated metadata, I > think > > > it > > > > is > > > > > > > > > possible > > > > > > > > > > to > > > > > > > > > > > > > ensure that client gets latest metadata by fetching > > > from > > > > > > > > > controller. > > > > > > > > > > > Note > > > > > > > > > > > > > that this requires extra logic in the controller > such > > > > that > > > > > > > > > controller > > > > > > > > > > > > > updates metadata directly in memory without > requiring > > > > > > > > > > > > > UpdateMetadataRequest. But I am not sure the main > > > > > motivation > > > > > > of > > > > > > > > > this > > > > > > > > > > at > > > > > > > > > > > > > this moment. But this makes controller more like a > > > > > bottleneck > > > > > > > in > > > > > > > > > the > > > > > > > > > > > > > cluster which we probably want to avoid. > > > > > > > > > > > > > > > > > > > > > > > > > > I think we can probably keep the current way of > > > ensuring > > > > > > > metadata > > > > > > > > > > > > > freshness. Currently client will be forced to > refresh > > > > > > metadata > > > > > > > if > > > > > > > > > > > broker > > > > > > > > > > > > > returns error (e.g. NotLeaderForPartition) due to > > > > outdated > > > > > > > > metadata > > > > > > > > > > or > > > > > > > > > > > if > > > > > > > > > > > > > the metadata does not contain the partition that > the > > > > client > > > > > > > > needs. > > > > > > > > > In > > > > > > > > > > > the > > > > > > > > > > > > > future, as you previously suggested, we can include > > > > > > > per-partition > > > > > > > > > > > > > leaderEpoch in the FetchRequest/ProduceRequest such > > > that > > > > > > broker > > > > > > > > can > > > > > > > > > > > > return > > > > > > > > > > > > > error if the epoch is smaller than cached epoch in > > the > > > > > > broker. > > > > > > > > > Given > > > > > > > > > > > that > > > > > > > > > > > > > this adds more complexity to Kafka, I think we can > > > > probably > > > > > > > think > > > > > > > > > > about > > > > > > > > > > > > > that leader when we have a specific use-case or > > problem > > > > to > > > > > > > solve > > > > > > > > > with > > > > > > > > > > > > > up-to-date metadata. Does this sound OK? > > > > > > > > > > > > > > > > > > > > > > > > > > Thanks, > > > > > > > > > > > > > Dong > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > On Fri, Dec 8, 2017 at 3:53 PM, Jun Rao < > > > > j...@confluent.io> > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > > > > > > > Hi, Dong, > > > > > > > > > > > > > > > > > > > > > > > > > > > > Thanks for the reply. A few more points below. > > > > > > > > > > > > > > > > > > > > > > > > > > > > For dealing with how to prevent a consumer > > switching > > > > > from a > > > > > > > new > > > > > > > > > > > leader > > > > > > > > > > > > to > > > > > > > > > > > > > > an old leader, you suggestion that refreshes > > metadata > > > > on > > > > > > > > consumer > > > > > > > > > > > > restart > > > > > > > > > > > > > > until it sees a metadata version >= the one > > > associated > > > > > with > > > > > > > the > > > > > > > > > > > offset > > > > > > > > > > > > > > works too, as long as we guarantee that the > cached > > > > > metadata > > > > > > > > > > versions > > > > > > > > > > > on > > > > > > > > > > > > > the > > > > > > > > > > > > > > brokers only go up. > > > > > > > > > > > > > > > > > > > > > > > > > > > > The second discussion point is on whether the > > > metadata > > > > > > > > versioning > > > > > > > > > > > > should > > > > > > > > > > > > > be > > > > > > > > > > > > > > per partition or global. For the partition level > > > > > > versioning, > > > > > > > > you > > > > > > > > > > were > > > > > > > > > > > > > > concerned about the performance. Given that > > metadata > > > > > > updates > > > > > > > > are > > > > > > > > > > > rare, > > > > > > > > > > > > I > > > > > > > > > > > > > am > > > > > > > > > > > > > > not sure if it's a big concern though. Doing a > > > million > > > > if > > > > > > > tests > > > > > > > > > is > > > > > > > > > > > > > probably > > > > > > > > > > > > > > going to take less than 1ms. Another thing is > that > > > the > > > > > > > metadata > > > > > > > > > > > version > > > > > > > > > > > > > > seems to need to survive controller failover. In > > your > > > > > > current > > > > > > > > > > > > approach, a > > > > > > > > > > > > > > consumer may not be able to wait on the right > > version > > > > of > > > > > > the > > > > > > > > > > metadata > > > > > > > > > > > > > after > > > > > > > > > > > > > > the consumer restart since the metadata version > may > > > > have > > > > > > been > > > > > > > > > > > recycled > > > > > > > > > > > > on > > > > > > > > > > > > > > the server side due to a controller failover > while > > > the > > > > > > > consumer > > > > > > > > > is > > > > > > > > > > > > down. > > > > > > > > > > > > > > The partition level leaderEpoch survives > controller > > > > > failure > > > > > > > and > > > > > > > > > > won't > > > > > > > > > > > > > have > > > > > > > > > > > > > > this issue. > > > > > > > > > > > > > > > > > > > > > > > > > > > > Lastly, neither your proposal nor mine addresses > > the > > > > > issue > > > > > > > how > > > > > > > > to > > > > > > > > > > > > > guarantee > > > > > > > > > > > > > > a consumer to detect that is metadata is > outdated. > > > > > > Currently, > > > > > > > > the > > > > > > > > > > > > > consumer > > > > > > > > > > > > > > is not guaranteed to fetch metadata from every > > broker > > > > > > within > > > > > > > > some > > > > > > > > > > > > bounded > > > > > > > > > > > > > > period of time. Maybe this is out of the scope of > > > your > > > > > KIP. > > > > > > > But > > > > > > > > > one > > > > > > > > > > > > idea > > > > > > > > > > > > > is > > > > > > > > > > > > > > force the consumer to refresh metadata from the > > > > > controller > > > > > > > > > > > > periodically. > > > > > > > > > > > > > > > > > > > > > > > > > > > > Jun > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > On Thu, Dec 7, 2017 at 11:25 AM, Dong Lin < > > > > > > > lindon...@gmail.com > > > > > > > > > > > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Hey Jun, > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Thanks much for the comments. Great point > > > > particularly > > > > > > > > > regarding > > > > > > > > > > > > (3). I > > > > > > > > > > > > > > > haven't thought about this before. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > It seems that there are two possible ways where > > the > > > > > > version > > > > > > > > > > number > > > > > > > > > > > > can > > > > > > > > > > > > > be > > > > > > > > > > > > > > > used. One solution is for client to check the > > > version > > > > > > > number > > > > > > > > at > > > > > > > > > > the > > > > > > > > > > > > > time > > > > > > > > > > > > > > it > > > > > > > > > > > > > > > receives MetadataResponse. And if the version > > > number > > > > in > > > > > > the > > > > > > > > > > > > > > > MetadataResponse is smaller than the version > > number > > > > in > > > > > > the > > > > > > > > > > client's > > > > > > > > > > > > > > cache, > > > > > > > > > > > > > > > the client will be forced to fetch metadata > > again. > > > > > > Another > > > > > > > > > > > solution, > > > > > > > > > > > > > as > > > > > > > > > > > > > > > you have suggested, is for broker to check the > > > > version > > > > > > > number > > > > > > > > > at > > > > > > > > > > > the > > > > > > > > > > > > > time > > > > > > > > > > > > > > > it receives a request from client. The broker > > will > > > > > reject > > > > > > > the > > > > > > > > > > > request > > > > > > > > > > > > > if > > > > > > > > > > > > > > > the version is smaller than the version in > > broker's > > > > > > cache. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > I am not very sure that the second solution can > > > > address > > > > > > the > > > > > > > > > > problem > > > > > > > > > > > > > here. > > > > > > > > > > > > > > > In the scenario described in the JIRA ticket, > > > > broker's > > > > > > > cache > > > > > > > > > may > > > > > > > > > > be > > > > > > > > > > > > > > > outdated because it has not processed the > > > > > > > LeaderAndIsrRequest > > > > > > > > > > from > > > > > > > > > > > > the > > > > > > > > > > > > > > > controller. Thus it may still process client's > > > > request > > > > > > even > > > > > > > > if > > > > > > > > > > the > > > > > > > > > > > > > > version > > > > > > > > > > > > > > > in client's request is actually outdated. Does > > this > > > > > make > > > > > > > > sense? > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > IMO, it seems that we can address problem (3) > by > > > > saving > > > > > > the > > > > > > > > > > > metadata > > > > > > > > > > > > > > > version together with the offset. After > consumer > > > > > starts, > > > > > > it > > > > > > > > > will > > > > > > > > > > > keep > > > > > > > > > > > > > > > fetching metadata until the metadata version >= > > the > > > > > > version > > > > > > > > > saved > > > > > > > > > > > > with > > > > > > > > > > > > > > the > > > > > > > > > > > > > > > offset of this partition. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Regarding problems (1) and (2): Currently we > use > > > the > > > > > > > version > > > > > > > > > > number > > > > > > > > > > > > in > > > > > > > > > > > > > > the > > > > > > > > > > > > > > > MetadataResponse to ensure that the metadata > does > > > not > > > > > go > > > > > > > back > > > > > > > > > in > > > > > > > > > > > > time. > > > > > > > > > > > > > > > There are two alternative solutions to address > > > > problems > > > > > > (1) > > > > > > > > and > > > > > > > > > > > (2). > > > > > > > > > > > > > One > > > > > > > > > > > > > > > solution is for client to enumerate all > > partitions > > > in > > > > > the > > > > > > > > > > > > > > MetadataResponse, > > > > > > > > > > > > > > > compare their epoch with those in the cached > > > > metadata, > > > > > > and > > > > > > > > > > rejects > > > > > > > > > > > > the > > > > > > > > > > > > > > > MetadataResponse iff any leader epoch is > smaller. > > > The > > > > > > main > > > > > > > > > > concern > > > > > > > > > > > is > > > > > > > > > > > > > > that > > > > > > > > > > > > > > > MetadataResponse currently cached information > of > > > all > > > > > > > > partitions > > > > > > > > > > in > > > > > > > > > > > > the > > > > > > > > > > > > > > > entire cluster. It may slow down client's > > > performance > > > > > if > > > > > > we > > > > > > > > > were > > > > > > > > > > to > > > > > > > > > > > > do > > > > > > > > > > > > > > it. > > > > > > > > > > > > > > > The other solution is for client to enumerate > > > > > partitions > > > > > > > for > > > > > > > > > only > > > > > > > > > > > > > topics > > > > > > > > > > > > > > > registered in the org.apache.kafka.clients. > > > Metadata, > > > > > > which > > > > > > > > > will > > > > > > > > > > be > > > > > > > > > > > > an > > > > > > > > > > > > > > > empty > > > > > > > > > > > > > > > set for producer and the set of subscribed > > > partitions > > > > > for > > > > > > > > > > consumer. > > > > > > > > > > > > But > > > > > > > > > > > > > > > this degrades to all topics if consumer > > subscribes > > > to > > > > > > > topics > > > > > > > > in > > > > > > > > > > the > > > > > > > > > > > > > > cluster > > > > > > > > > > > > > > > by pattern. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Note that client will only be forced to update > > > > metadata > > > > > > if > > > > > > > > the > > > > > > > > > > > > version > > > > > > > > > > > > > in > > > > > > > > > > > > > > > the MetadataResponse is smaller than the > version > > in > > > > the > > > > > > > > cached > > > > > > > > > > > > > metadata. > > > > > > > > > > > > > > In > > > > > > > > > > > > > > > general it should not be a problem. It can be a > > > > problem > > > > > > > only > > > > > > > > if > > > > > > > > > > > some > > > > > > > > > > > > > > broker > > > > > > > > > > > > > > > is particularly slower than other brokers in > > > > processing > > > > > > > > > > > > > > > UpdateMetadataRequest. When this is the case, > it > > > > means > > > > > > that > > > > > > > > the > > > > > > > > > > > > broker > > > > > > > > > > > > > is > > > > > > > > > > > > > > > also particularly slower in processing > > > > > > LeaderAndIsrRequest, > > > > > > > > > which > > > > > > > > > > > can > > > > > > > > > > > > > > cause > > > > > > > > > > > > > > > problem anyway because some partition will > > probably > > > > > have > > > > > > no > > > > > > > > > > leader > > > > > > > > > > > > > during > > > > > > > > > > > > > > > this period. I am not sure problems (1) and (2) > > > cause > > > > > > more > > > > > > > > > > problem > > > > > > > > > > > > than > > > > > > > > > > > > > > > what we already have. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Thanks, > > > > > > > > > > > > > > > Dong > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > On Wed, Dec 6, 2017 at 6:42 PM, Jun Rao < > > > > > > j...@confluent.io> > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Hi, Dong, > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Great finding on the issue. It's a real > > problem. > > > A > > > > > few > > > > > > > > > comments > > > > > > > > > > > > about > > > > > > > > > > > > > > the > > > > > > > > > > > > > > > > KIP. (1) I am not sure about updating > > > > > > > > > controller_metadata_epoch > > > > > > > > > > > on > > > > > > > > > > > > > > every > > > > > > > > > > > > > > > > UpdateMetadataRequest. Currently, the > > controller > > > > can > > > > > > send > > > > > > > > > > > > > > > > UpdateMetadataRequest when there is no actual > > > > > metadata > > > > > > > > > change. > > > > > > > > > > > > Doing > > > > > > > > > > > > > > this > > > > > > > > > > > > > > > > may require unnecessary metadata refresh on > the > > > > > client. > > > > > > > (2) > > > > > > > > > > > > > > > > controller_metadata_epoch is global across > all > > > > > topics. > > > > > > > This > > > > > > > > > > means > > > > > > > > > > > > > that > > > > > > > > > > > > > > a > > > > > > > > > > > > > > > > client may be forced to update its metadata > > even > > > > when > > > > > > the > > > > > > > > > > > metadata > > > > > > > > > > > > > for > > > > > > > > > > > > > > > the > > > > > > > > > > > > > > > > topics that it cares haven't changed. (3) It > > > > doesn't > > > > > > seem > > > > > > > > > that > > > > > > > > > > > the > > > > > > > > > > > > > KIP > > > > > > > > > > > > > > > > handles the corner case when a consumer is > > > > restarted. > > > > > > > Say a > > > > > > > > > > > > consumer > > > > > > > > > > > > > > > reads > > > > > > > > > > > > > > > > from the new leader, commits the offset and > > then > > > is > > > > > > > > > restarted. > > > > > > > > > > On > > > > > > > > > > > > > > > restart, > > > > > > > > > > > > > > > > the consumer gets an outdated metadata and > > > fetches > > > > > from > > > > > > > the > > > > > > > > > old > > > > > > > > > > > > > leader. > > > > > > > > > > > > > > > > Then, the consumer will get into the offset > out > > > of > > > > > > range > > > > > > > > > issue. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Given the above, I am thinking of the > following > > > > > > approach. > > > > > > > > We > > > > > > > > > > > > actually > > > > > > > > > > > > > > > > already have metadata versioning at the > > partition > > > > > > level. > > > > > > > > Each > > > > > > > > > > > > leader > > > > > > > > > > > > > > has > > > > > > > > > > > > > > > a > > > > > > > > > > > > > > > > leader epoch which is monotonically > increasing. > > > We > > > > > can > > > > > > > > > > > potentially > > > > > > > > > > > > > > > > propagate leader epoch back in the metadata > > > > response > > > > > > and > > > > > > > > the > > > > > > > > > > > > clients > > > > > > > > > > > > > > can > > > > > > > > > > > > > > > > cache that. This solves the issue of (1) and > > (2). > > > > To > > > > > > > solve > > > > > > > > > (3), > > > > > > > > > > > > when > > > > > > > > > > > > > > > saving > > > > > > > > > > > > > > > > an offset, we could save both an offset and > the > > > > > > > > corresponding > > > > > > > > > > > > leader > > > > > > > > > > > > > > > epoch. > > > > > > > > > > > > > > > > When fetching the data, the consumer provides > > > both > > > > > the > > > > > > > > offset > > > > > > > > > > and > > > > > > > > > > > > the > > > > > > > > > > > > > > > > leader epoch. A leader will only serve the > > > request > > > > if > > > > > > its > > > > > > > > > > leader > > > > > > > > > > > > > epoch > > > > > > > > > > > > > > is > > > > > > > > > > > > > > > > equal to or greater than the leader epoch > from > > > the > > > > > > > > consumer. > > > > > > > > > To > > > > > > > > > > > > > achieve > > > > > > > > > > > > > > > > this, we need to change the fetch request > > > protocol > > > > > and > > > > > > > the > > > > > > > > > > offset > > > > > > > > > > > > > > commit > > > > > > > > > > > > > > > > api, which requires some more thoughts. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Thanks, > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Jun > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > On Wed, Dec 6, 2017 at 10:57 AM, Dong Lin < > > > > > > > > > lindon...@gmail.com > > > > > > > > > > > > > > > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Bump up the thread. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > It will be great to have more comments on > > > whether > > > > > we > > > > > > > > should > > > > > > > > > > do > > > > > > > > > > > it > > > > > > > > > > > > > or > > > > > > > > > > > > > > > > > whether there is better way to address the > > > > > motivation > > > > > > > of > > > > > > > > > this > > > > > > > > > > > > KIP. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > On Mon, Dec 4, 2017 at 3:09 PM, Dong Lin < > > > > > > > > > > lindon...@gmail.com> > > > > > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > I don't have an interesting rejected > > > > alternative > > > > > > > > solution > > > > > > > > > > to > > > > > > > > > > > > put > > > > > > > > > > > > > in > > > > > > > > > > > > > > > the > > > > > > > > > > > > > > > > > > KIP. If there is good alternative > solution > > > from > > > > > > > anyone > > > > > > > > in > > > > > > > > > > > this > > > > > > > > > > > > > > > thread, > > > > > > > > > > > > > > > > I > > > > > > > > > > > > > > > > > am > > > > > > > > > > > > > > > > > > happy to discuss this and update the KIP > > > > > > accordingly. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Thanks, > > > > > > > > > > > > > > > > > > Dong > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > On Mon, Dec 4, 2017 at 1:12 PM, Ted Yu < > > > > > > > > > > yuzhih...@gmail.com> > > > > > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > >> It is clearer now. > > > > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > >> I noticed that Rejected Alternatives > > section > > > > is > > > > > > > empty. > > > > > > > > > > > > > > > > > >> Have you considered any alternative ? > > > > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > >> Cheers > > > > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > >> On Mon, Dec 4, 2017 at 1:07 PM, Dong > Lin < > > > > > > > > > > > lindon...@gmail.com > > > > > > > > > > > > > > > > > > > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > >> > Ted, thanks for catching this. I have > > > > updated > > > > > > the > > > > > > > > > > sentence > > > > > > > > > > > > to > > > > > > > > > > > > > > make > > > > > > > > > > > > > > > > it > > > > > > > > > > > > > > > > > >> > readable. > > > > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > >> > Thanks, > > > > > > > > > > > > > > > > > >> > Dong > > > > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > >> > On Sat, Dec 2, 2017 at 3:05 PM, Ted > Yu < > > > > > > > > > > > yuzhih...@gmail.com > > > > > > > > > > > > > > > > > > > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > >> > > bq. It the controller_epoch of the > > > > incoming > > > > > > > > > > > > > MetadataResponse, > > > > > > > > > > > > > > or > > > > > > > > > > > > > > > > if > > > > > > > > > > > > > > > > > >> the > > > > > > > > > > > > > > > > > >> > > controller_epoch is the same but the > > > > > > > > > > > > > controller_metadata_epoch > > > > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > > >> > > Can you update the above sentence so > > > that > > > > > the > > > > > > > > > > intention > > > > > > > > > > > is > > > > > > > > > > > > > > > > clearer ? > > > > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > > >> > > Thanks > > > > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > > >> > > On Fri, Dec 1, 2017 at 6:33 PM, Dong > > > Lin < > > > > > > > > > > > > > lindon...@gmail.com > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > > >> > > > Hi all, > > > > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > > > >> > > > I have created KIP-232: Detect > > > outdated > > > > > > > metadata > > > > > > > > > by > > > > > > > > > > > > adding > > > > > > > > > > > > > > > > > >> > > > ControllerMetadataEpoch field: > > > > > > > > > > > > > > > > > >> > > > https://cwiki.apache.org/ > > > > > > > > > > > confluence/display/KAFKA/KIP- > > > > > > > > > > > > > > > > > >> > > > 232%3A+Detect+outdated+ > > > > > metadata+by+adding+ > > > > > > > > > > > > > > > > > >> > ControllerMetadataEpoch+field > > > > > > > > > > > > > > > > > >> > > > . > > > > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > > > >> > > > The KIP proposes to add fields in > > > > > > > > MetadataResponse > > > > > > > > > > and > > > > > > > > > > > > > > > > > >> > > > UpdateMetadataRequest so that > client > > > can > > > > > > > reject > > > > > > > > > > > outdated > > > > > > > > > > > > > > > > metadata > > > > > > > > > > > > > > > > > >> and > > > > > > > > > > > > > > > > > >> > > avoid > > > > > > > > > > > > > > > > > >> > > > unnecessary > > OffsetOutOfRangeException. > > > > > > > Otherwise > > > > > > > > > > there > > > > > > > > > > > > is > > > > > > > > > > > > > > > > > currently > > > > > > > > > > > > > > > > > >> > race > > > > > > > > > > > > > > > > > >> > > > condition that can cause consumer > to > > > > reset > > > > > > > > offset > > > > > > > > > > > which > > > > > > > > > > > > > > > > negatively > > > > > > > > > > > > > > > > > >> > affect > > > > > > > > > > > > > > > > > >> > > > the consumer's availability. > > > > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > > > >> > > > Feedback and suggestions are > > welcome! > > > > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > > > >> > > > Regards, > > > > > > > > > > > > > > > > > >> > > > Dong > > > > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > >
