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 > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > >> > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > >
