Hey Jason, Thanks for the update! I agree with the current proposal overall. I have some minor comments related to naming etc.
1) I am not strong and will just leave it here for discussion. Would it be better to rename "CurrentLeaderEpoch" to "ExpectedLeaderEpoch" for the new field in the OffsetsForLeaderEpochRequest? The reason is that "CurrentLeaderEpoch" may not necessarily be true current leader epoch if the consumer has stale metadata. "ExpectedLeaderEpoch" shows that this epoch is what consumer expects on the broker which may or may not be the true value. 2) Currently we add the field "LeaderEpoch" to FetchRequest and the field "CurrentLeaderEpoch" to OffsetsForLeaderEpochRequest. Given that both fields are compared with the leaderEpoch in the broker, would it be better to give them the same name? 3) Currently LogTruncationException.truncationOffset() returns Optional<OffsetAndMetadata> to user. Should it return Optional<Map<TopicPartition, OffsetAndMetadata>> to handle the scenario where leaderEpoch of multiple partitions are different from the leaderEpoch in the broker? 4) Currently LogTruncationException.truncationOffset() returns an Optional value. Could you explain a bit more when it will return Optional.empty()? I am trying to understand whether it is simpler and reasonable to replace Optional.empty() with OffsetMetadata(offset=last_fetched_offset, leaderEpoch=-1). 5) Do we also need to add a new retriable exception for error code FENCED_LEADER_EPOCH? And do we need to define both FENCED_LEADER_EPOCH and UNKNOWN_LEADER_EPOCH. It seems that the current KIP uses these two error codes in the same way and the exception for these two error codes is not exposed to the user. Maybe we should combine them into one error, e.g. INVALID_LEADER_EPOCH? 6) For users who has turned off auto offset reset, when consumer.poll() throw LogTruncationException, it seems that user will most likely call seekToCommitted(offset, leaderEpoch) where offset and leaderEpoch are obtained from LogTruncationException.truncationOffset(). In this case, the offset used here is not committed, which is inconsistent from the method name seekToCommitted(...). Would it be better to rename the method to e.g. seekToLastConsumedMessage()? 7) Per point 3 in Jun's comment, would it be useful to explicitly specify in the KIP that we will log the truncation event if user has turned on auto offset reset policy? Thanks, Dong On Fri, Jul 27, 2018 at 12:39 PM, Jason Gustafson <ja...@confluent.io> wrote: > Thanks Anna, you are right on both points. I updated the KIP. > > -Jason > > On Thu, Jul 26, 2018 at 2:08 PM, Anna Povzner <a...@confluent.io> wrote: > > > Hi Jason, > > > > Thanks for the update. I agree with the current proposal. > > > > Two minor comments: > > 1) In “API Changes” section, first paragraph says that “users can catch > the > > more specific exception type and use the new `seekToNearest()` API > defined > > below.”. Since LogTruncationException “will include the partitions that > > were truncated and the offset of divergence”., shouldn’t the client use > > seek(offset) to seek to the offset of divergence in response to the > > exception? > > 2) In “Protocol Changes” section, OffsetsForLeaderEpoch subsection says > > “Note > > that consumers will send a sentinel value (-1) for the current epoch and > > the broker will simply disregard that validation.”. Is that still true > with > > MetadataResponse containing leader epoch? > > > > Thanks, > > Anna > > > > On Wed, Jul 25, 2018 at 1:44 PM Jason Gustafson <ja...@confluent.io> > > wrote: > > > > > Hi All, > > > > > > I have made some updates to the KIP. As many of you know, a side > project > > of > > > mine has been specifying the Kafka replication protocol in TLA. You can > > > check out the code here if you are interested: > > > https://github.com/hachikuji/kafka-specification. In addition to > > > uncovering > > > a couple unknown bugs in the replication protocol (e.g. > > > https://issues.apache.org/jira/browse/KAFKA-7128), this has helped me > > > validate the behavior in this KIP. In fact, the original version I > > proposed > > > had a weakness. I initially suggested letting the leader validate the > > > expected epoch at the fetch offset. This made sense for the consumer in > > the > > > handling of unclean leader election, but it was not strong enough to > > > protect the follower in all cases. In order to make advancement of the > > high > > > watermark safe, for example, the leader actually needs to be sure that > > > every follower in the ISR matches its own epoch. > > > > > > I attempted to fix this problem by treating the epoch in the fetch > > request > > > slightly differently for consumers and followers. For consumers, it > would > > > be the expected epoch of the record at the fetch offset, and the leader > > > would raise a LOG_TRUNCATION error if the expectation failed. For > > > followers, it would be the current epoch and the leader would require > > that > > > it match its own epoch. This was unsatisfying both because of the > > > inconsistency in behavior and because the consumer was left with the > > weaker > > > fencing that we already knew was insufficient for the replicas. > > Ultimately > > > I decided that we should make the behavior consistent and that meant > that > > > the consumer needed to act more like a following replica. Instead of > > > checking for truncation while fetching, the consumer should check for > > > truncation after leader changes. After checking for truncation, the > > > consumer can then use the current epoch when fetching and get the > > stronger > > > protection that it provides. What this means is that the Metadata API > > must > > > include the current leader epoch. Given the problems we have had around > > > stale metadata and how challenging they have been to debug, I'm > convinced > > > that this is a good idea in any case and it resolves the inconsistent > > > behavior in the Fetch API. The downside is that there will be some > > > additional overhead upon leader changes, but I don't think it is a > major > > > concern since leader changes are rare and the OffsetForLeaderEpoch > > request > > > is cheap. > > > > > > This approach leaves the door open for some interesting follow up > > > improvements. For example, now that we have the leader epoch in the > > > Metadata request, we can implement similar fencing for the Produce API. > > And > > > now that the consumer can reason about truncation, we could consider > > having > > > a configuration to expose records beyond the high watermark. This would > > let > > > users trade lower end-to-end latency for weaker durability semantics. > It > > is > > > sort of like having an acks=0 option for the consumer. Neither of these > > > options are included in this KIP, I am just mentioning them as > potential > > > work for the future. > > > > > > Finally, based on the discussion in this thread, I have added the > > > seekToCommitted API for the consumer. Please take a look and let me > know > > > what you think. > > > > > > Thanks, > > > Jason > > > > > > On Fri, Jul 20, 2018 at 2:34 PM, Guozhang Wang <wangg...@gmail.com> > > wrote: > > > > > > > Hi Jason, > > > > > > > > The proposed API seems reasonable to me too. Could you please also > > update > > > > the wiki page ( > > > > https://cwiki.apache.org/confluence/display/KAFKA/KIP- > > > > 320%3A+Allow+fetchers+to+detect+and+handle+log+truncation) > > > > with a section say "workflow" on how the proposed API will be co-used > > > with > > > > others to: > > > > > > > > 1. consumer callers handling a LogTruncationException. > > > > 2. consumer internals for handling a retriable > > > UnknownLeaderEpochException. > > > > > > > > > > > > Guozhang > > > > > > > > > > > > On Tue, Jul 17, 2018 at 10:23 AM, Anna Povzner <a...@confluent.io> > > > wrote: > > > > > > > > > Hi Jason, > > > > > > > > > > > > > > > I also like your proposal and agree that > > > KafkaConsumer#seekToCommitted() > > > > > is > > > > > more intuitive as a way to initialize both consumer's position and > > its > > > > > fetch state. > > > > > > > > > > > > > > > My understanding that KafkaConsumer#seekToCommitted() is purely > for > > > > > clients > > > > > who store their offsets externally, right? And we are still going > to > > > > > add KafkaConsumer#findOffsets() > > > > > in this KIP as we discussed, so that the client can handle > > > > > LogTruncationException? > > > > > > > > > > > > > > > Thanks, > > > > > > > > > > Anna > > > > > > > > > > > > > > > On Thu, Jul 12, 2018 at 3:57 PM Dong Lin <lindon...@gmail.com> > > wrote: > > > > > > > > > > > Hey Jason, > > > > > > > > > > > > It is a great summary. The solution sounds good. I might have > minor > > > > > > comments regarding the method name. But we can discuss that minor > > > > points > > > > > > later after we reach consensus on the high level API. > > > > > > > > > > > > Thanks, > > > > > > Dong > > > > > > > > > > > > > > > > > > On Thu, Jul 12, 2018 at 11:41 AM, Jason Gustafson < > > > ja...@confluent.io> > > > > > > wrote: > > > > > > > > > > > > > Hey Anna and Dong, > > > > > > > > > > > > > > Thanks a lot for the great discussion. I've been hanging back a > > bit > > > > > > because > > > > > > > honestly the best option hasn't seemed clear. I agree with > Anna's > > > > > general > > > > > > > observation that there is a distinction between the position of > > the > > > > > > > consumer and its fetch state up to that position. If you think > > > about > > > > > it, > > > > > > a > > > > > > > committed offset actually represents both of these. The > metadata > > is > > > > > used > > > > > > to > > > > > > > initialize the state of the consumer application and the offset > > > > > > initializes > > > > > > > the position. Additionally, we are extending the offset commit > in > > > > this > > > > > > KIP > > > > > > > to also include the last epoch fetched by the consumer, which > is > > > used > > > > > to > > > > > > > initialize the internal fetch state. Of course if you do an > > > arbitrary > > > > > > > `seek` and immediately commit offsets, then there won't be a > last > > > > epoch > > > > > > to > > > > > > > commit. This seems intuitive since there is no fetch state in > > this > > > > > case. > > > > > > We > > > > > > > only commit fetch state when we have it. > > > > > > > > > > > > > > So if we think about a committed offset as initializing both > the > > > > > > consumer's > > > > > > > position and its fetch state, then the gap in the API is > > evidently > > > > that > > > > > > we > > > > > > > don't have a way to initialize the consumer to a committed > > offset. > > > We > > > > > do > > > > > > it > > > > > > > implicitly of course for offsets stored in Kafka, but since > > > external > > > > > > > storage is a use case we support, then we should have an > explicit > > > API > > > > > as > > > > > > > well. Perhaps something like this: > > > > > > > > > > > > > > seekToCommitted(TopicPartition, OffsetAndMetadata) > > > > > > > > > > > > > > In this KIP, we are proposing to allow the `OffsetAndMetadata` > > > object > > > > > to > > > > > > > include the leader epoch, so I think this would have the same > > > effect > > > > as > > > > > > > Anna's suggested `seekToRecord`. But perhaps it is a more > natural > > > fit > > > > > > given > > > > > > > the current API? Furthermore, if we find a need for additional > > > > metadata > > > > > > in > > > > > > > the offset commit API in the future, then we will just need to > > > modify > > > > > the > > > > > > > `OffsetAndMetadata` object and we will not need a new `seek` > API. > > > > > > > > > > > > > > With this approach, I think then we can leave the `position` > API > > as > > > > it > > > > > > is. > > > > > > > The position of the consumer is still just the next expected > > fetch > > > > > > offset. > > > > > > > If a user needs to record additional state based on previous > > fetch > > > > > > > progress, then they would use the result of the previous fetch > to > > > > > obtain > > > > > > > it. This makes the dependence on fetch progress explicit. I > think > > > we > > > > > > could > > > > > > > make this a little more convenience with a helper in the > > > > > > `ConsumerRecords` > > > > > > > object, but I think that's more of a nice-to-have. > > > > > > > > > > > > > > Thoughts? > > > > > > > > > > > > > > By the way, I have been iterating a little bit on the replica > > side > > > of > > > > > > this > > > > > > > KIP. My initial proposal in fact did not have strong enough > > fencing > > > > to > > > > > > > protect all of the edge cases. I believe the current proposal > > fixes > > > > the > > > > > > > problems, but I am still verifying the model. > > > > > > > > > > > > > > Thanks, > > > > > > > Jason > > > > > > > > > > > > > > > > > > > > > On Wed, Jul 11, 2018 at 10:45 AM, Dong Lin < > lindon...@gmail.com> > > > > > wrote: > > > > > > > > > > > > > > > Hey Anna, > > > > > > > > > > > > > > > > Thanks much for the explanation. Approach 1 also sounds good > to > > > > me. I > > > > > > > think > > > > > > > > findOffsets() is useful for users who don't use automatic > > offset > > > > > reset > > > > > > > > policy. > > > > > > > > > > > > > > > > Just one more question. Since users who store offsets > > externally > > > > need > > > > > > to > > > > > > > > provide leaderEpoch to findOffsets(...), do we need an extra > > API > > > > for > > > > > > user > > > > > > > > to get both offset and leaderEpoch, e.g. recordPosition()? > > > > > > > > > > > > > > > > Thanks, > > > > > > > > Dong > > > > > > > > > > > > > > > > On Wed, Jul 11, 2018 at 10:12 AM, Anna Povzner < > > > a...@confluent.io> > > > > > > > wrote: > > > > > > > > > > > > > > > > > Hi Dong, > > > > > > > > > > > > > > > > > > > > > > > > > > > What I called “not covering all use cases” is what you call > > > > > > best-effort > > > > > > > > > (not guaranteeing some corner cases). I think we are on the > > > same > > > > > page > > > > > > > > here. > > > > > > > > > > > > > > > > > > > > > > > > > > > I wanted to be clear in the API whether the consumer seeks > > to a > > > > > > > position > > > > > > > > > (offset) or to a record (offset, leader epoch). The only > > > use-case > > > > > of > > > > > > > > > seeking to a record is seeking to a committed offset for a > > user > > > > who > > > > > > > > stores > > > > > > > > > committed offsets externally. (Unless users find some other > > > > reason > > > > > to > > > > > > > > seek > > > > > > > > > to a record.) I thought it was possible to provide this > > > > > functionality > > > > > > > > with > > > > > > > > > findOffset(offset, leader epoch) followed by a > seek(offset). > > > > > However, > > > > > > > you > > > > > > > > > are right that this will not handle the race condition > where > > > > > > > > non-divergent > > > > > > > > > offset found by findOffset() could change again before the > > > > consumer > > > > > > > does > > > > > > > > > the first fetch. > > > > > > > > > > > > > > > > > > > > > > > > > > > Regarding position() — if we add position that returns > > (offset, > > > > > > leader > > > > > > > > > epoch), this is specifically a position after a record that > > was > > > > > > > actually > > > > > > > > > consumed or position of a committed record. In which case, > I > > > > still > > > > > > > think > > > > > > > > > it’s cleaner to get a record position of consumed message > > from > > > a > > > > > new > > > > > > > > helper > > > > > > > > > method in ConsumerRecords() or from committed offsets. > > > > > > > > > > > > > > > > > > > > > > > > > > > I think all the use-cases could be then covered with: > > > > > > > > > > > > > > > > > > (Approach 1) > > > > > > > > > > > > > > > > > > seekToRecord(offset, leaderEpoch) — this will just > > > initialize/set > > > > > the > > > > > > > > > consumer state; > > > > > > > > > > > > > > > > > > findOffsets(offset, leaderEpoch) returns {offset, > > leaderEpoch} > > > > > > > > > > > > > > > > > > > > > > > > > > > If we agree that the race condition is also a corner case, > > > then I > > > > > > think > > > > > > > > we > > > > > > > > > can cover use-cases with: > > > > > > > > > > > > > > > > > > (Approach 2) > > > > > > > > > > > > > > > > > > findOffsets(offset, leaderEpoch) returns offset — we still > > want > > > > > > leader > > > > > > > > > epoch as a parameter for the users who store their > committed > > > > > offsets > > > > > > > > > externally. > > > > > > > > > > > > > > > > > > > > > > > > > > > I am actually now leaning more to approach 1, since it is > > more > > > > > > > explicit, > > > > > > > > > and maybe there are more use cases for it. > > > > > > > > > > > > > > > > > > > > > > > > > > > Thanks, > > > > > > > > > > > > > > > > > > Anna > > > > > > > > > > > > > > > > > > > > > > > > > > > On Tue, Jul 10, 2018 at 3:47 PM Dong Lin < > > lindon...@gmail.com> > > > > > > wrote: > > > > > > > > > > > > > > > > > > > Hey Anna, > > > > > > > > > > > > > > > > > > > > Thanks for the comment. To answer your question, it seems > > > that > > > > we > > > > > > can > > > > > > > > > cover > > > > > > > > > > all case in this KIP. As stated in "Consumer Handling" > > > section, > > > > > > > KIP-101 > > > > > > > > > > based approach will be used to derive the truncation > offset > > > > from > > > > > > the > > > > > > > > > > 2-tuple (offset, leaderEpoch). This approach is best > effort > > > and > > > > > it > > > > > > is > > > > > > > > > > inaccurate only in very rare scenarios (as described in > > > > KIP-279). > > > > > > > > > > > > > > > > > > > > By using seek(offset, leaderEpoch), consumer will still > be > > > able > > > > > to > > > > > > > > follow > > > > > > > > > > this best-effort approach to detect log truncation and > > > > determine > > > > > > the > > > > > > > > > > truncation offset. On the other hand, if we use > > seek(offset), > > > > > > > consumer > > > > > > > > > will > > > > > > > > > > not detect log truncation in some cases which weakens the > > > > > guarantee > > > > > > > of > > > > > > > > > this > > > > > > > > > > KIP. Does this make sense? > > > > > > > > > > > > > > > > > > > > Thanks, > > > > > > > > > > Dong > > > > > > > > > > > > > > > > > > > > On Tue, Jul 10, 2018 at 3:14 PM, Anna Povzner < > > > > a...@confluent.io > > > > > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > Sorry, I hit "send" before finishing. Continuing... > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > 2) Hiding most of the consumer handling log truncation > > > logic > > > > > with > > > > > > > > > minimal > > > > > > > > > > > exposure in KafkaConsumer API. I was proposing this > > path. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Before answering your specific questions… I want to > > answer > > > to > > > > > > your > > > > > > > > > > comment > > > > > > > > > > > “In general, maybe we should discuss the final solution > > > that > > > > > > covers > > > > > > > > all > > > > > > > > > > > cases?”. With current KIP, we don’t cover all cases of > > > > consumer > > > > > > > > > detecting > > > > > > > > > > > log truncation because the KIP proposes a leader epoch > > > cache > > > > in > > > > > > > > > consumer > > > > > > > > > > > that does not persist across restarts. Plus, we only > > store > > > > last > > > > > > > > > committed > > > > > > > > > > > offset (either internally or users can store > externally). > > > > This > > > > > > has > > > > > > > a > > > > > > > > > > > limitation that the consumer will not always be able to > > > find > > > > > > point > > > > > > > of > > > > > > > > > > > truncation just because we have a limited history (just > > one > > > > > data > > > > > > > > > point). > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > So, maybe we should first agree on whether we accept > that > > > > > storing > > > > > > > > last > > > > > > > > > > > committed offset/leader epoch has a limitation that the > > > > > consumer > > > > > > > will > > > > > > > > > not > > > > > > > > > > > be able to detect log truncation in all cases? > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Thanks, > > > > > > > > > > > > > > > > > > > > > > Anna > > > > > > > > > > > > > > > > > > > > > > On Tue, Jul 10, 2018 at 2:20 PM Anna Povzner < > > > > > a...@confluent.io> > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > > > Hi Dong, > > > > > > > > > > > > > > > > > > > > > > > > Thanks for the follow up! I finally have much more > > clear > > > > > > > > > understanding > > > > > > > > > > of > > > > > > > > > > > > where you are coming from. > > > > > > > > > > > > > > > > > > > > > > > > You are right. The success of findOffsets()/finding a > > > point > > > > > of > > > > > > > > > > > > non-divergence depends on whether we have enough > > entries > > > in > > > > > the > > > > > > > > > > > consumer's > > > > > > > > > > > > leader epoch cache. However, I think this is a > > > fundamental > > > > > > > > limitation > > > > > > > > > > of > > > > > > > > > > > > having a leader epoch cache that does not persist > > across > > > > > > consumer > > > > > > > > > > > restarts. > > > > > > > > > > > > > > > > > > > > > > > > If we consider the general case where consumer may or > > may > > > > not > > > > > > > have > > > > > > > > > this > > > > > > > > > > > > cache, then I see two paths: > > > > > > > > > > > > 1) Letting the user to track the leader epoch history > > > > > > externally, > > > > > > > > and > > > > > > > > > > > have > > > > > > > > > > > > more exposure to leader epoch and finding point of > > > > > > non-divergence > > > > > > > > in > > > > > > > > > > > > KafkaConsumer API. I understand this is the case you > > were > > > > > > talking > > > > > > > > > > about. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > On Tue, Jul 10, 2018 at 12:16 PM Dong Lin < > > > > > lindon...@gmail.com > > > > > > > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > > > >> Hey Anna, > > > > > > > > > > > >> > > > > > > > > > > > >> Thanks much for your detailed explanation and > example! > > > It > > > > > does > > > > > > > > help > > > > > > > > > me > > > > > > > > > > > >> understand the difference between our understanding. > > > > > > > > > > > >> > > > > > > > > > > > >> So it seems that the solution based on findOffsets() > > > > > currently > > > > > > > > > focuses > > > > > > > > > > > >> mainly on the scenario that consumer has cached > > > > leaderEpoch > > > > > -> > > > > > > > > > offset > > > > > > > > > > > >> mapping whereas I was thinking about the general > case > > > > where > > > > > > > > consumer > > > > > > > > > > may > > > > > > > > > > > >> or > > > > > > > > > > > >> may not have this cache. I guess that is why we have > > > > > different > > > > > > > > > > > >> understanding here. I have some comments below. > > > > > > > > > > > >> > > > > > > > > > > > >> > > > > > > > > > > > >> 3) The proposed solution using findOffsets(offset, > > > > > > leaderEpoch) > > > > > > > > > > followed > > > > > > > > > > > >> by > > > > > > > > > > > >> seek(offset) works if consumer has the cached > > > leaderEpoch > > > > -> > > > > > > > > offset > > > > > > > > > > > >> mapping. But if we assume consumer has this cache, > do > > we > > > > > need > > > > > > to > > > > > > > > > have > > > > > > > > > > > >> leaderEpoch in the findOffsets(...)? Intuitively, > the > > > > > > > > > > > findOffsets(offset) > > > > > > > > > > > >> can also derive the leaderEpoch using offset just > like > > > the > > > > > > > > proposed > > > > > > > > > > > >> solution does with seek(offset). > > > > > > > > > > > >> > > > > > > > > > > > >> > > > > > > > > > > > >> 4) If consumer does not have cached leaderEpoch -> > > > offset > > > > > > > mapping, > > > > > > > > > > which > > > > > > > > > > > >> is > > > > > > > > > > > >> the case if consumer is restarted on a new machine, > > then > > > > it > > > > > is > > > > > > > not > > > > > > > > > > clear > > > > > > > > > > > >> what leaderEpoch would be included in the > FetchRequest > > > if > > > > > > > consumer > > > > > > > > > > does > > > > > > > > > > > >> seek(offset). This is the case that motivates the > > first > > > > > > question > > > > > > > > of > > > > > > > > > > the > > > > > > > > > > > >> previous email. In general, maybe we should discuss > > the > > > > > final > > > > > > > > > solution > > > > > > > > > > > >> that > > > > > > > > > > > >> covers all cases? > > > > > > > > > > > >> > > > > > > > > > > > >> > > > > > > > > > > > >> 5) The second question in my previous email is > related > > > to > > > > > the > > > > > > > > > > following > > > > > > > > > > > >> paragraph: > > > > > > > > > > > >> > > > > > > > > > > > >> "... In some cases, offsets returned from position() > > > could > > > > > be > > > > > > > > actual > > > > > > > > > > > >> consumed messages by this consumer identified by > > > {offset, > > > > > > leader > > > > > > > > > > epoch}. > > > > > > > > > > > >> In > > > > > > > > > > > >> other cases, position() returns offset that was not > > > > actually > > > > > > > > > consumed. > > > > > > > > > > > >> Suppose, the user calls position() for the last > > > > offset...". > > > > > > > > > > > >> > > > > > > > > > > > >> I guess my point is that, if user calls position() > for > > > the > > > > > > last > > > > > > > > > offset > > > > > > > > > > > and > > > > > > > > > > > >> uses that offset in seek(...), then user can > probably > > > just > > > > > > call > > > > > > > > > > > >> Consumer#seekToEnd() without calling position() and > > > > > seek(...). > > > > > > > > > > Similarly > > > > > > > > > > > >> user can call Consumer#seekToBeginning() to the seek > > to > > > > the > > > > > > > > earliest > > > > > > > > > > > >> position without calling position() and seek(...). > > Thus > > > > > > > position() > > > > > > > > > > only > > > > > > > > > > > >> needs to return the actual consumed messages > > identified > > > by > > > > > > > > {offset, > > > > > > > > > > > leader > > > > > > > > > > > >> epoch}. Does this make sense? > > > > > > > > > > > >> > > > > > > > > > > > >> > > > > > > > > > > > >> Thanks, > > > > > > > > > > > >> Dong > > > > > > > > > > > >> > > > > > > > > > > > >> > > > > > > > > > > > >> On Mon, Jul 9, 2018 at 6:47 PM, Anna Povzner < > > > > > > a...@confluent.io > > > > > > > > > > > > > > > > > > wrote: > > > > > > > > > > > >> > > > > > > > > > > > >> > Hi Dong, > > > > > > > > > > > >> > > > > > > > > > > > > >> > > > > > > > > > > > > >> > Thanks for considering my suggestions. > > > > > > > > > > > >> > > > > > > > > > > > > >> > > > > > > > > > > > > >> > Based on your comments, I realized that my > > suggestion > > > > was > > > > > > not > > > > > > > > > > complete > > > > > > > > > > > >> with > > > > > > > > > > > >> > regard to KafkaConsumer API vs. consumer-broker > > > > protocol. > > > > > > > While > > > > > > > > I > > > > > > > > > > > >> propose > > > > > > > > > > > >> > to keep KafkaConsumer#seek() unchanged and take > > offset > > > > > only, > > > > > > > the > > > > > > > > > > > >> underlying > > > > > > > > > > > >> > consumer will send the next FetchRequest() to > broker > > > > with > > > > > > > offset > > > > > > > > > and > > > > > > > > > > > >> > leaderEpoch if it is known (based on leader epoch > > > cache > > > > in > > > > > > > > > > consumer) — > > > > > > > > > > > >> note > > > > > > > > > > > >> > that this is different from the current KIP, which > > > > > suggests > > > > > > to > > > > > > > > > > always > > > > > > > > > > > >> send > > > > > > > > > > > >> > unknown leader epoch after seek(). This way, if > the > > > > > consumer > > > > > > > > and a > > > > > > > > > > > >> broker > > > > > > > > > > > >> > agreed on the point of non-divergence, which is > some > > > > > > {offset, > > > > > > > > > > > >> leaderEpoch} > > > > > > > > > > > >> > pair, the new leader which causes another > truncation > > > > (even > > > > > > > > further > > > > > > > > > > > back) > > > > > > > > > > > >> > will be able to detect new divergence and restart > > the > > > > > > process > > > > > > > of > > > > > > > > > > > finding > > > > > > > > > > > >> > the new point of non-divergence. So, to answer > your > > > > > > question, > > > > > > > If > > > > > > > > > the > > > > > > > > > > > >> > truncation happens just after the user calls > > > > > > > > > > > >> > KafkaConsumer#findOffsets(offset, leaderEpoch) > > > followed > > > > > by > > > > > > > > > > > seek(offset), > > > > > > > > > > > >> > the user will not seek to the wrong position > without > > > > > knowing > > > > > > > > that > > > > > > > > > > > >> > truncation has happened, because the consumer will > > get > > > > > > another > > > > > > > > > > > >> truncation > > > > > > > > > > > >> > error, and seek again. > > > > > > > > > > > >> > > > > > > > > > > > > >> > > > > > > > > > > > > >> > I am afraid, I did not understand your second > > > question. > > > > > Let > > > > > > me > > > > > > > > > > > >> summarize my > > > > > > > > > > > >> > suggestions again, and then give an example to > > > hopefully > > > > > > make > > > > > > > my > > > > > > > > > > > >> > suggestions more clear. Also, the last part of my > > > > example > > > > > > > shows > > > > > > > > > how > > > > > > > > > > > the > > > > > > > > > > > >> > use-case in your first question will work. If it > > does > > > > not > > > > > > > answer > > > > > > > > > > your > > > > > > > > > > > >> > second question, would you mind clarifying? I am > > also > > > > > > focusing > > > > > > > > on > > > > > > > > > > the > > > > > > > > > > > >> case > > > > > > > > > > > >> > of a consumer having enough entries in the cache. > > The > > > > case > > > > > > of > > > > > > > > > > > restarting > > > > > > > > > > > >> > from committed offset either stored externally or > > > > > internally > > > > > > > > will > > > > > > > > > > > >> probably > > > > > > > > > > > >> > need to be discussed more. > > > > > > > > > > > >> > > > > > > > > > > > > >> > > > > > > > > > > > > >> > Let me summarize my suggestion again: > > > > > > > > > > > >> > > > > > > > > > > > > >> > 1) KafkaConsumer#seek() and > KafkaConsumer#position() > > > > > remains > > > > > > > > > > unchanged > > > > > > > > > > > >> > > > > > > > > > > > > >> > 2) New KafkaConsumer#findOffsets() takes {offset, > > > > > > leaderEpoch} > > > > > > > > > pair > > > > > > > > > > > per > > > > > > > > > > > >> > topic partition and returns offset per topic > > > partition. > > > > > > > > > > > >> > > > > > > > > > > > > >> > 3) FetchRequest() to broker after > > KafkaConsumer#seek() > > > > > will > > > > > > > > > contain > > > > > > > > > > > the > > > > > > > > > > > >> > offset set by seek and leaderEpoch that > corresponds > > to > > > > the > > > > > > > > offset > > > > > > > > > > > based > > > > > > > > > > > >> on > > > > > > > > > > > >> > leader epoch cache in the consumer. > > > > > > > > > > > >> > > > > > > > > > > > > >> > > > > > > > > > > > > >> > The rest of this e-mail is a long and contrived > > > example > > > > > with > > > > > > > > > several > > > > > > > > > > > log > > > > > > > > > > > >> > truncations and unclean leader elections to > > illustrate > > > > the > > > > > > API > > > > > > > > and > > > > > > > > > > > your > > > > > > > > > > > >> > first use-case. Suppose we have three brokers. > > > > Initially, > > > > > > > Broker > > > > > > > > > A, > > > > > > > > > > B, > > > > > > > > > > > >> and > > > > > > > > > > > >> > C has one message at offset 0 with leader epoch 0. > > > Then, > > > > > > > Broker > > > > > > > > A > > > > > > > > > > goes > > > > > > > > > > > >> down > > > > > > > > > > > >> > for some time. Broker B becomes a leader with > epoch > > 1, > > > > and > > > > > > > > writes > > > > > > > > > > > >> messages > > > > > > > > > > > >> > to offsets 1 and 2. Broker C fetches offset 1, but > > > > before > > > > > > > > fetching > > > > > > > > > > > >> offset > > > > > > > > > > > >> > 2, becomes a leader with leader epoch 2 and > writes a > > > > > message > > > > > > > at > > > > > > > > > > offset > > > > > > > > > > > >> 2. > > > > > > > > > > > >> > Here is the state of brokers at this point: > > > > > > > > > > > >> > > > > > > > > > > > > >> > > Broker A: > > > > > > > > > > > >> > > offset 0, epoch 0 <— leader > > > > > > > > > > > >> > > goes down… > > > > > > > > > > > >> > > > > > > > > > > > > >> > > > > > > > > > > > > >> > > Broker B: > > > > > > > > > > > >> > > offset 0, epoch 0 > > > > > > > > > > > >> > > offset 1, epoch 1 <- leader > > > > > > > > > > > >> > > offset 2, epoch 1 > > > > > > > > > > > >> > > > > > > > > > > > > >> > > > > > > > > > > > > >> > > > > > > > > > > > > >> > Broker C: > > > > > > > > > > > >> > > offset 0, epoch 0 > > > > > > > > > > > >> > > offset 1, epoch 1 > > > > > > > > > > > >> > > offset 2, epoch 2 <— leader > > > > > > > > > > > >> > > > > > > > > > > > > >> > > > > > > > > > > > > >> > Before Broker C becomes a leader with leader epoch > > 2, > > > > the > > > > > > > > consumer > > > > > > > > > > > >> consumed > > > > > > > > > > > >> > the following messages from broker A and broker B: > > > > > > > > > > > >> > > > > > > > > > > > > >> > {offset=0, leaderEpoch=0}, {offset=1, > > leaderEpoch=1}, > > > > > > > {offset=2, > > > > > > > > > > > >> > leaderEpoch=1}. > > > > > > > > > > > >> > > > > > > > > > > > > >> > Consumer’s leader epoch cache at this point > contains > > > the > > > > > > > > following > > > > > > > > > > > >> entries: > > > > > > > > > > > >> > > > > > > > > > > > > >> > (leaderEpoch=0, startOffset=0) > > > > > > > > > > > >> > > > > > > > > > > > > >> > (leaderEpoch=1, startOffset=1) > > > > > > > > > > > >> > > > > > > > > > > > > >> > endOffset = 3 > > > > > > > > > > > >> > > > > > > > > > > > > >> > > > > > > > > > > > > >> > Then, broker B becomes the follower of broker C, > > > > truncates > > > > > > and > > > > > > > > > > starts > > > > > > > > > > > >> > fetching from offset 2. > > > > > > > > > > > >> > > > > > > > > > > > > >> > Consumer sends fetchRequest(offset=3, > leaderEpoch=1) > > > and > > > > > > gets > > > > > > > > > > > >> > LOG_TRUNCATION > > > > > > > > > > > >> > error from broker C. > > > > > > > > > > > >> > > > > > > > > > > > > >> > In response, the client calls > > > KafkaConsumer#findOffsets( > > > > > > > > offset=3, > > > > > > > > > > > >> > leaderEpoch=1). The underlying consumer sends > > > > > > > > > > > >> > OffsetsForLeaderEpoch(leaderEpoch=1), broker C > > > responds > > > > > with > > > > > > > > > > > >> > {leaderEpoch=1, endOffset=2}. So, > > > > > > > > > > KafkaConsumer#findOffsets(offset=3, > > > > > > > > > > > >> > leaderEpoch=1) returns offset=2. > > > > > > > > > > > >> > > > > > > > > > > > > >> > In response, consumer calls KafkaConsumer@seek > > > > (offset=2) > > > > > > > > followed > > > > > > > > > > by > > > > > > > > > > > >> > poll(), which results in FetchRequest(offset=2, > > > > > > leaderEpoch=1) > > > > > > > > to > > > > > > > > > > > >> broker C. > > > > > > > > > > > >> > > > > > > > > > > > > >> > > > > > > > > > > > > >> > I will continue with this example with the goal to > > > > answer > > > > > > your > > > > > > > > > first > > > > > > > > > > > >> > question about truncation just after findOffsets() > > > > > followed > > > > > > by > > > > > > > > > > seek(): > > > > > > > > > > > >> > > > > > > > > > > > > >> > Suppose, brokers B and C go down, and broker A > comes > > > up > > > > > and > > > > > > > > > becomes > > > > > > > > > > a > > > > > > > > > > > >> > leader with leader epoch 3, and writes a message > to > > > > offset > > > > > > 1. > > > > > > > > > > Suppose, > > > > > > > > > > > >> this > > > > > > > > > > > >> > happens before the consumer gets response from > > broker > > > C > > > > to > > > > > > the > > > > > > > > > > > previous > > > > > > > > > > > >> > fetch request: FetchRequest(offset=2, > > leaderEpoch=1). > > > > > > > > > > > >> > > > > > > > > > > > > >> > Consumer re-sends FetchRequest(offset=2, > > > leaderEpoch=1) > > > > to > > > > > > > > broker > > > > > > > > > A, > > > > > > > > > > > >> which > > > > > > > > > > > >> > returns LOG_TRUNCATION error, because broker A has > > > > leader > > > > > > > epoch > > > > > > > > 3 > > > > > > > > > > > > > > > > > > > > > >> leader > > > > > > > > > > > >> > epoch in FetchRequest with starting offset = 1 < > > > offset > > > > 2 > > > > > in > > > > > > > > > > > >> > FetchRequest(). > > > > > > > > > > > >> > > > > > > > > > > > > >> > In response, the user calls > > KafkaConsumer#findOffsets( > > > > > > > offset=2, > > > > > > > > > > > >> > leaderEpoch=1). The underlying consumer sends > > > > > > > > > > > >> > OffsetsForLeaderEpoch(leaderEpoch=1), broker A > > > responds > > > > > with > > > > > > > > > > > >> > {leaderEpoch=0, endOffset=1}; the underlying > > consumer > > > > > finds > > > > > > > > > > > leaderEpoch > > > > > > > > > > > >> = 0 > > > > > > > > > > > >> > in its cache with end offset == 1, which results > in > > > > > > > > > > > >> > KafkaConsumer#findOffsets(offset=2, > leaderEpoch=1) > > > > > returning > > > > > > > > > offset > > > > > > > > > > > = 1. > > > > > > > > > > > >> > > > > > > > > > > > > >> > In response, the user calls KafkaConsumer@seek > > > > (offset=1) > > > > > > > > followed > > > > > > > > > > by > > > > > > > > > > > >> > poll(), which results in FetchRequest(offset=1, > > > > > > leaderEpoch=0) > > > > > > > > to > > > > > > > > > > > >> broker A, > > > > > > > > > > > >> > which responds with message at offset 1, leader > > epoch > > > 3. > > > > > > > > > > > >> > > > > > > > > > > > > >> > > > > > > > > > > > > >> > I will think some more about consumers restarting > > from > > > > > > > committed > > > > > > > > > > > >> offsets, > > > > > > > > > > > >> > and send a follow up. > > > > > > > > > > > >> > > > > > > > > > > > > >> > > > > > > > > > > > > >> > Thanks, > > > > > > > > > > > >> > > > > > > > > > > > > >> > Anna > > > > > > > > > > > >> > > > > > > > > > > > > >> > > > > > > > > > > > > >> > On Sat, Jul 7, 2018 at 1:36 AM Dong Lin < > > > > > > lindon...@gmail.com> > > > > > > > > > > wrote: > > > > > > > > > > > >> > > > > > > > > > > > > >> > > Hey Anna, > > > > > > > > > > > >> > > > > > > > > > > > > > >> > > Thanks much for the thoughtful reply. It makes > > sense > > > > to > > > > > > > > > different > > > > > > > > > > > >> between > > > > > > > > > > > >> > > "seeking to a message" and "seeking to a > > position". > > > I > > > > > have > > > > > > > to > > > > > > > > > > > >> questions > > > > > > > > > > > >> > > here: > > > > > > > > > > > >> > > > > > > > > > > > > > >> > > - For "seeking to a message" use-case, with the > > > > proposed > > > > > > > > > approach > > > > > > > > > > > user > > > > > > > > > > > >> > > needs to call findOffset(offset, leaderEpoch) > > > followed > > > > > by > > > > > > > > > > > >> seek(offset). > > > > > > > > > > > >> > If > > > > > > > > > > > >> > > message truncation and message append happen > > > > immediately > > > > > > > after > > > > > > > > > > > >> > > findOffset(offset, > > > > > > > > > > > >> > > leaderEpoch) but before seek(offset), it seems > > that > > > > user > > > > > > > will > > > > > > > > > seek > > > > > > > > > > > to > > > > > > > > > > > >> the > > > > > > > > > > > >> > > wrong message without knowing the truncation has > > > > > happened. > > > > > > > > Would > > > > > > > > > > > this > > > > > > > > > > > >> be > > > > > > > > > > > >> > a > > > > > > > > > > > >> > > problem? > > > > > > > > > > > >> > > > > > > > > > > > > > >> > > - For "seeking to a position" use-case, it seems > > > that > > > > > > there > > > > > > > > can > > > > > > > > > be > > > > > > > > > > > two > > > > > > > > > > > >> > > positions, i.e. earliest and latest. So these > two > > > > cases > > > > > > can > > > > > > > be > > > > > > > > > > > >> > > Consumer.fulfilled by seekToBeginning() and > > > > > > > > > Consumer.seekToEnd(). > > > > > > > > > > > >> Then it > > > > > > > > > > > >> > > seems that user will only need to call > position() > > > and > > > > > > seek() > > > > > > > > for > > > > > > > > > > > >> "seeking > > > > > > > > > > > >> > > to a message" use-case? > > > > > > > > > > > >> > > > > > > > > > > > > > >> > > Thanks, > > > > > > > > > > > >> > > Dong > > > > > > > > > > > >> > > > > > > > > > > > > > >> > > > > > > > > > > > > > >> > > On Wed, Jul 4, 2018 at 12:33 PM, Anna Povzner < > > > > > > > > > a...@confluent.io> > > > > > > > > > > > >> wrote: > > > > > > > > > > > >> > > > > > > > > > > > > > >> > > > Hi Jason and Dong, > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > I’ve been thinking about your suggestions and > > > > > discussion > > > > > > > > > > regarding > > > > > > > > > > > >> > > > position(), seek(), and new proposed API. > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > Here is my thought process why we should keep > > > > > position() > > > > > > > and > > > > > > > > > > > seek() > > > > > > > > > > > >> API > > > > > > > > > > > >> > > > unchanged. > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > I think we should separate {offset, leader > > epoch} > > > > that > > > > > > > > > uniquely > > > > > > > > > > > >> > > identifies > > > > > > > > > > > >> > > > a message from an offset that is a position. > In > > > some > > > > > > > cases, > > > > > > > > > > > offsets > > > > > > > > > > > >> > > > returned from position() could be actual > > consumed > > > > > > messages > > > > > > > > by > > > > > > > > > > this > > > > > > > > > > > >> > > consumer > > > > > > > > > > > >> > > > identified by {offset, leader epoch}. In other > > > > cases, > > > > > > > > > position() > > > > > > > > > > > >> > returns > > > > > > > > > > > >> > > > offset that was not actually consumed. > Suppose, > > > the > > > > > user > > > > > > > > calls > > > > > > > > > > > >> > position() > > > > > > > > > > > >> > > > for the last offset. Suppose we return > {offset, > > > > leader > > > > > > > > epoch} > > > > > > > > > of > > > > > > > > > > > the > > > > > > > > > > > >> > > > message currently in the log. Then, the > message > > > gets > > > > > > > > truncated > > > > > > > > > > > >> before > > > > > > > > > > > >> > > > consumer’s first poll(). It does not make > sense > > > for > > > > > > poll() > > > > > > > > to > > > > > > > > > > fail > > > > > > > > > > > >> in > > > > > > > > > > > >> > > this > > > > > > > > > > > >> > > > case, because the log truncation did not > > actually > > > > > happen > > > > > > > > from > > > > > > > > > > the > > > > > > > > > > > >> > > consumer > > > > > > > > > > > >> > > > perspective. On the other hand, as the KIP > > > proposes, > > > > > it > > > > > > > > makes > > > > > > > > > > > sense > > > > > > > > > > > >> for > > > > > > > > > > > >> > > the > > > > > > > > > > > >> > > > committed() method to return {offset, leader > > > epoch} > > > > > > > because > > > > > > > > > > those > > > > > > > > > > > >> > offsets > > > > > > > > > > > >> > > > represent actual consumed messages. > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > The same argument applies to the seek() > method — > > > we > > > > > are > > > > > > > not > > > > > > > > > > > seeking > > > > > > > > > > > >> to > > > > > > > > > > > >> > a > > > > > > > > > > > >> > > > message, we are seeking to a position. > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > I like the proposal to add > > > > KafkaConsumer#findOffsets() > > > > > > > API. > > > > > > > > I > > > > > > > > > am > > > > > > > > > > > >> > assuming > > > > > > > > > > > >> > > > something like: > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > Map<TopicPartition, Long> > > > > > > findOffsets(Map<TopicPartition, > > > > > > > > > > > >> > OffsetAndEpoch> > > > > > > > > > > > >> > > > offsetsToSearch) > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > Similar to seek() and position(), I think > > > > > findOffsets() > > > > > > > > should > > > > > > > > > > > >> return > > > > > > > > > > > >> > > > offset without leader epoch, because what we > > want > > > is > > > > > the > > > > > > > > > offset > > > > > > > > > > > >> that we > > > > > > > > > > > >> > > > think is closest to the not divergent message > > from > > > > the > > > > > > > given > > > > > > > > > > > >> consumed > > > > > > > > > > > >> > > > message. Until the consumer actually fetches > the > > > > > > message, > > > > > > > we > > > > > > > > > > > should > > > > > > > > > > > >> not > > > > > > > > > > > >> > > let > > > > > > > > > > > >> > > > the consumer store the leader epoch for a > > message > > > it > > > > > did > > > > > > > not > > > > > > > > > > > >> consume. > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > So, the workflow will be: > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > 1) The user gets LogTruncationException with > > > > {offset, > > > > > > > leader > > > > > > > > > > epoch > > > > > > > > > > > >> of > > > > > > > > > > > >> > the > > > > > > > > > > > >> > > > previous message} (whatever we send with new > > > > > > FetchRecords > > > > > > > > > > > request). > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > 2) offset = findOffsets(tp -> {offset, leader > > > > epoch}) > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > 3) seek(offset) > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > For the use-case where the users store > committed > > > > > offsets > > > > > > > > > > > externally: > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > 1) Such users would have to track the leader > > epoch > > > > > > > together > > > > > > > > > with > > > > > > > > > > > an > > > > > > > > > > > >> > > offset. > > > > > > > > > > > >> > > > Otherwise, there is no way to detect later > what > > > > leader > > > > > > > epoch > > > > > > > > > was > > > > > > > > > > > >> > > associated > > > > > > > > > > > >> > > > with the message. I think it’s reasonable to > ask > > > > that > > > > > > from > > > > > > > > > users > > > > > > > > > > > if > > > > > > > > > > > >> > they > > > > > > > > > > > >> > > > want to detect log truncation. Otherwise, they > > > will > > > > > get > > > > > > > the > > > > > > > > > > > current > > > > > > > > > > > >> > > > behavior. > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > If the users currently get an offset to be > > stored > > > > > using > > > > > > > > > > > position(), > > > > > > > > > > > >> I > > > > > > > > > > > >> > see > > > > > > > > > > > >> > > > two possibilities. First, they call save > offset > > > > > returned > > > > > > > > from > > > > > > > > > > > >> > position() > > > > > > > > > > > >> > > > that they call before poll(). In that case, it > > > would > > > > > not > > > > > > > be > > > > > > > > > > > correct > > > > > > > > > > > >> to > > > > > > > > > > > >> > > > store {offset, leader epoch} if we would have > > > > changed > > > > > > > > > position() > > > > > > > > > > > to > > > > > > > > > > > >> > > return > > > > > > > > > > > >> > > > {offset, leader epoch} since actual fetched > > > message > > > > > > could > > > > > > > be > > > > > > > > > > > >> different > > > > > > > > > > > >> > > > (from the example I described earlier). So, it > > > would > > > > > be > > > > > > > more > > > > > > > > > > > >> correct to > > > > > > > > > > > >> > > > call position() after poll(). However, the > user > > > > > already > > > > > > > gets > > > > > > > > > > > >> > > > ConsumerRecords at this point, from which the > > user > > > > can > > > > > > > > extract > > > > > > > > > > > >> {offset, > > > > > > > > > > > >> > > > leader epoch} of the last message. > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > So, I like the idea of adding a helper method > to > > > > > > > > > > ConsumerRecords, > > > > > > > > > > > as > > > > > > > > > > > >> > > Jason > > > > > > > > > > > >> > > > proposed, something like: > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > public OffsetAndEpoch > > lastOffsetWithLeaderEpoch(), > > > > > where > > > > > > > > > > > >> OffsetAndEpoch > > > > > > > > > > > >> > > is > > > > > > > > > > > >> > > > a data struct holding {offset, leader epoch}. > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > In this case, we would advise the user to > follow > > > the > > > > > > > > workflow: > > > > > > > > > > > >> poll(), > > > > > > > > > > > >> > > get > > > > > > > > > > > >> > > > {offset, leader epoch} from > > > > > > ConsumerRecords#lastOffsetWith > > > > > > > > > > > >> > LeaderEpoch(), > > > > > > > > > > > >> > > > save offset and leader epoch, process records. > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > 2) When the user needs to seek to the last > > > committed > > > > > > > offset, > > > > > > > > > > they > > > > > > > > > > > >> call > > > > > > > > > > > >> > > new > > > > > > > > > > > >> > > > findOffsets(saved offset, leader epoch), and > > then > > > > > > > > > seek(offset). > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > What do you think? > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > Thanks, > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > Anna > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > On Tue, Jul 3, 2018 at 4:06 PM Dong Lin < > > > > > > > > lindon...@gmail.com> > > > > > > > > > > > >> wrote: > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > > Hey Jason, > > > > > > > > > > > >> > > > > > > > > > > > > > > > >> > > > > Thanks much for your thoughtful explanation. > > > > > > > > > > > >> > > > > > > > > > > > > > > > >> > > > > Yes the solution using findOffsets(offset, > > > > > > leaderEpoch) > > > > > > > > also > > > > > > > > > > > >> works. > > > > > > > > > > > >> > The > > > > > > > > > > > >> > > > > advantage of this solution it adds only one > > API > > > > > > instead > > > > > > > of > > > > > > > > > two > > > > > > > > > > > >> APIs. > > > > > > > > > > > >> > > The > > > > > > > > > > > >> > > > > concern is that its usage seems a bit more > > > clumsy > > > > > for > > > > > > > > > advanced > > > > > > > > > > > >> users. > > > > > > > > > > > >> > > > More > > > > > > > > > > > >> > > > > specifically, advanced users who store > offsets > > > > > > > externally > > > > > > > > > will > > > > > > > > > > > >> always > > > > > > > > > > > >> > > > need > > > > > > > > > > > >> > > > > to call findOffsets() before calling > > > seek(offset) > > > > > > during > > > > > > > > > > > consumer > > > > > > > > > > > >> > > > > initialization. And those advanced users > will > > > need > > > > > to > > > > > > > > > manually > > > > > > > > > > > >> keep > > > > > > > > > > > >> > > track > > > > > > > > > > > >> > > > > of the leaderEpoch of the last > ConsumerRecord. > > > > > > > > > > > >> > > > > > > > > > > > > > > > >> > > > > The other solution may be more user-friendly > > for > > > > > > > advanced > > > > > > > > > > users > > > > > > > > > > > >> is to > > > > > > > > > > > >> > > add > > > > > > > > > > > >> > > > > two APIs, `void seek(offset, leaderEpoch)` > and > > > > > > `(offset, > > > > > > > > > > epoch) > > > > > > > > > > > = > > > > > > > > > > > >> > > > > offsetEpochs(topicPartition)`. > > > > > > > > > > > >> > > > > > > > > > > > > > > > >> > > > > I kind of prefer the second solution because > > it > > > is > > > > > > > easier > > > > > > > > to > > > > > > > > > > use > > > > > > > > > > > >> for > > > > > > > > > > > >> > > > > advanced users. If we need to expose > > leaderEpoch > > > > > > anyway > > > > > > > to > > > > > > > > > > > safely > > > > > > > > > > > >> > > > identify > > > > > > > > > > > >> > > > > a message, it may be conceptually simpler to > > > > expose > > > > > it > > > > > > > > > > directly > > > > > > > > > > > in > > > > > > > > > > > >> > > > > seek(...) rather than requiring one more > > > > translation > > > > > > > using > > > > > > > > > > > >> > > > > findOffsets(...). But I am also OK with the > > > first > > > > > > > solution > > > > > > > > > if > > > > > > > > > > > >> other > > > > > > > > > > > >> > > > > developers also favor that one :) > > > > > > > > > > > >> > > > > > > > > > > > > > > > >> > > > > Thanks, > > > > > > > > > > > >> > > > > Dong > > > > > > > > > > > >> > > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > >> > > > > On Mon, Jul 2, 2018 at 11:10 AM, Jason > > > Gustafson < > > > > > > > > > > > >> ja...@confluent.io > > > > > > > > > > > >> > > > > > > > > > > > > > >> > > > > wrote: > > > > > > > > > > > >> > > > > > > > > > > > > > > > >> > > > > > Hi Dong, > > > > > > > > > > > >> > > > > > > > > > > > > > > > > >> > > > > > Thanks, I've been thinking about your > > > > suggestions > > > > > a > > > > > > > bit. > > > > > > > > > It > > > > > > > > > > is > > > > > > > > > > > >> > > > > challenging > > > > > > > > > > > >> > > > > > to make this work given the current APIs. > > One > > > of > > > > > the > > > > > > > > > > > >> difficulties > > > > > > > > > > > >> > is > > > > > > > > > > > >> > > > that > > > > > > > > > > > >> > > > > > we don't have an API to find the leader > > epoch > > > > for > > > > > a > > > > > > > > given > > > > > > > > > > > >> offset at > > > > > > > > > > > >> > > the > > > > > > > > > > > >> > > > > > moment. So if the user does a seek to > offset > > > 5, > > > > > then > > > > > > > > we'll > > > > > > > > > > > need > > > > > > > > > > > >> a > > > > > > > > > > > >> > new > > > > > > > > > > > >> > > > API > > > > > > > > > > > >> > > > > > to find the corresponding epoch in order > to > > > > > fulfill > > > > > > > the > > > > > > > > > new > > > > > > > > > > > >> > > position() > > > > > > > > > > > >> > > > > API. > > > > > > > > > > > >> > > > > > Potentially we could modify ListOffsets to > > > > enable > > > > > > > > finding > > > > > > > > > > the > > > > > > > > > > > >> > leader > > > > > > > > > > > >> > > > > epoch, > > > > > > > > > > > >> > > > > > but I am not sure it is worthwhile. > Perhaps > > it > > > > is > > > > > > > > > reasonable > > > > > > > > > > > for > > > > > > > > > > > >> > > > advanced > > > > > > > > > > > >> > > > > > usage to expect that the epoch > information, > > if > > > > > > needed, > > > > > > > > > will > > > > > > > > > > be > > > > > > > > > > > >> > > > extracted > > > > > > > > > > > >> > > > > > from the records directly? It might make > > sense > > > > to > > > > > > > > expose a > > > > > > > > > > > >> helper > > > > > > > > > > > >> > in > > > > > > > > > > > >> > > > > > `ConsumerRecords` to make this a little > > easier > > > > > > though. > > > > > > > > > > > >> > > > > > > > > > > > > > > > > >> > > > > > Alternatively, if we think it is important > > to > > > > have > > > > > > > this > > > > > > > > > > > >> information > > > > > > > > > > > >> > > > > exposed > > > > > > > > > > > >> > > > > > directly, we could create batch APIs to > > solve > > > > the > > > > > > > naming > > > > > > > > > > > >> problem. > > > > > > > > > > > >> > For > > > > > > > > > > > >> > > > > > example: > > > > > > > > > > > >> > > > > > > > > > > > > > > > > >> > > > > > Map<TopicPartition, OffsetAndEpoch> > > > positions(); > > > > > > > > > > > >> > > > > > void seek(Map<TopicPartition, > > OffsetAndEpoch> > > > > > > > > positions); > > > > > > > > > > > >> > > > > > > > > > > > > > > > > >> > > > > > However, I'm actually leaning toward > leaving > > > the > > > > > > > seek() > > > > > > > > > and > > > > > > > > > > > >> > > position() > > > > > > > > > > > >> > > > > APIs > > > > > > > > > > > >> > > > > > unchanged. Instead, we can add a new API > to > > > > search > > > > > > for > > > > > > > > > > offset > > > > > > > > > > > by > > > > > > > > > > > >> > > > > timestamp > > > > > > > > > > > >> > > > > > or by offset/leader epoch. Let's say we > call > > > it > > > > > > > > > > `findOffsets`. > > > > > > > > > > > >> If > > > > > > > > > > > >> > the > > > > > > > > > > > >> > > > > user > > > > > > > > > > > >> > > > > > hits a log truncation error, they can use > > this > > > > API > > > > > > to > > > > > > > > find > > > > > > > > > > the > > > > > > > > > > > >> > > closest > > > > > > > > > > > >> > > > > > offset and then do a seek(). At the same > > time, > > > > we > > > > > > > > > deprecate > > > > > > > > > > > the > > > > > > > > > > > >> > > > > > `offsetsForTimes` APIs. We now have two > use > > > > cases > > > > > > > which > > > > > > > > > > > require > > > > > > > > > > > >> > > finding > > > > > > > > > > > >> > > > > > offsets, so I think we should make this > API > > > > > general > > > > > > > and > > > > > > > > > > leave > > > > > > > > > > > >> the > > > > > > > > > > > >> > > door > > > > > > > > > > > >> > > > > open > > > > > > > > > > > >> > > > > > for future extensions. > > > > > > > > > > > >> > > > > > > > > > > > > > > > > >> > > > > > By the way, I'm unclear about the desire > to > > > move > > > > > > part > > > > > > > of > > > > > > > > > > this > > > > > > > > > > > >> > > > > functionality > > > > > > > > > > > >> > > > > > to AdminClient. Guozhang suggested this > > > > > previously, > > > > > > > but > > > > > > > > I > > > > > > > > > > > think > > > > > > > > > > > >> it > > > > > > > > > > > >> > > only > > > > > > > > > > > >> > > > > > makes sense for cross-cutting capabilities > > > such > > > > as > > > > > > > topic > > > > > > > > > > > >> creation. > > > > > > > > > > > >> > If > > > > > > > > > > > >> > > > we > > > > > > > > > > > >> > > > > > have an API which is primarily useful by > > > > > consumers, > > > > > > > > then I > > > > > > > > > > > think > > > > > > > > > > > >> > > that's > > > > > > > > > > > >> > > > > > where it should be exposed. The > AdminClient > > > also > > > > > has > > > > > > > its > > > > > > > > > own > > > > > > > > > > > API > > > > > > > > > > > >> > > > > integrity > > > > > > > > > > > >> > > > > > and should not become a dumping ground for > > > > > advanced > > > > > > > use > > > > > > > > > > cases. > > > > > > > > > > > >> I'll > > > > > > > > > > > >> > > > > update > > > > > > > > > > > >> > > > > > the KIP with the `findOffsets` API > > suggested > > > > > above > > > > > > > and > > > > > > > > we > > > > > > > > > > can > > > > > > > > > > > >> see > > > > > > > > > > > >> > if > > > > > > > > > > > >> > > > it > > > > > > > > > > > >> > > > > > does a good enough job of keeping the API > > > simple > > > > > for > > > > > > > > > common > > > > > > > > > > > >> cases. > > > > > > > > > > > >> > > > > > > > > > > > > > > > > >> > > > > > Thanks, > > > > > > > > > > > >> > > > > > Jason > > > > > > > > > > > >> > > > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > >> > > > > > On Sat, Jun 30, 2018 at 4:39 AM, Dong Lin > < > > > > > > > > > > > lindon...@gmail.com> > > > > > > > > > > > >> > > wrote: > > > > > > > > > > > >> > > > > > > > > > > > > > > > > >> > > > > > > Hey Jason, > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > >> > > > > > > Regarding seek(...), it seems that we > want > > > an > > > > > API > > > > > > > for > > > > > > > > > user > > > > > > > > > > > to > > > > > > > > > > > >> > > > > initialize > > > > > > > > > > > >> > > > > > > consumer with (offset, leaderEpoch) and > > that > > > > API > > > > > > > > should > > > > > > > > > > > allow > > > > > > > > > > > >> > > > throwing > > > > > > > > > > > >> > > > > > > PartitionTruncationException. Suppose we > > > agree > > > > > on > > > > > > > > this, > > > > > > > > > > then > > > > > > > > > > > >> > > > > > > seekToNearest() is not sufficient > because > > it > > > > > will > > > > > > > > always > > > > > > > > > > > >> swallow > > > > > > > > > > > >> > > > > > > PartitionTruncationException. Here we > have > > > two > > > > > > > > options. > > > > > > > > > > The > > > > > > > > > > > >> first > > > > > > > > > > > >> > > > > option > > > > > > > > > > > >> > > > > > is > > > > > > > > > > > >> > > > > > > to add API offsetsForLeaderEpochs() to > > > > translate > > > > > > > > > > > (leaderEpoch, > > > > > > > > > > > >> > > > offset) > > > > > > > > > > > >> > > > > to > > > > > > > > > > > >> > > > > > > offset. The second option is to have add > > > > > > > seek(offset, > > > > > > > > > > > >> > leaderEpoch). > > > > > > > > > > > >> > > > It > > > > > > > > > > > >> > > > > > > seems that second option may be more > > simpler > > > > > > because > > > > > > > > it > > > > > > > > > > > makes > > > > > > > > > > > >> it > > > > > > > > > > > >> > > > clear > > > > > > > > > > > >> > > > > > that > > > > > > > > > > > >> > > > > > > (offset, leaderEpoch) will be used to > > > identify > > > > > > > > > consumer's > > > > > > > > > > > >> > position > > > > > > > > > > > >> > > > in a > > > > > > > > > > > >> > > > > > > partition. And user only needs to handle > > > > > > > > > > > >> > > PartitionTruncationException > > > > > > > > > > > >> > > > > > from > > > > > > > > > > > >> > > > > > > the poll(). In comparison the first > option > > > > > seems a > > > > > > > bit > > > > > > > > > > > harder > > > > > > > > > > > >> to > > > > > > > > > > > >> > > use > > > > > > > > > > > >> > > > > > > because user have to also handle the > > > > > > > > > > > >> PartitionTruncationException > > > > > > > > > > > >> > > if > > > > > > > > > > > >> > > > > > > offsetsForLeaderEpochs() returns > different > > > > > offset > > > > > > > from > > > > > > > > > > > >> > > user-provided > > > > > > > > > > > >> > > > > > > offset. What do you think? > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > >> > > > > > > If we decide to add API seek(offset, > > > > > leaderEpoch), > > > > > > > > then > > > > > > > > > we > > > > > > > > > > > can > > > > > > > > > > > >> > > decide > > > > > > > > > > > >> > > > > > > whether and how to add API to translate > > > > (offset, > > > > > > > > > > > leaderEpoch) > > > > > > > > > > > >> to > > > > > > > > > > > >> > > > > offset. > > > > > > > > > > > >> > > > > > It > > > > > > > > > > > >> > > > > > > seems that this API will be needed by > > > advanced > > > > > > user > > > > > > > to > > > > > > > > > > don't > > > > > > > > > > > >> want > > > > > > > > > > > >> > > > auto > > > > > > > > > > > >> > > > > > > offset reset (so that it can be > notified) > > > but > > > > > > still > > > > > > > > > wants > > > > > > > > > > to > > > > > > > > > > > >> > reset > > > > > > > > > > > >> > > > > offset > > > > > > > > > > > >> > > > > > > to closest. For those users if probably > > > makes > > > > > > sense > > > > > > > to > > > > > > > > > > only > > > > > > > > > > > >> have > > > > > > > > > > > >> > > the > > > > > > > > > > > >> > > > > API > > > > > > > > > > > >> > > > > > in > > > > > > > > > > > >> > > > > > > AdminClient. offsetsForTimes() seems > like > > a > > > > > common > > > > > > > API > > > > > > > > > > that > > > > > > > > > > > >> will > > > > > > > > > > > >> > be > > > > > > > > > > > >> > > > > > needed > > > > > > > > > > > >> > > > > > > by user's of consumer in general, so it > > may > > > be > > > > > > more > > > > > > > > > > > >> reasonable to > > > > > > > > > > > >> > > > stay > > > > > > > > > > > >> > > > > in > > > > > > > > > > > >> > > > > > > the consumer API. I don't have a strong > > > > opinion > > > > > on > > > > > > > > > whether > > > > > > > > > > > >> > > > > > > offsetsForTimes() should be replaced by > > API > > > in > > > > > > > > > > AdminClient. > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > >> > > > > > > Though (offset, leaderEpoch) is needed > to > > > > > uniquely > > > > > > > > > > identify > > > > > > > > > > > a > > > > > > > > > > > >> > > message > > > > > > > > > > > >> > > > > in > > > > > > > > > > > >> > > > > > > general, it is only needed for advanced > > > users > > > > > who > > > > > > > has > > > > > > > > > > turned > > > > > > > > > > > >> on > > > > > > > > > > > >> > > > unclean > > > > > > > > > > > >> > > > > > > leader election, need to use seek(..), > and > > > > don't > > > > > > > want > > > > > > > > > auto > > > > > > > > > > > >> offset > > > > > > > > > > > >> > > > > reset. > > > > > > > > > > > >> > > > > > > Most other users probably just want to > > > enable > > > > > auto > > > > > > > > > offset > > > > > > > > > > > >> reset > > > > > > > > > > > >> > and > > > > > > > > > > > >> > > > > store > > > > > > > > > > > >> > > > > > > offset in Kafka. Thus we might want to > > keep > > > > the > > > > > > > > existing > > > > > > > > > > > >> > > offset-only > > > > > > > > > > > >> > > > > APIs > > > > > > > > > > > >> > > > > > > (e.g. seek() and position()) for most > > users > > > > > while > > > > > > > > adding > > > > > > > > > > new > > > > > > > > > > > >> APIs > > > > > > > > > > > >> > > for > > > > > > > > > > > >> > > > > > > advanced users. And yes, it seems that > we > > > need > > > > > new > > > > > > > > name > > > > > > > > > > for > > > > > > > > > > > >> > > > position(). > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > >> > > > > > > Though I think we need new APIs to carry > > the > > > > new > > > > > > > > > > information > > > > > > > > > > > >> > (e.g. > > > > > > > > > > > >> > > > > > > leaderEpoch), I am not very sure how > that > > > > should > > > > > > > look > > > > > > > > > > like. > > > > > > > > > > > >> One > > > > > > > > > > > >> > > > > possible > > > > > > > > > > > >> > > > > > > option is those APIs in KIP-232. Another > > > > option > > > > > is > > > > > > > > > > something > > > > > > > > > > > >> like > > > > > > > > > > > >> > > > this: > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > >> > > > > > > ````` > > > > > > > > > > > >> > > > > > > class OffsetEpochs { > > > > > > > > > > > >> > > > > > > long offset; > > > > > > > > > > > >> > > > > > > int leaderEpoch; > > > > > > > > > > > >> > > > > > > int partitionEpoch; // This may be > > > needed > > > > > > later > > > > > > > as > > > > > > > > > > > >> discussed > > > > > > > > > > > >> > in > > > > > > > > > > > >> > > > > > KIP-232 > > > > > > > > > > > >> > > > > > > ... // Hopefully these are all we need > > to > > > > > > identify > > > > > > > > > > message > > > > > > > > > > > >> in > > > > > > > > > > > >> > > > Kafka. > > > > > > > > > > > >> > > > > > But > > > > > > > > > > > >> > > > > > > if we need more then we can add new > fields > > > in > > > > > this > > > > > > > > > class. > > > > > > > > > > > >> > > > > > > } > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > >> > > > > > > OffsetEpochs > offsetEpochs(TopicPartition); > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > >> > > > > > > void seek(TopicPartition, OffsetEpochs); > > > > > > > > > > > >> > > > > > > `````` > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > >> > > > > > > Thanks, > > > > > > > > > > > >> > > > > > > Dong > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > >> > > > > > > On Fri, Jun 29, 2018 at 11:13 AM, Jason > > > > > Gustafson > > > > > > < > > > > > > > > > > > >> > > > ja...@confluent.io> > > > > > > > > > > > >> > > > > > > wrote: > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > >> > > > > > > > Hey Dong, > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > >> > > > > > > > Thanks for the feedback. The first > three > > > > > points > > > > > > > are > > > > > > > > > > easy: > > > > > > > > > > > >> > > > > > > > > > > > > > > > > > > >> > > > > > > > 1. Yes, we should be consistent. > > > > > > > > > > > >> > > > > > > > 2. Yes, I will add this. > > > > > > > > > > > >> > > > > > > > 3. Yes, I think we should document the > > > > changes > > > > > > to > > > > > > > > the > > > > > > > > > > > >> committed > > > > > > > > > > > >> > > > > offset > > > > > > > > > > > >> > > > > > > >
