Hey Jason,
Thanks for the reply. Regarding 3), I am thinking that both "Offset" and
"LastLeaderEpoch" in the OffsetCommitRequest are associated with the last
consumed messages. Value of "Offset" is not necessarily the offset of the
next message due to log compaction. Since we are naming "Offset" as e.g.
"NextOffset", it may be simpler to use "LeaderEpoch".
I am not strong on this. If we decide to name the new field as
"LastLeaderEpoch", would it be more consistent to also name the new field
as "LastLeaderEpoch" in the Offset Commit Value Schema?
Thanks,
Dong
On Tue, Aug 7, 2018 at 1:23 PM, Jason Gustafson <ja...@confluent.io> wrote:
> Hi Dong,
>
> Thanks for the comments.
>
> 1) Yes, makes sense.
>
> 2) This is an interesting point. The suggestion made more sense in the
> initial version of the KIP, but I think you are right that we should use
> the same fencing semantics we use for the Fetch and OffsetForLeaderEpoch
> APIs. Just like a following replica, we need to protect the initial fetch
> to a leader. If the initial fetch position was obtained from a broker with
> the same epoch, then we can begin fetching. Otherwise, we need the
> OffsetForLeaderEpoch dance. And it is better to avoid using offsets
> obtained from stale leaders in the first place. I'll update the KIP.
>
> 3) The committed offset is actually the next offset that a consumer will
> read, right? The leader epoch on the other hand is the last one that was
> consumed.
>
> 4) Yes, that is right. And it is a good point that the epoch we use for log
> reconciliation must be less than or equal to the current leader epoch. I
> will mention this.
>
> Thanks,
> Jason
>
> On Tue, Aug 7, 2018 at 12:43 PM, Jason Gustafson <ja...@confluent.io>
> wrote:
>
> > Hey Jun,
> >
> > 57. It's a fair point. I could go either way, but I'm slightly inclined
> to
> > just document the new API for now. We'll still support seeking to an
> offset
> > with corresponding epoch information, so deprecating the old seek() seems
> > like overkill.
> >
> > 60. The phrasing was a little confusing. Does this sound better?
> >
> > "Log truncation is detected if there exists a leader epoch which is
> > larger than this epoch and begins at an offset earlier than the committed
> > offset."
> >
> > Thanks,
> > Jason
> >
> >
> > On Tue, Aug 7, 2018 at 12:11 PM, Dong Lin <lindon...@gmail.com> wrote:
> >
> >> Hey Jason,
> >>
> >> Thanks for the update. I have some comments below:
> >>
> >> 1) Since FencedLeaderEpochException indicates that the metadata in the
> >> client is outdated, should it extend InvalidMetadataException?
> >>
> >> 2) It is mentioned that "To fix the problem with KIP-232, we will add
> the
> >> leader epoch the ListOffsets response. The consumer will use this in its
> >> first fetch request after resetting offsets". If consumer sends
> >> ListOffsetRequest to the broker who is no longer the leader, and the
> >> broker
> >> still think it is the leader, then the broker may return
> >> ListOffsetResponse
> >> whose leaderEpoch is smaller than the leaderEpoch in the metadata of the
> >> consumer. In this case consumer probably should not just send
> FetchRequest
> >> with the leaderEpoch of the ListOffsetResponse, right? I am wondering
> >> whether we should also include CurrentLeaderEpoch in the
> >> ListOffsetRequest.
> >>
> >> 3) Currently the new field added in the OffsetCommitRequest/
> >> OffsetFetchResponse is named LastLeaderEpoch. For the same reason that
> we
> >> are not naming the existing field "Offset" as "LastOffset", would it be
> >> more consistent to just name the new field as LeaderEpoch? Same for the
> >> new
> >> API in the class OffsetAndMetadata.
> >>
> >> 4) Could we clarify in the KIP where the value of CurrentLeaderEpoch in
> >> the
> >> FetchRequest comes from? I suppose this value can be updated by the
> >> MetadataResponse, right? If so, maybe we should also clarify that client
> >> should reject MetadataResponse if the leaderEpoch in the metadata
> response
> >> is smaller than what the client also knows from e.g.
> >> seek(...), OffsetFetchResponse?
> >>
> >>
> >> Thanks,
> >> Dong
> >>
> >>
> >> On Mon, Aug 6, 2018 at 5:30 PM, Jun Rao <j...@confluent.io> wrote:
> >>
> >> > Hi, Jason,
> >> >
> >> > Thanks for the reply. They all make sense. Just a couple of more minor
> >> > comments.
> >> >
> >> > 57. I was thinking that if will be useful to encourage people to use
> the
> >> > new seek() api to get better semantics. Deprecating the old seek api
> is
> >> one
> >> > way. I guess we could also just document it for now.
> >> >
> >> > 60. "Log truncation is detected if the first offset of the epoch for
> the
> >> > committed offset is larger than this epoch and begins at an earlier
> >> > offset." It seems that we should add "that" before "is larger than"?
> >> >
> >> > Thanks,
> >> >
> >> > Jun
> >> >
> >> >
> >> > On Mon, Aug 6, 2018 at 3:07 PM, Jason Gustafson <ja...@confluent.io>
> >> > wrote:
> >> >
> >> > > Hi Jun,
> >> > >
> >> > > I spent a little more time looking at the usage in WorkerSinkTask. I
> >> > think
> >> > > actually the initialization of the positions in the assignment
> >> callback
> >> > is
> >> > > not strictly necessary. We keep a map of the current consumed
> offsets
> >> > which
> >> > > is updated as we consume the data. As far as I can tell, we could
> >> either
> >> > > skip the initialization and wait until the first fetched records
> come
> >> in
> >> > or
> >> > > we could use the committed() API to initialize positions. I think
> the
> >> > root
> >> > > of it is the argument Anna made previously. The leader epoch lets us
> >> > track
> >> > > the history of records that we have consumed. It is only useful when
> >> we
> >> > > want to tell whether records we have consumed were lost. So getting
> >> the
> >> > > leader epoch of an arbitrary position that was seeked doesn't really
> >> make
> >> > > sense. The dependence on the consumed records is most explicit if we
> >> only
> >> > > expose the leader epoch inside the fetched records. We might
> consider
> >> > > adding a `lastConsumedLeaderEpoch` API to expose it directly, but
> I'm
> >> > > inclined to leave that as potential future work.
> >> > >
> >> > > A couple additional notes:
> >> > >
> >> > > 1. I've renamed OffsetAndMetadata.leaderEpoch to
> >> > > OffsetAndMetadata.lastLeaderEpoch. In fact, the consumer doesn't
> know
> >> > what
> >> > > the leader epoch of the committed offset should be, so this just
> >> > clarifies
> >> > > the expected usage.
> >> > >
> >> > > 2. I decided to add a helper to ConsumerRecords to get the next
> >> offsets.
> >> > We
> >> > > would use this in WorkerSinkTask and external storage use cases to
> >> > simplify
> >> > > the commit logic. If we are consuming batch by batch, then we don't
> >> need
> >> > > the message-level bookkeeping.
> >> > >
> >> > > Thanks,
> >> > > Jason
> >> > >
> >> > > On Mon, Aug 6, 2018 at 10:28 AM, Jason Gustafson <
> ja...@confluent.io>
> >> > > wrote:
> >> > >
> >> > > > Hey Jun,
> >> > > >
> >> > > > Thanks for the review. Responses below:
> >> > > >
> >> > > > 50. Yes, that is right. I clarified this in the KIP.
> >> > > >
> >> > > > 51. Yes, updated the KIP to mention.
> >> > > >
> >> > > > 52. Yeah, this was a reference to a previous iteration. I've fixed
> >> it.
> >> > > >
> >> > > > 53. I changed the API to use an `Optional<Integer>` for the leader
> >> > epoch
> >> > > > and added a note about the default value. Does that seem
> reasonable?
> >> > > >
> >> > > > 54. We discussed this above, but could not find a great option.
> The
> >> > > > options are to add a new API (e.g. positionAndEpoch) or to rely on
> >> the
> >> > > user
> >> > > > to get the epoch from the fetched records. We were leaning toward
> >> the
> >> > > > latter, but I admit it was not fully satisfying. In this case,
> >> Connect
> >> > > > would need to track the last consumed offsets manually instead of
> >> > relying
> >> > > > on the consumer. We also considered adding a convenience method to
> >> > > > ConsumerRecords to get the offset to commit for all fetched
> >> partitions.
> >> > > > This makes the additional bookkeeping pretty minimal. What do you
> >> > think?
> >> > > >
> >> > > > 55. I clarified in the KIP. I was mainly thinking of situations
> >> where a
> >> > > > previously valid offset becomes out of range.
> >> > > >
> >> > > > 56. Yeah, that's a bit annoying. I decided to keep LeaderEpoch as
> >> it is
> >> > > > and use CurrentLeaderEpoch for both OffsetForLeaderEpoch and the
> >> Fetch
> >> > > > APIs. I think Dong suggested this previously as well.
> >> > > >
> >> > > > 57. We could, but I'm not sure there's a strong reason to do so. I
> >> was
> >> > > > thinking we would leave it around for convenience, but let me know
> >> if
> >> > you
> >> > > > think we should do otherwise.
> >> > > >
> >> > > >
> >> > > > Thanks,
> >> > > > Jason
> >> > > >
> >> > > >
> >> > > > On Fri, Aug 3, 2018 at 4:49 PM, Jun Rao <j...@confluent.io> wrote:
> >> > > >
> >> > > >> Hi, Jason,
> >> > > >>
> >> > > >> Thanks for the updated KIP. Well thought-through. Just a few
> minor
> >> > > >> comments
> >> > > >> below.
> >> > > >>
> >> > > >> 50. For seek(TopicPartition partition, OffsetAndMetadata
> offset), I
> >> > > guess
> >> > > >> under the cover, it will make OffsetsForLeaderEpoch request to
> >> > determine
> >> > > >> if
> >> > > >> the seeked offset is still valid before fetching? If so, it will
> be
> >> > > useful
> >> > > >> document this in the wiki.
> >> > > >>
> >> > > >> 51. Similarly, if the consumer fetch request gets
> >> > FENCED_LEADER_EPOCH, I
> >> > > >> guess the consumer will also make OffsetsForLeaderEpoch request
> to
> >> > > >> determine if the last consumed offset is still valid before
> >> fetching?
> >> > If
> >> > > >> so, it will be useful document this in the wiki.
> >> > > >>
> >> > > >> 52. "If the consumer seeks to the middle of the log, for example,
> >> then
> >> > > we
> >> > > >> will use the sentinel value -1 and the leader will skip the epoch
> >> > > >> validation. " Is this true? If the consumer seeks using
> >> > > >> seek(TopicPartition
> >> > > >> partition, OffsetAndMetadata offset) and the seeked offset is
> >> valid,
> >> > the
> >> > > >> consumer can/should use the leaderEpoch in the cached metadata
> for
> >> > > >> fetching?
> >> > > >>
> >> > > >> 53. OffsetAndMetadata. For backward compatibility, we need to
> >> support
> >> > > >> constructing OffsetAndMetadata without providing leaderEpoch.
> >> Could we
> >> > > >> define the default value of leaderEpoch if not provided and the
> >> > > semantics
> >> > > >> of that (e.g., skipping the epoch validation)?
> >> > > >>
> >> > > >> 54. I saw the following code in WorkerSinkTask in Connect. It
> saves
> >> > the
> >> > > >> offset obtained through position(), which can be committed
> latter.
> >> > Since
> >> > > >> position() doesn't return the leaderEpoch, this can lead to
> >> committed
> >> > > >> offset without leaderEpoch. Not sure how common this usage is,
> but
> >> > > what's
> >> > > >> the recommendation for such users?
> >> > > >>
> >> > > >> private class HandleRebalance implements
> ConsumerRebalanceListener
> >> {
> >> > > >> @Override
> >> > > >> public void onPartitionsAssigned(Collection<TopicPartition>
> >> > > >> partitions) {
> >> > > >> log.debug("{} Partitions assigned {}",
> WorkerSinkTask.this,
> >> > > >> partitions);
> >> > > >> lastCommittedOffsets = new HashMap<>();
> >> > > >> currentOffsets = new HashMap<>();
> >> > > >> for (TopicPartition tp : partitions) {
> >> > > >> long pos = consumer.position(tp);
> >> > > >> lastCommittedOffsets.put(tp, new
> >> OffsetAndMetadata(pos));
> >> > > >>
> >> > > >> 55. "With this KIP, the only case in which this is possible is if
> >> the
> >> > > >> consumer fetches from an offset earlier than the log start
> >> offset." Is
> >> > > >> that
> >> > > >> true? I guess a user could seek to a large offset without
> providing
> >> > > >> leaderEpoch, which can cause the offset to be larger than the log
> >> end
> >> > > >> offset during fetch?
> >> > > >>
> >> > > >> 56. In the schema for OffsetForLeaderEpochRequest, LeaderEpoch
> >> seems
> >> > to
> >> > > be
> >> > > >> an existing field. Is LeaderEpochQuery the new field? The name is
> >> not
> >> > > very
> >> > > >> intuitive. It will be useful to document its meaning.
> >> > > >>
> >> > > >> 57. Should we deprecate the following api?
> >> > > >> void seek(TopicPartition partition, long offset);
> >> > > >>
> >> > > >> Thanks,
> >> > > >>
> >> > > >> Jun
> >> > > >>
> >> > > >>
> >> > > >> On Fri, Aug 3, 2018 at 9:32 AM, Jason Gustafson <
> >> ja...@confluent.io>
> >> > > >> wrote:
> >> > > >>
> >> > > >> > Hey All,
> >> > > >> >
> >> > > >> > I think I've addressed all pending review. If there is no
> >> additional
> >> > > >> > feedback, I'll plan to start a vote thread next week.
> >> > > >> >
> >> > > >> > Thanks,
> >> > > >> > Jason
> >> > > >> >
> >> > > >> > On Tue, Jul 31, 2018 at 9:46 AM, Dong Lin <lindon...@gmail.com
> >
> >> > > wrote:
> >> > > >> >
> >> > > >> > > Hey Jason,
> >> > > >> > >
> >> > > >> > > Thanks for your reply. I will comment below.
> >> > > >> > >
> >> > > >> > > Regarding 1, we probably can not simply rename both to
> >> > `LeaderEpoch`
> >> > > >> > > because we already have a LeaderEpoch field in
> >> > > OffsetsForLeaderEpoch.
> >> > > >> > >
> >> > > >> > > Regarding 5, I am not strong on this. I agree with the two
> >> > benefits
> >> > > of
> >> > > >> > > having two error codes: 1) not having to refresh metadata
> when
> >> > > >> consumer
> >> > > >> > > sees UNKNOWN_LEADER_EPOCH and 2) provide more information in
> >> the
> >> > log
> >> > > >> for
> >> > > >> > > debugging. Whether or not these two benefits are useful
> enough
> >> for
> >> > > one
> >> > > >> > more
> >> > > >> > > error code may be subjective. I will let you and others
> >> determine
> >> > > >> this.
> >> > > >> > >
> >> > > >> > > Regarding 6, yeah overloading seek() looks good to me.
> >> > > >> > >
> >> > > >> > >
> >> > > >> > > Thanks,
> >> > > >> > > Dong
> >> > > >> > >
> >> > > >> > >
> >> > > >> > > On Mon, Jul 30, 2018 at 9:33 AM, Jason Gustafson <
> >> > > ja...@confluent.io>
> >> > > >> > > wrote:
> >> > > >> > >
> >> > > >> > > > Hey Dong,
> >> > > >> > > >
> >> > > >> > > > Thanks for the detailed review. Responses below:
> >> > > >> > > >
> >> > > >> > > > 1/2: Thanks for noticing the inconsistency. Would it be
> >> > reasonable
> >> > > >> to
> >> > > >> > > > simply call it LeaderEpoch for both APIs?
> >> > > >> > > >
> >> > > >> > > > 3: I agree it should be a map. I will update.
> >> > > >> > > >
> >> > > >> > > > 4: Fair point. I think we should always be able to identify
> >> an
> >> > > >> offset.
> >> > > >> > > > Let's remove the Optional for now and reconsider if we find
> >> an
> >> > > >> > unhandled
> >> > > >> > > > case during implementation.
> >> > > >> > > >
> >> > > >> > > > 5: Yeah, I was thinking about this. The two error codes
> >> could be
> >> > > >> > handled
> >> > > >> > > > similarly, so we might merge them. Mainly I was thinking
> >> that it
> >> > > >> will
> >> > > >> > be
> >> > > >> > > > useful for consumers/replicas to know whether they are
> ahead
> >> or
> >> > > >> behind
> >> > > >> > > the
> >> > > >> > > > leader. For example, if a consumer sees
> >> UNKNOWN_LEADER_EPOCH, it
> >> > > >> need
> >> > > >> > not
> >> > > >> > > > refresh metadata. Or if a replica sees a
> FENCED_LEADER_EPOCH
> >> > > error,
> >> > > >> it
> >> > > >> > > > could just stop fetching and await the LeaderAndIsr request
> >> that
> >> > > it
> >> > > >> is
> >> > > >> > > > missing. It probably also makes debugging a little bit
> >> easier. I
> >> > > >> guess
> >> > > >> > > I'm
> >> > > >> > > > a bit inclined to keep both error codes, but I'm open to
> >> > > >> > reconsideration
> >> > > >> > > if
> >> > > >> > > > you feel strongly. Another point to consider is whether we
> >> > should
> >> > > >> > > continue
> >> > > >> > > > using NOT_LEADER_FOR_PARTITION if a follower receives an
> >> > > unexpected
> >> > > >> > > fetch.
> >> > > >> > > > The leader epoch would be different in this case so we
> could
> >> use
> >> > > >> one of
> >> > > >> > > the
> >> > > >> > > > invalid epoch error codes instead since they contain more
> >> > > >> information.
> >> > > >> > > >
> >> > > >> > > > 6: I agree the name is not ideal in that scenario. What if
> we
> >> > > >> > overloaded
> >> > > >> > > > `seek`?
> >> > > >> > > >
> >> > > >> > > > 7: Sure, I will mention this.
> >> > > >> > > >
> >> > > >> > > >
> >> > > >> > > > Thanks,
> >> > > >> > > > Jason
> >> > > >> > > >
> >> > > >> > > > On Fri, Jul 27, 2018 at 6:17 PM, Dong Lin <
> >> lindon...@gmail.com>
> >> > > >> wrote:
> >> > > >> > > >
> >> > > >> > > > > 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/confl
> >> > uence/display/KAFKA/KIP-
> >> > > >> > > > > > > > > 320%3A+Allow+fetchers+to+detec
> >> > > >> t+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
> >> > > >> > > > > > > >
> >
> > ...
> >
> > [Message clipped]
>
