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
> > schema. I meant to do this, but it slipped my mind.
> >
> > The latter questions are tougher. One option I was considering is to have
> > only `offsetsForLeaderEpochs` exposed from the consumer and to drop the
> new
> > seek() API. That seems more consistent with the current use of
> > `offsetsForTimes` (we don't have a separate `seekToTimestamp` API). An
> > alternative might be to take a page from the AdminClient API and add a
> new
> > method to generalize offset lookup. For example, we could have
> > `lookupOffsets(LookupOptions)`. We could then deprecate
> `offsetsForTimes`
> > and this would open the door for future extensions without needing new
> > APIs.
> >
> > The case of position() is a little more annoying. It would have been
> better
> > had we let this return an object so that it is easier to extend. This is
> > the only reason I didn't add the API to the KIP. Maybe we should bite the
> > bullet and fix this now? Unfortunately we'll have to come up with a new
> > name. Maybe `currentPosition`?
> >
> > Thoughts?
> >
> > -Jason
> >
> >
> > On Fri, Jun 29, 2018 at 10:40 AM, Dong Lin <lindon...@gmail.com> wrote:
> >
> > > Regarding points 4) and 5) above, motivation for the alternative APIs
> is
> > > that, if we decide that leaderEpoch is equally important as offset in
> > > identifying a message, then it may be reasonable to always specify it
> > > wherever offset is currently required in the consumer API to identify a
> > > message, e.g. position(), seek(). For example, since we allow user to
> > > retrieve offset using position() instead of asking user to keep track
> of
> > > the offset of the latest ConsumerRecord, may be it will be more
> > consistent
> > > for user to also retrieve leaderEpoch using position()?
> > >
> > >
> > >
> > > On Fri, Jun 29, 2018 at 10:30 AM, Dong Lin <lindon...@gmail.com>
> wrote:
> > >
> > > > Hey Jason,
> > > >
> > > > Thanks for the update. It looks pretty good. Just some minor comments
> > > > below:
> > > >
> > > > 1) The KIP adds new error code "LOG_TRUNCATION" and new exception
> > > TruncatedPartitionException.
> > > > Can we make the name more consistent, e.g. LogTruncationException?
> > > >
> > > > 2) Do we need to add UnknownLeaderEpochException as part of API
> change?
> > > >
> > > > 3) Not sure if the offset topic schema is also public API. If so,
> maybe
> > > we
> > > > should also include the schema change in the API?
> > > >
> > > > 4) For users who store offset externally, currently they get offset
> > using
> > > > position(..), store the offset externally, and use seek(..) to
> > initialize
> > > > the consumer next time. After this KIP they will need to store and
> use
> > > the
> > > > leaderEpoch together with the offset. Should we also update the API
> so
> > > that
> > > > user can also get leaderEpoch from position(...)? Not sure if it is
> OK
> > to
> > > > ask user to track the latest leaderEpoch of ConsumerRecord by
> > themselves.
> > > >
> > > > 5) Also for users who store offset externally, they need to call
> > seek(..)
> > > > with leaderEpoch to initialize consumer. With current KIP users need
> to
> > > > call seekToNearest(), whose name suggests that the final position may
> > be
> > > > different from what was requested. However, if users may want to
> avoid
> > > auto
> > > > offset reset and be notified explicitly when there is log truncation,
> > > then seekToNearest()
> > > > probably does not help here. Would it make sense to replace
> > > seekToNearest()
> > > > with seek(offset, leaderEpoch) + AminClient.
> > offsetsForLeaderEpochs(...)?
> > > >
> > > >
> > > > Thanks,
> > > > Dong
> > > >
> > > >
> > > > On Wed, Jun 27, 2018 at 3:57 PM, Jason Gustafson <ja...@confluent.io
> >
> > > > wrote:
> > > >
> > > >> Hey Guozhang,
> > > >>
> > > >> That's fair. In fact, perhaps we do not need this API at all. We
> > already
> > > >> have the new seek() in this KIP which can do the lookup based on
> epoch
> > > for
> > > >> this use case. I guess we should probably call it seekToNearest()
> > though
> > > >> to
> > > >> make it clear that the final position may be different from what was
> > > >> requested.
> > > >>
> > > >> Thanks,
> > > >> Jason
> > > >>
> > > >> On Wed, Jun 27, 2018 at 3:20 PM, Guozhang Wang <wangg...@gmail.com>
> > > >> wrote:
> > > >>
> > > >> > Hi Jason,
> > > >> >
> > > >> > I think it is less worthwhile to add
> KafkaConsumer#offsetsForLeader
> > > >> Epochs,
> > > >> > since probably only very advanced users are aware of the
> > leaderEpoch,
> > > >> and
> > > >> > hence ever care to use it anyways. It is more like an admin client
> > > >> > operation than a consumer client operation: if the motivation is
> to
> > > >> > facility customized reset policy, maybe adding it as
> > > >> > AdminClient#offsetsForLeaderEpochs
> > > >> > is better as it is not an aggressive assumption that for such
> > advanced
> > > >> > users they are willing to use some admin client to get further
> > > >> information?
> > > >> >
> > > >> >
> > > >> > Guozhang
> > > >> >
> > > >> >
> > > >> > On Wed, Jun 27, 2018 at 2:07 PM, Jason Gustafson <
> > ja...@confluent.io>
> > > >> > wrote:
> > > >> >
> > > >> > > Thanks for the feedback. I've updated the KIP. Specifically I
> > > removed
> > > >> the
> > > >> > > "closest" reset option and the proposal to reset by timestamp
> when
> > > the
> > > >> > > precise truncation point cannot be determined. Instead, I
> proposed
> > > >> that
> > > >> > we
> > > >> > > always reset using the nearest epoch when a reset policy is
> > defined
> > > >> > (either
> > > >> > > "earliest" or "latest"). Does that sound reasonable?
> > > >> > >
> > > >> > > One thing I am still debating is whether it would be better to
> > have
> > > a
> > > >> > > separate API to find the closest offset using the leader epoch.
> In
> > > the
> > > >> > > current KIP, I suggested to piggyback this information on an
> > > >> exception,
> > > >> > but
> > > >> > > I'm beginning to think it would be better not to hide the
> lookup.
> > It
> > > >> is
> > > >> > > awkward to implement since it means delaying the exception and
> the
> > > API
> > > >> > may
> > > >> > > actually be useful when customizing reset logic if no auto reset
> > > >> policy
> > > >> > is
> > > >> > > defined. I was thinking we can add an API like the following:
> > > >> > >
> > > >> > > Map<TopicPartition, OffsetAndEpoch>
> > > >> > > offsetsForLeaderEpochs(Map<TopicPartition, Integer>
> > epochsToSearch)
> > > >> > >
> > > >> > > Thoughts?
> > > >> > >
> > > >> > > -Jason
> > > >> > >
> > > >> > >
> > > >> > >
> > > >> > >
> > > >> > > On Wed, Jun 27, 2018 at 11:12 AM, Jason Gustafson <
> > > ja...@confluent.io
> > > >> >
> > > >> > > wrote:
> > > >> > >
> > > >> > > > @Dong
> > > >> > > >
> > > >> > > > Those are fair points. Both approaches require some fuzziness
> to
> > > >> reset
> > > >> > > the
> > > >> > > > offset in these pathological scenarios and we cannot guarantee
> > > >> > > > at-least-once delivery either way unless we have the full
> > history
> > > of
> > > >> > > leader
> > > >> > > > epochs that were consumed. The KIP-101 logic may actually be
> > more
> > > >> > > accurate
> > > >> > > > than using timestamps because it does not depend on the
> messages
> > > >> which
> > > >> > > are
> > > >> > > > written after the unclean leader election. The case we're
> > talking
> > > >> about
> > > >> > > > should be extremely rare in practice anyway. I also agree that
> > we
> > > >> may
> > > >> > not
> > > >> > > > want to add new machinery if it only helps the old message
> > format.
> > > >> Ok,
> > > >> > > > let's go ahead and drop the timestamp.
> > > >> > > >
> > > >> > > > @Guozhang
> > > >> > > >
> > > >> > > > * My current understanding is that, with unclean leader
> election
> > > >> turned
> > > >> > > on,
> > > >> > > >> exactly-once is out of the window since we cannot guarantee
> > that
> > > >> all
> > > >> > > >> committed message markers will not be lost. And hence there
> is
> > no
> > > >> need
> > > >> > > to
> > > >> > > >> have special handling logic for LOG_TRUNCATED or OOR error
> > codes
> > > >> with
> > > >> > > >> read.committed turned on. Is that right?
> > > >> > > >
> > > >> > > >
> > > >> > > > Yes, that's right. EoS and unclean leader election don't mix
> > well.
> > > >> It
> > > >> > may
> > > >> > > > be worth considering separately whether we should try to
> > reconcile
> > > >> the
> > > >> > > > transaction log following an unclean leader election. At least
> > we
> > > >> may
> > > >> > be
> > > >> > > > able to prevent dangling transactions from blocking consumers.
> > > This
> > > >> KIP
> > > >> > > > does not address this problem.
> > > >> > > >
> > > >> > > > * MINOR: "if the epoch is greater than the minimum expected
> > epoch,
> > > >> that
> > > >> > > the
> > > >> > > >> new epoch does not begin at an earlier offset than the fetch
> > > >> offset.
> > > >> > In
> > > >> > > >> the latter case, the leader can respond with a new
> > LOG_TRUNCATION
> > > >> > error
> > > >> > > >> code" should it be "does not begin at a later offset than the
> > > fetch
> > > >> > > >> offset"?
> > > >> > > >
> > > >> > > >
> > > >> > > > I think the comment is correct, though the phrasing may be
> > > >> confusing.
> > > >> > We
> > > >> > > > know truncation has occurred if there exists a larger epoch
> > with a
> > > >> > > starting
> > > >> > > > offset that is lower than the fetch offset. Let me try to
> > rephrase
> > > >> > this.
> > > >> > > >
> > > >> > > > Thanks,
> > > >> > > > Jason
> > > >> > > >
> > > >> > > > On Wed, Jun 27, 2018 at 9:17 AM, Guozhang Wang <
> > > wangg...@gmail.com>
> > > >> > > wrote:
> > > >> > > >
> > > >> > > >> Jason, thanks for the KIP. A few comments:
> > > >> > > >>
> > > >> > > >> * I think Dong's question about whether to use
> timestamp-based
> > > >> > approach
> > > >> > > >> v.s. start-offset-of-first-larger-epoch is valid; more
> > > >> specifically,
> > > >> > > with
> > > >> > > >> timestamp-based approach we may still be reseting to an
> offset
> > > >> falling
> > > >> > > >> into
> > > >> > > >> the truncated interval, and hence we may still miss some
> data,
> > > i.e.
> > > >> > not
> > > >> > > >> guaranteeing at-least-once still. With the
> > > >> > > >> start-offset-of-first-larger-epoch, I'm not sure if it will
> > > >> guarantee
> > > >> > > no
> > > >> > > >> valid data is missed when we have consecutive log truncations
> > > >> (maybe
> > > >> > we
> > > >> > > >> need to look back into details of KIP-101 to figure it out).
> If
> > > the
> > > >> > > latter
> > > >> > > >> can indeed guarantee at least once, we could consider using
> > that
> > > >> > > approach.
> > > >> > > >>
> > > >> > > >> * My current understanding is that, with unclean leader
> > election
> > > >> > turned
> > > >> > > >> on,
> > > >> > > >> exactly-once is out of the window since we cannot guarantee
> > that
> > > >> all
> > > >> > > >> committed message markers will not be lost. And hence there
> is
> > no
> > > >> need
> > > >> > > to
> > > >> > > >> have special handling logic for LOG_TRUNCATED or OOR error
> > codes
> > > >> with
> > > >> > > >> read.committed turned on. Is that right?
> > > >> > > >>
> > > >> > > >> * MINOR: "if the epoch is greater than the minimum expected
> > > epoch,
> > > >> > that
> > > >> > > >> the
> > > >> > > >> new epoch does not begin at an earlier offset than the fetch
> > > >> offset.
> > > >> > In
> > > >> > > >> the latter case, the leader can respond with a new
> > LOG_TRUNCATION
> > > >> > error
> > > >> > > >> code" should it be "does not begin at a later offset than the
> > > fetch
> > > >> > > >> offset"?
> > > >> > > >>
> > > >> > > >>
> > > >> > > >>
> > > >> > > >> Guozhang
> > > >> > > >>
> > > >> > > >>
> > > >> > > >> On Tue, Jun 26, 2018 at 6:51 PM, Dong Lin <
> lindon...@gmail.com
> > >
> > > >> > wrote:
> > > >> > > >>
> > > >> > > >> > Hey Jason,
> > > >> > > >> >
> > > >> > > >> > Thanks for the explanation.
> > > >> > > >> >
> > > >> > > >> > Please correct me if this is wrong. The "unknown truncation
> > > >> offset"
> > > >> > > >> > scenario happens when consumer does not have the full
> > > >> leaderEpoch ->
> > > >> > > >> offset
> > > >> > > >> > mapping. In this case we can still use the KIP-101-based
> > > >> approach to
> > > >> > > >> > truncate offset to "start offset of the first Leader Epoch
> > > larger
> > > >> > than
> > > >> > > >> last
> > > >> > > >> > epoch of the consumer" but it may be inaccurate. So the KIP
> > > >> chooses
> > > >> > to
> > > >> > > >> use
> > > >> > > >> > the timestamp-based approach which is also best-effort.
> > > >> > > >> >
> > > >> > > >> > If this understanding is correct, for "closest" offset
> reset
> > > >> policy
> > > >> > > and
> > > >> > > >> > "unknown truncation offset" scenario, I am wondering
> whether
> > it
> > > >> > maybe
> > > >> > > >> > better to replace timestamp-based approach with KIP-101
> based
> > > >> > > approach.
> > > >> > > >> In
> > > >> > > >> > comparison to timestamp-based approach, the KIP-101-based
> > > >> approach
> > > >> > > >> seems to
> > > >> > > >> > simplify the API a bit since user does not need to
> understand
> > > >> > > timestamp.
> > > >> > > >> > Similar to the timestamp-based approach, both approaches
> are
> > > >> > > best-effort
> > > >> > > >> > and do not guarantee that consumer can consume all
> messages.
> > It
> > > >> is
> > > >> > not
> > > >> > > >> like
> > > >> > > >> > KIP-279 which guarantees that follower broker can consume
> all
> > > >> > messages
> > > >> > > >> from
> > > >> > > >> > the leader.
> > > >> > > >> >
> > > >> > > >> > Then it seems that the remaining difference is mostly about
> > > >> > accuracy,
> > > >> > > >> i.e.
> > > >> > > >> > how much message will be duplicated or missed in the
> "unknown
> > > >> > > truncation
> > > >> > > >> > offset" scenario. Not sure either one is clearly better
> than
> > > the
> > > >> > > other.
> > > >> > > >> > Note that there are two scenarios mentioned in KIP-279
> which
> > > are
> > > >> not
> > > >> > > >> > addressed by KIP-101. Both scenarios require quick
> leadership
> > > >> change
> > > >> > > >> > between brokers, which seems to suggest that the offset
> based
> > > >> > obtained
> > > >> > > >> > by "start
> > > >> > > >> > offset of the first Leader Epoch larger than last epoch of
> > the
> > > >> > > consumer"
> > > >> > > >> > under these two scenarios may be very close to the offset
> > > >> obtained
> > > >> > by
> > > >> > > >> the
> > > >> > > >> > message timestamp. Does this sound reasonable?
> > > >> > > >> >
> > > >> > > >> > Good point that users on v1 format can get benefit with
> > > timestamp
> > > >> > > based
> > > >> > > >> > approach. On the other hand it seems like a short term
> > benefit
> > > >> for
> > > >> > > users
> > > >> > > >> > who have not migrated. I am just not sure whether it is
> more
> > > >> > important
> > > >> > > >> than
> > > >> > > >> > designing a better API.
> > > >> > > >> >
> > > >> > > >> > Also, for both "latest" and "earliest" reset policy, do you
> > > >> think it
> > > >> > > >> would
> > > >> > > >> > make sense to also use the KIP-101 based approach to
> truncate
> > > >> offset
> > > >> > > for
> > > >> > > >> > the "unknown truncation offset" scenario?
> > > >> > > >> >
> > > >> > > >> >
> > > >> > > >> > Thanks,
> > > >> > > >> > Dong
> > > >> > > >> >
> > > >> > > >>
> > > >> > > >>
> > > >> > > >>
> > > >> > > >> --
> > > >> > > >> -- Guozhang
> > > >> > > >>
> > > >> > > >
> > > >> > > >
> > > >> > >
> > > >> >
> > > >> >
> > > >> >
> > > >> > --
> > > >> > -- Guozhang
> > > >> >
> > > >>
> > > >
> > > >
> > >
> >
>
