Hi Boyang/Jason,
I've also thought about this (i.e. using CommitFailed for all non-fatal),
but what I'm pondering is that, in the catch (CommitFailed) block, what
would happen if the `producer.abortTransaction();` throws again? should
that be captured as a fatal and cause the client to close again.
If yes, then naively the pattern would be:
...
catch (CommitFailedException e) {
// Transaction commit failed with abortable error, user could reset
// the application state and resume with a new transaction. The root
// cause was wrapped in the thrown exception.
resetToLastCommittedPositions(consumer);
try {
producer.abortTransaction();
} catch (KafkaException e) {
producer.close();
consumer.close();
throw e;
}
} catch (KafkaException e) {
producer.close();
consumer.close();
throw e;
}
...
Guozhang
On Fri, Jan 22, 2021 at 10:47 AM Boyang Chen <reluctanthero...@gmail.com>
wrote:
> Hey Guozhang,
>
> Jason and I were discussing the new API offline and decided to take another
> approach. Firstly, the reason not to invent a new API with returned boolean
> flag is for compatibility consideration, since old EOS code would not know
> that a given transaction commit was failed internally as they don't listen
> to the output flag. Our proposed alternative solution is to let
> *commitTransaction
> throw CommitFailedException whenever the commit failed with non-fatal
> exception*, so that on the caller side the handling logic becomes:
>
> try {
> if (shouldCommit) {
> producer.commitTransaction();
> } else {
> resetToLastCommittedPositions(consumer);
> producer.abortTransaction();
> }
> } catch (CommitFailedException e) {
> // Transaction commit failed with abortable error, user could reset
> // the application state and resume with a new transaction. The
> root
> // cause was wrapped in the thrown exception.
> resetToLastCommittedPositions(consumer);
> producer.abortTransaction();
> } catch (KafkaException e) {
> producer.close();
> consumer.close();
> throw e;
> }
>
> This approach looks cleaner as all exception types other than CommitFailed
> will doom to be fatal, which is very easy to adopt for users. In the
> meantime, we still maintain the commitTxn behavior to throw instead of
> silently failing.
>
> In addition, we decided to drop the recommendation to handle
> TimeoutException and leave it to the users to make the call. The downside
> for blindly calling abortTxn upon timeout is that we could result in an
> illegal state when the commit was already successful on the broker
> side. Without a good guarantee on the outcome, overcomplicating the
> template should not be encouraged IMHO.
>
> Let me know your thoughts on the new approach here, thank you!
>
> Best,
> Boyang
>
> On Tue, Jan 19, 2021 at 11:11 AM Guozhang Wang <wangg...@gmail.com> wrote:
>
> > Thanks for your clarification on 2)/3), that makes sense.
> >
> > On Tue, Jan 19, 2021 at 10:16 AM Boyang Chen <reluctanthero...@gmail.com
> >
> > wrote:
> >
> > > Thanks for the input Guozhang, replied inline.
> > >
> > > On Mon, Jan 18, 2021 at 8:57 PM Guozhang Wang <wangg...@gmail.com>
> > wrote:
> > >
> > > > Hello Boyang,
> > > >
> > > > Thanks for the updated KIP. I read it again and have the following
> > > > thoughts:
> > > >
> > > > 0. I'm a bit concerned that if commitTxn does not throw any non-fatal
> > > > exception, and instead we rely on the subsequent beginTxn call to
> > throw,
> > > it
> > > > may violate some callers with a pattern that relying on commitTxn to
> > > > succeed to make some non-rollback operations. For example:
> > > >
> > > > beginTxn()
> > > > // do some read-write on my local DB
> > > > commitTxn()
> > > > // if commitTxn succeeds, then commit the DB
> > > >
> > > > -------------
> > > >
> > > > The issue is that, committing DB is a non-rollback operation, and
> users
> > > may
> > > > just rely on commitTxn to return without error to make this
> > non-rollback
> > > > call. Of course we can just claim this pattern is not legitimate and
> is
> > > not
> > > > the right way of doing things, but many users may naturally adopt
> this
> > > > pattern.
> > > >
> > > > So maybe we should still let commitTxn also throw non-fatal
> exceptions,
> > > in
> > > > which case we would then call abortTxn again.
> > > >
> > > > But if we do this, the pattern becomes:
> > > >
> > > > try {
> > > > beginTxn()
> > > > // do something
> > > > } catch (Exception) {
> > > > shouldCommit = false;
> > > > }
> > > >
> > > > if (shouldCommit) {
> > > > try {
> > > > commitTxn()
> > > > } catch (...) { // enumerate all fatal exceptions
> > > > shutdown()
> > > > } catch (KafkaException) {
> > > > // non-fatal
> > > > shouldCommit = false;
> > > > }
> > > > }
> > > >
> > > > if (!shouldCommit && running()) {
> > > > try {
> > > > abortTxn()
> > > > } catch (KafkaException) {
> > > > // only throw fatal
> > > > shutdown()
> > > > }
> > > > }
> > > >
> > > > ---------------------
> > > >
> > > > Which is much more complicated.
> > > >
> > > > Thank makes me think, the alternative we have discussed offline may
> be
> > > > better: let commitTxn() to return a boolean flag.
> > > >
> > > > * If it returns true, it means the commit succeeded. Users can
> > > comfortably
> > > > continue and do any external non-rollback operations if they like.
> > > > * If it returns false, it means the commit failed with non-fatal
> error
> > > *AND
> > > > the txn has been aborted*. Users do not need to call abortTxn again.
> > > > * If it throws, then it means fatal errors. Users should shut down
> the
> > > > client.
> > > >
> > > > In this case, the pattern becomes:
> > > >
> > > > try {
> > > > beginTxn()
> > > > // do something
> > > > } catch (Exception) {
> > > > shouldCommit = false;
> > > > }
> > > >
> > > > try {
> > > > if (shouldCommit) {
> > > > commitSucceeded = commitTxn()
> > > > } else {
> > > > // reset offsets, rollback operations, etc
> > > > abortTxn()
> > > > }
> > > > } catch (KafkaException) {
> > > > // only throw fatal
> > > > shutdown()
> > > > }
> > > >
> > > > if (commitSucceeded)
> > > > // do other non-rollbackable things
> > > > else
> > > > // reset offsets, rollback operations, etc
> > > >
> > > > -------------------------
> > > >
> > > > Of course, if we want to have better visibility into what caused the
> > > commit
> > > > to fail and txn to abort. We can let the return type be an enum
> instead
> > > of
> > > > a flag. But my main idea is to still let the commitTxn be the final
> > point
> > > > users can tell whether this txn succeeded or not, instead of relying
> on
> > > the
> > > > next beginTxn() call.
> > > >
> > > > I agree that adding a boolean flag is indeed useful in this case.
> Will
> > > update the KIP.
> > >
> > > 1. Re: "while maintaining the behavior to throw fatal exception in raw"
> > not
> > > > sure what do you mean by "throw" here. Are you proposing the callback
> > > would
> > > > *pass* (not throw) in any fatal exceptions when triggered without
> > > wrapping?
> > > >
> > > > Yes, I want to say *pass*, the benefit is to make the end user's
> > > expectation consistent
> > > regarding exception handling.
> > >
> > >
> > > > 2. I'm not sure I understand the claim regarding the callback that
> "In
> > > EOS
> > > > setup, it is not required to handle the exception". Are you proposing
> > > that,
> > > > e.g. in Streams, we do not try to handle any exceptions if EOS is
> > enabled
> > > > in the callback anymore, but just let commitTxn() itself to fail to
> be
> > > > notified about the problem? Personally I think in Streams we should
> > just
> > > > make the handling logic of the callback to be consistent regardless
> of
> > > the
> > > > EOS settings (today we have different logic depending on this logic,
> > > which
> > > > I think could be unified as well).
> > > >
> > > > My idea originates from the claim on send API:
> > > "When used as part of a transaction, it is not necessary to define a
> > > callback or check the result of the future in order to detect errors
> > from
> > > <code>send</code>. "
> > > My understanding is that for EOS, the exception will be detected by
> > calling
> > > transactional APIs either way, so it is a duplicate handling to track
> > > all the sendExceptions in RecordCollector. However, I looked up
> > > sendException is being used today as follow:
> > >
> > > 1. Pass to "ProductionExceptionHandler" for any default or customized
> > > exception handler to handle
> > > 2. Stop collecting offset info or new exceptions
> > > 3. Check and rethrow exceptions in close(), flush() or new send() calls
> > >
> > > To my understanding, we should still honor the commitment to #1 for any
> > > user customized implementation. The #2 does not really affect our
> > decision
> > > upon EOS. The #3 here is still valuable as it could help us fail fast
> in
> > > new send() instead of waiting to later stage of processing. In that
> > sense,
> > > I agree we should continue to record send exceptions even under EOS
> case
> > to
> > > ensure the strength of stream side Producer logic. On the safer side,
> we
> > no
> > > longer need to wrap certain fatal exceptions like ProducerFenced as
> > > TaskMigrated, since they should not crash the stream thread if thrown
> in
> > > raw format, once we adopt the new processing model in the send phase.
> > >
> > >
> > > >
> > > > Guozhang
> > > >
> > > >
> > > >
> > > > On Thu, Dec 17, 2020 at 8:42 PM Boyang Chen <
> > reluctanthero...@gmail.com>
> > > > wrote:
> > > >
> > > > > Thanks for everyone's feedback so far. I have polished the KIP
> after
> > > > > offline discussion with some folks working on EOS to make the
> > exception
> > > > > handling more lightweight. The essential change is that we are not
> > > > > inventing a new intermediate exception type, but instead
> separating a
> > > > full
> > > > > transaction into two phases:
> > > > >
> > > > > 1. The data transmission phase
> > > > > 2. The commit phase
> > > > >
> > > > > This way for any exception thrown from phase 1, will be an
> indicator
> > in
> > > > > phase 2 whether we should do commit or abort, and from now on
> > > > > `commitTransaction` should only throw fatal exceptions, similar to
> > > > > `abortTransaction`, so that any KafkaException caught in the commit
> > > phase
> > > > > will be definitely fatal to crash the app. For more advanced users
> > such
> > > > as
> > > > > Streams, we have the ability to further wrap selected types of
> fatal
> > > > > exceptions to trigger task migration if necessary.
> > > > >
> > > > > More details in the KIP, feel free to take another look, thanks!
> > > > >
> > > > > On Thu, Dec 17, 2020 at 8:36 PM Boyang Chen <
> > > reluctanthero...@gmail.com>
> > > > > wrote:
> > > > >
> > > > > > Thanks Bruno for the feedback.
> > > > > >
> > > > > > On Mon, Dec 7, 2020 at 5:26 AM Bruno Cadonna <br...@confluent.io
> >
> > > > wrote:
> > > > > >
> > > > > >> Thanks Boyang for the KIP!
> > > > > >>
> > > > > >> Like Matthias, I do also not know the producer internal well
> > enough
> > > to
> > > > > >> comment on the categorization. However, I think having a super
> > > > exception
> > > > > >> (e.g. RetriableException) that encodes if an exception is fatal
> > or
> > > > not
> > > > > >> is cleaner, better understandable and less error-prone, because
> > > > ideally
> > > > > >> when you add a new non-fatal exception in future you just need
> to
> > > > think
> > > > > >> about letting it inherit from the super exception and all the
> rest
> > > of
> > > > > >> the code will just behave correctly without the need to wrap the
> > new
> > > > > >> exception into another exception each time it is thrown (maybe
> it
> > is
> > > > > >> thrown at different location in the code).
> > > > > >>
> > > > > >> As far as I understand the following statement from your
> previous
> > > > e-mail
> > > > > >> is the reason that currently such a super exception is not
> > possible:
> > > > > >>
> > > > > >> "Right now we have RetriableException type, if we are going to
> > add a
> > > > > >> `ProducerRetriableException` type, we have to put this new
> > interface
> > > > as
> > > > > >> the parent of the RetriableException, because not all thrown
> > > non-fatal
> > > > > >> exceptions are `retriable` in general for producer"
> > > > > >>
> > > > > >>
> > > > > >> In the list of exceptions in your KIP, I found non-fatal
> > exceptions
> > > > that
> > > > > >> do not inherit from RetriableException. I guess those are the
> ones
> > > you
> > > > > >> are referring to in your statement:
> > > > > >>
> > > > > >> InvalidProducerEpochException
> > > > > >> InvalidPidMappingException
> > > > > >> TransactionAbortedException
> > > > > >>
> > > > > >> All of those exceptions are non-fatal and do not inherit from
> > > > > >> RetriableException. Is there a reason for that? If they depended
> > > from
> > > > > >> RetriableException we would be a bit closer to a super
> exception I
> > > > > >> mention above.
> > > > > >>
> > > > > >> The reason is that sender may catch those exceptions in the
> > > > > > ProduceResponse, and it currently does infinite
> > > > > > retries on RetriableException. To make sure we could trigger the
> > > > > > abortTransaction() by catching non-fatal thrown
> > > > > > exceptions and reinitialize the txn state, we chose not to let
> > those
> > > > > > exceptions inherit RetriableException so that
> > > > > > they won't cause infinite retry on sender.
> > > > > >
> > > > > >
> > > > > >> With OutOfOrderSequenceException and
> UnknownProducerIdException, I
> > > > think
> > > > > >> to understand that their fatality depends on the type (i.e.
> > > > > >> configuration) of the producer. That makes it difficult to have
> a
> > > > super
> > > > > >> exception that encodes the retriability as mentioned above.
> Would
> > it
> > > > be
> > > > > >> possible to introduce exceptions that inherit from
> > > RetriableException
> > > > > >> and that are thrown when those exceptions are caught from the
> > > brokers
> > > > > >> and the type of the producer is such that the exceptions are
> > > > retriable?
> > > > > >>
> > > > > >> Yea, I think in general the exception type mixing is difficult
> to
> > > get
> > > > > > them all right. I have already proposed another solution based on
> > my
> > > > > > offline discussion with some folks working on EOS
> > > > > > to make the handling more straightforward for end users without
> the
> > > > need
> > > > > > to distinguish exception fatality.
> > > > > >
> > > > > >> Best,
> > > > > >> Bruno
> > > > > >>
> > > > > >>
> > > > > >> On 04.12.20 19:34, Guozhang Wang wrote:
> > > > > >> > Thanks Boyang for the proposal! I made a pass over the list
> and
> > > here
> > > > > are
> > > > > >> > some thoughts:
> > > > > >> >
> > > > > >> > 0) Although this is not part of the public API, I think we
> > should
> > > > make
> > > > > >> sure
> > > > > >> > that the suggested pattern (i.e. user can always call
> abortTxn()
> > > > when
> > > > > >> > handling non-fatal errors) are indeed supported. E.g. if the
> txn
> > > is
> > > > > >> already
> > > > > >> > aborted by the broker side, then users can still call abortTxn
> > > which
> > > > > >> would
> > > > > >> > not throw another exception but just be treated as a no-op.
> > > > > >> >
> > > > > >> > 1) *ConcurrentTransactionsException*: I think this error can
> > also
> > > be
> > > > > >> > returned but not documented yet. This should be a non-fatal
> > error.
> > > > > >> >
> > > > > >> > 2) *InvalidTxnStateException*: this error is returned from
> > broker
> > > > when
> > > > > >> txn
> > > > > >> > state transition failed (e.g. it is trying to transit to
> > > > > complete-commit
> > > > > >> > while the current state is not prepare-commit). This error
> could
> > > > > >> indicates
> > > > > >> > a bug on the client internal code or the broker code, OR a
> user
> > > > error
> > > > > >> --- a
> > > > > >> > similar error is ConcurrentTransactionsException, i.e. if
> Kafka
> > is
> > > > > >> bug-free
> > > > > >> > these exceptions would only be returned if users try to do
> > > something
> > > > > >> wrong,
> > > > > >> > e.g. calling abortTxn right after a commitTxn, etc. So I'm
> > > thinking
> > > > it
> > > > > >> > should be a non-fatal error instead of a fatal error, wdyt?
> > > > > >> >
> > > > > >> > 3) *KafkaException*: case i "indicates fatal transactional
> > > sequence
> > > > > >> > (Fatal)", this is a bit conflicting with the
> > > > *OutOfSequenceException*
> > > > > >> that
> > > > > >> > is treated as non-fatal. I guess your proposal is that
> > > > > >> > OutOfOrderSequenceException would be treated either as fatal
> > with
> > > > > >> > transactional producer, or non-fatal with idempotent producer,
> > is
> > > > that
> > > > > >> > right? If the producer is only configured with idempotency but
> > not
> > > > > >> > transaction, then throwing a
> TransactionStateCorruptedException
> > > for
> > > > > >> > non-fatal errors would be confusing since users are not using
> > > > > >> transactions
> > > > > >> > at all.. So I suggest we always throw OutOfSequenceException
> > as-is
> > > > > (i.e.
> > > > > >> > not wrapped) no matter how the producer is configured, and let
> > the
> > > > > >> caller
> > > > > >> > decide how to handle it based on whether it is only idempotent
> > or
> > > > > >> > transactional itself.
> > > > > >> >
> > > > > >> > 4) Besides all the txn APIs, the `send()` callback / future
> can
> > > also
> > > > > >> throw
> > > > > >> > txn-related exceptions, I think this KIP should also cover
> this
> > > API
> > > > as
> > > > > >> well?
> > > > > >> >
> > > > > >> > 5) This is related to 1/2) above: sometimes those non-fatal
> > errors
> > > > > like
> > > > > >> > ConcurrentTxn or InvalidTxnState are not due to the state
> being
> > > > > >> corrupted
> > > > > >> > at the broker side, but maybe users are doing something wrong.
> > So
> > > > I'm
> > > > > >> > wondering if we should further distinguish those non-fatal
> > errors
> > > > > >> between
> > > > > >> > a) those that are caused by Kafka itself, e.g. a broker timed
> > out
> > > > and
> > > > > >> > aborted a txn and later an endTxn request is received, and b)
> > the
> > > > > user's
> > > > > >> > API call pattern is incorrect, causing the request to be
> > rejected
> > > > with
> > > > > >> an
> > > > > >> > error code from the broker.
> *TransactionStateCorruptedException*
> > > > feels
> > > > > >> to
> > > > > >> > me more like for case a), but not case b).
> > > > > >> >
> > > > > >> >
> > > > > >> > Guozhang
> > > > > >> >
> > > > > >> >
> > > > > >> > On Wed, Dec 2, 2020 at 4:50 PM Boyang Chen <
> > > > > reluctanthero...@gmail.com>
> > > > > >> > wrote:
> > > > > >> >
> > > > > >> >> Thanks Matthias, I think your proposal makes sense as well,
> on
> > > the
> > > > > pro
> > > > > >> side
> > > > > >> >> we could have a universally agreed exception type to be
> caught
> > by
> > > > the
> > > > > >> user,
> > > > > >> >> without having an extra layer on top of the actual
> exceptions.
> > I
> > > > > could
> > > > > >> see
> > > > > >> >> some issue on downsides:
> > > > > >> >>
> > > > > >> >> 1. The exception hierarchy will be more complex. Right now we
> > > have
> > > > > >> >> RetriableException type, if we are going to add a
> > > > > >> >> `ProducerRetriableException` type, we have to put this new
> > > > interface
> > > > > >> as the
> > > > > >> >> parent of the RetriableException, because not all thrown
> > > non-fatal
> > > > > >> >> exceptions are `retriable` in general for producer, for
> example
> > > > > >> >> <
> > > > > >> >>
> > > > > >>
> > > > >
> > > >
> > >
> >
> https://github.com/apache/kafka/blob/e275742f850af4a1b79b0d1bd1ac9a1d2e89c64e/clients/src/main/java/org/apache/kafka/clients/producer/internals/Sender.java#L745
> > > > > >> >>> .
> > > > > >> >> We could have an empty interface `ProducerRetriableException`
> > to
> > > > let
> > > > > >> all
> > > > > >> >> the thrown exceptions implement for sure, but it's a bit
> > > unorthodox
> > > > > in
> > > > > >> the
> > > > > >> >> way we deal with exceptions in general.
> > > > > >> >>
> > > > > >> >> 2. There are cases where we throw a KafkaException wrapping
> > > another
> > > > > >> >> KafkaException as either fatal or non-fatal. If we use an
> > > interface
> > > > > to
> > > > > >> >> solve #1, it is also required to implement another bloated
> > > > exception
> > > > > >> class
> > > > > >> >> which could replace KafkaException type, as we couldn't mark
> > > > > >> KafkaException
> > > > > >> >> as retriable for sure.
> > > > > >> >>
> > > > > >> >> 3. In terms of the encapsulation, wrapping means we could
> limit
> > > the
> > > > > >> scope
> > > > > >> >> of affection to the producer only, which is important since
> we
> > > > don't
> > > > > >> want
> > > > > >> >> shared exception types to implement a producer-related
> > interface,
> > > > > such
> > > > > >> >> as UnknownTopicOrPartitionException.
> > > > > >> >>
> > > > > >> >> Best,
> > > > > >> >> Boyang
> > > > > >> >>
> > > > > >> >> On Wed, Dec 2, 2020 at 3:38 PM Matthias J. Sax <
> > mj...@apache.org
> > > >
> > > > > >> wrote:
> > > > > >> >>
> > > > > >> >>> Thanks for the KIP Boyang!
> > > > > >> >>>
> > > > > >> >>> Overall, categorizing exceptions makes a lot of sense. As I
> > > don't
> > > > > know
> > > > > >> >>> the producer internals well enough, I cannot comment on the
> > > > > >> >>> categorization in detail though.
> > > > > >> >>>
> > > > > >> >>> What I am wondering is, if we should introduce an exception
> > > > > interface
> > > > > >> >>> that non-fatal exception would implement instead of
> creating a
> > > new
> > > > > >> class
> > > > > >> >>> that will wrap non-fatal exceptions? What would be the
> > pros/cons
> > > > for
> > > > > >> >>> both designs?
> > > > > >> >>>
> > > > > >> >>>
> > > > > >> >>> -Matthias
> > > > > >> >>>
> > > > > >> >>>
> > > > > >> >>> On 12/2/20 11:35 AM, Boyang Chen wrote:
> > > > > >> >>>> Hey there,
> > > > > >> >>>>
> > > > > >> >>>> I would like to start a discussion thread for KIP-691:
> > > > > >> >>>>
> > > > > >> >>>
> > > > > >> >>
> > > > > >>
> > > > >
> > > >
> > >
> >
> https://cwiki.apache.org/confluence/display/KAFKA/KIP-691%3A+Enhance+Transactional+Producer+Exception+Handling
> > > > > >> >>>>
> > > > > >> >>>> The KIP is aiming to simplify the exception handling logic
> > for
> > > > > >> >>>> transactional Producer users by classifying fatal and
> > non-fatal
> > > > > >> >>> exceptions
> > > > > >> >>>> and throw them correspondingly for easier catch and retry.
> > Let
> > > me
> > > > > >> know
> > > > > >> >>> what
> > > > > >> >>>> you think.
> > > > > >> >>>>
> > > > > >> >>>> Best,
> > > > > >> >>>> Boyang
> > > > > >> >>>>
> > > > > >> >>>
> > > > > >> >>
> > > > > >> >
> > > > > >> >
> > > > > >>
> > > > > >
> > > > >
> > > >
> > > >
> > > > --
> > > > -- Guozhang
> > > >
> > >
> >
> >
> > --
> > -- Guozhang
> >
>
--
-- Guozhang
