Yeah, the person who was working on it originally stopped working on Beam, and nobody else ever finished it. I think it is important to finish though. Many of the existing Sinks are only fully correct for Dataflow today, because they generate either Reshuffle or GroupByKey to ensure input stability before outputting (in many cases this code was inherited from before Beam existed). On Flink today, these sinks might occasionally produce duplicate output in the case of failures.
Reuven On Fri, Mar 1, 2019 at 7:18 AM Maximilian Michels <m...@apache.org> wrote: > Circling back to the RequiresStableInput annotation[1]. I've done some > protoyping to see how this could be integrated into Flink. I'm currently > writing a test based on RequiresStableInput. > > I found out there are already checks in place at the Runners to throw in > case transforms use RequiresStableInput and its not supported. However, > not a single transform actually uses the annotation. > > It seems that the effort stopped at some point? Would it make sense to > start annotating KafkaExactlyOnceSink with @RequiresStableInput? We > could then get rid of the whitelist. > > -Max > > [1] > > https://docs.google.com/document/d/117yRKbbcEdm3eIKB_26BHOJGmHSZl1YNoF0RqWGtqAM > > > On 01.03.19 14:28, Maximilian Michels wrote: > > Just realized that transactions do not spawn multiple elements in > > KafkaExactlyOnceSink. So the proposed solution to stop processing > > elements while a snapshot is pending would work. > > > > It is certainly not optimal in terms of performance for Flink and poses > > problems when checkpoints take long to complete, but it would be > > worthwhile to implement this to make use of the EOS feature. > > > > Thanks, > > Max > > > > On 01.03.19 12:23, Maximilian Michels wrote: > >> Thanks you for the prompt replies. It's great to see that there is > >> good understanding of how EOS in Flink works. > >> > >>> This is exactly what RequiresStableInput is supposed to do. On the > >>> Flink runner, this would be implemented by delaying processing until > >>> the current checkpoint is done. > >> > >> I don't think that works because we have no control over the Kafka > >> transactions. Imagine: > >> > >> 1) ExactlyOnceWriter writes records to Kafka and commits, then starts > >> a new transaction. > >> 2) Flink checkpoints, delaying the processing of elements, the > >> checkpoint fails. > >> 3) We restore from an old checkpoint and will start writing duplicate > >> data to Kafka. The de-duplication that the sink performs does not > >> help, especially because the random shards ids might be assigned > >> differently. > >> > >> IMHO we have to have control over commit to be able to provide EOS. > >> > >>> When we discussed this in Aug 2017, the understanding was that 2 > >>> Phase commit utility in Flink used to implement Flink's Kafka EOS > >>> could not be implemented in Beam's context. > >> > >> That's also my understanding, unless we change the interface. > >> > >>> I don't see how SDF solves this problem.. > >> > >> SDF has a checkpoint method which the Runner can call, but I think > >> that you are right, that the above problem would be the same. > >> > >>> Absolutely. I would love to support EOS in KakaIO for Flink. I think > >>> that will help many future exactly-once sinks.. and address > >>> fundamental incompatibility between Beam model and Flink's horizontal > >>> checkpointing for such applications. > >> > >> Great :) > >> > >>> The FlinkRunner would need to insert the "wait until checkpoint > >>> finalization" logic wherever it sees @RequiresStableInput, which is > >>> already what it would have to do. > >> > >> I don't think that fixes the problem. See above example. > >> > >> Thanks, > >> Max > >> > >> On 01.03.19 00:04, Raghu Angadi wrote: > >>> > >>> > >>> On Thu, Feb 28, 2019 at 2:42 PM Raghu Angadi <ang...@gmail.com > >>> <mailto:ang...@gmail.com>> wrote: > >>> > >>> On Thu, Feb 28, 2019 at 2:34 PM Kenneth Knowles <k...@apache.org > >>> <mailto:k...@apache.org>> wrote: > >>> > >>> I'm not sure what a hard fail is. I probably have a shallow > >>> understanding, but doesn't @RequiresStableInput work for 2PC? > >>> The preCommit() phase should establish the transaction and > >>> commit() is not called until after checkpoint finalization. Can > >>> you describe the way that it does not work a little bit more? > >>> > >>> > >>> - preCommit() is called before checkpoint. Kafka EOS in Flink > starts > >>> the transaction before this and makes sure it flushes all records > in > >>> preCommit(). So far good. > >>> - commit is called after checkpoint is persisted. Now, imagine > >>> commit() fails for some reason. There is no option to rerun the 1st > >>> phase to write the records again in a new transaction. This is a > >>> hard failure for the the job. In practice Flink might attempt to > >>> commit again (not sure how many times), which is likely to fail and > >>> eventually results in job failure. > >>> > >>> > >>> In Apache Beam, the records could be stored in state, and can be > >>> written inside commit() to work around this issue. It could have > >>> scalability issues if checkpoints are not frequent enough in Flink > >>> runner. > >>> > >>> Raghu. > >>> > >>> > >>> > >>> Kenn > >>> > >>> On Thu, Feb 28, 2019 at 1:25 PM Raghu Angadi <ang...@gmail.com > >>> <mailto:ang...@gmail.com>> wrote: > >>> > >>> On Thu, Feb 28, 2019 at 11:01 AM Kenneth Knowles > >>> <k...@apache.org <mailto:k...@apache.org>> wrote: > >>> > >>> I believe the way you would implement the logic behind > >>> Flink's KafkaProducer would be to have two steps: > >>> > >>> 1. Start transaction > >>> 2. @RequiresStableInput Close transaction > >>> > >>> > >>> I see. What happens if closing the transaction fails in > >>> (2)? Flink's 2PC requires that commit() should never hard > >>> fail once preCommit() succeeds. I think that is cost of not > >>> having an extra shuffle. It is alright since this policy > has > >>> worked well for Flink so far. > >>> > >>> Overall, it will be great to have @RequiresStableInput > >>> support in Flink runner. > >>> > >>> Raghu. > >>> > >>> The FlinkRunner would need to insert the "wait until > >>> checkpoint finalization" logic wherever it > >>> sees @RequiresStableInput, which is already what it > >>> would have to do. > >>> > >>> This matches the KafkaProducer's logic - delay closing > >>> the transaction until checkpoint finalization. This > >>> answers my main question, which is "is > >>> @RequiresStableInput expressive enough to allow > >>> Beam-on-Flink to have exactly once behavior with the > >>> same performance characteristics as native Flink > >>> checkpoint finalization?" > >>> > >>> Kenn > >>> > >>> [1] https://github.com/apache/beam/pull/7955 > >>> > >>> On Thu, Feb 28, 2019 at 10:43 AM Reuven Lax > >>> <re...@google.com <mailto:re...@google.com>> wrote: > >>> > >>> > >>> > >>> On Thu, Feb 28, 2019 at 10:41 AM Raghu Angadi > >>> <ang...@gmail.com <mailto:ang...@gmail.com>> > wrote: > >>> > >>> > >>> Now why does the Flink Runner not support > >>> KafkaIO EOS? Flink's native > >>> KafkaProducer supports exactly-once. It > >>> simply commits the pending > >>> transaction once it has completed a > >>> checkpoint. > >>> > >>> > >>> > >>> On Thu, Feb 28, 2019 at 9:59 AM Maximilian > >>> Michels <m...@apache.org <mailto:m...@apache.org > >> > >>> wrote: > >>> > >>> Hi, > >>> > >>> I came across KafkaIO's Runner whitelist > [1] > >>> for enabling exactly-once > >>> semantics (EOS). I think it is questionable > >>> to exclude Runners from > >>> inside a transform, but I see that the > >>> intention was to save users from > >>> surprises. > >>> > >>> Now why does the Flink Runner not support > >>> KafkaIO EOS? Flink's native > >>> KafkaProducer supports exactly-once. It > >>> simply commits the pending > >>> transaction once it has completed a > >>> checkpoint. > >>> > >>> > >>> > >>> When we discussed this in Aug 2017, the > >>> understanding was that 2 Phase commit utility > in > >>> Flink used to implement Flink's Kafka EOS could > >>> not be implemented in Beam's context. > >>> See this message > >>> <https://www.mail-archive.com/dev@beam.apache.org/msg02664.html> in > >>> that dev thread. Has anything changed in this > >>> regard? The whole thread is relevant to this > >>> topic and worth going through. > >>> > >>> I think that TwoPhaseCommit utility class wouldn't > >>> work. The Flink runner would probably want to > >>> directly use notifySnapshotComplete in order to > >>> implement @RequiresStableInput. > >>> > >>> > >>> A checkpoint is realized by sending > barriers > >>> through all channels > >>> starting from the source until reaching all > >>> sinks. Every operator > >>> persists its state once it has received a > >>> barrier on all its input > >>> channels, it then forwards it to the > >>> downstream operators. > >>> > >>> The architecture of Beam's > >>> KafkaExactlyOnceSink is as follows[2]: > >>> > >>> Input -> AssignRandomShardIds -> GroupByKey > >>> -> AssignSequenceIds -> > >>> GroupByKey -> ExactlyOnceWriter > >>> > >>> As I understood, Spark or Dataflow use the > >>> GroupByKey stages to persist > >>> the input. That is not required in Flink to > >>> be able to take a consistent > >>> snapshot of the pipeline. > >>> > >>> Basically, for Flink we don't need any of > >>> that magic that KafkaIO does. > >>> What we would need to support EOS is a way > >>> to tell the ExactlyOnceWriter > >>> (a DoFn) to commit once a checkpoint has > >>> completed. > >>> > >>> I know that the new version of SDF supports > >>> checkpointing which should > >>> solve this issue. But there is still a lot > >>> of work to do to make this > >>> reality. > >>> > >>> > >>> I don't see how SDF solves this problem.. May > be > >>> pseudo code would make more clear. But if > >>> helps, that is great! > >>> > >>> So I think it would make sense to think > >>> about a way to make KafkaIO's > >>> EOS more accessible to Runners which > support > >>> a different way of > >>> checkpointing. > >>> > >>> > >>> Absolutely. I would love to support EOS in > >>> KakaIO for Flink. I think that will help many > >>> future exactly-once sinks.. and address > >>> fundamental incompatibility between Beam model > >>> and Flink's horizontal checkpointing for such > >>> applications. > >>> > >>> Raghu. > >>> > >>> Cheers, > >>> Max > >>> > >>> PS: I found this document about > >>> RequiresStableInput [3], but IMHO > >>> defining an annotation only manifests the > >>> conceptual difference between > >>> the Runners. > >>> > >>> > >>> [1] > >>> > https://github.com/apache/beam/blob/988a22f01bb133dd65b3cc75e05978d695aed76c/sdks/java/io/kafka/src/main/java/org/apache/beam/sdk/io/kafka/KafkaIO.java#L1144 > >>> > >>> [2] > >>> > https://github.com/apache/beam/blob/988a22f01bb133dd65b3cc75e05978d695aed76c/sdks/java/io/kafka/src/main/java/org/apache/beam/sdk/io/kafka/KafkaExactlyOnceSink.java#L166 > >>> > >>> [3] > >>> > https://docs.google.com/document/d/117yRKbbcEdm3eIKB_26BHOJGmHSZl1YNoF0RqWGtqAM > >>> > >>> >
