Hi Kaymak, To answer your last question: there will be no data loss in that scenario you described, but there could be duplicate processed records.
With checkpointing enabled, the Flink Kafka consumer does not commit offsets back to Kafka until offsets in Flink checkpoints have been persisted. That external offset commit, however, is not guaranteed to happen, and always "lag" behind the offsets maintained internally in Flink checkpoints. That is the reason for why there may be duplicate consumed records if you rely on those on startup, instead of the offsets maintained within Flink. The rule of thumb is: Committed offsets back to Kafka by the Flink Kafka consumer is only a means to expose progress to the outside world, and there is no guarantee that those committed offsets are consistent with operator states in the streaming job. BR, Gordon On Mon, Mar 2, 2020, 11:18 PM Kaymak, Tobias <tobias.kay...@ricardo.ch> wrote: > Thank you! One last question regarding Gordons response. When a pipeline > stops consuming and cleanly shuts down and there is no error during that > process, and then it gets started again and uses the last committed offset > in Kafka - there should be no data loss - or am I missing something? > > In what scenario should I expect a data loss? (I can only think of the > jobmanager or taskmanager getting killed before the shutdown is done.) > > Best, > Tobi > > On Mon, Mar 2, 2020 at 1:45 PM Piotr Nowojski <pi...@ververica.com> wrote: > >> Hi, >> >> Sorry for my previous slightly confusing response, please take a look at >> the response from Gordon. >> >> Piotrek >> >> On 2 Mar 2020, at 12:05, Kaymak, Tobias <tobias.kay...@ricardo.ch> wrote: >> >> Hi, >> >> let me refine my question: My pipeline is generated from Beam, so the >> Flink pipeline is a translated Beam pipeline. When I update my Apache Beam >> pipeline code, working with a snapshot in Flink to stop the pipeline is not >> an option, as the snapshot will use the old representation of the the Flink >> pipeline when resuming from that snapshot. >> >> Meaning that I am looking for a way to drain the pipeline cleanly and >> using the last committed offset in Kafka to resume processing after I >> started it again (launching it through Beam will regenerate the Flink >> pipeline and it should resume at the offset where it left of, that is the >> latest committed offset in Kafka). >> >> Can this be achieved with a cancel or stop of the Flink pipeline? >> >> Best, >> Tobias >> >> On Mon, Mar 2, 2020 at 11:09 AM Piotr Nowojski <pi...@ververica.com> >> wrote: >> >>> Hi Tobi, >>> >>> No, FlinkKafkaConsumer is not using committed Kafka’s offsets for >>> recovery. Offsets where to start from are stored in the checkpoint itself. >>> Updating the offsets back to Kafka is an optional, purely cosmetic thing >>> from the Flink’s perspective, so the job will start from the correct >>> offsets. >>> >>> However, if you for whatever the reason re-start the job from a >>> savepoint/checkpoint that’s not the latest one, this will violate >>> exactly-once guarantees - there will be some duplicated records committed >>> two times in the sinks, as simply some records would be processed and >>> committed twice. Committing happens on checkpoint, so if you are recovering >>> to some previous checkpoint, there is nothing Flink can do - some records >>> were already committed before. >>> >>> Piotrek >>> >>> On 2 Mar 2020, at 10:12, Kaymak, Tobias <tobias.kay...@ricardo.ch> >>> wrote: >>> >>> Thank you Piotr! >>> >>> One last question - let's assume my source is a Kafka topic - if I stop >>> via the CLI with a savepoint in Flink 1.9, but do not use that savepoint >>> when restarting my job - the job would continue from the last offset that >>> has been committed in Kafka and thus I would also not experience a loss of >>> data in my sink. Is that correct? >>> >>> Best, >>> Tobi >>> >>> On Fri, Feb 28, 2020 at 3:17 PM Piotr Nowojski <pi...@ververica.com> >>> wrote: >>> >>>> Yes, that’s correct. There shouldn’t be any data loss. Stop with >>>> savepoint is a solution to make sure, that if you are stopping a job >>>> (either permanently or temporarily) that all of the results are >>>> published/committed to external systems before you actually stop the job. >>>> >>>> If you just cancel/kill/crash a job, in some rare cases (if a >>>> checkpoint was completing at the time cluster was crashing), some records >>>> might not be committed before the cancellation/kill/crash happened. Also >>>> note that doesn’t mean there is a data loss, just those records will be >>>> published once you restore your job from a checkpoint. If you want to stop >>>> the job permanently, that might not happen, hence we need stop with >>>> savepoint. >>>> >>>> Piotrek >>>> >>>> On 28 Feb 2020, at 15:02, Kaymak, Tobias <tobias.kay...@ricardo.ch> >>>> wrote: >>>> >>>> Thank you! For understanding the matter: When I have a streaming >>>> pipeline (reading from Kafka, writing somewhere) and I click "cancel" and >>>> after that I restart the pipeline - I should not expect any data to be lost >>>> - is that correct? >>>> >>>> Best, >>>> Tobias >>>> >>>> On Fri, Feb 28, 2020 at 2:51 PM Piotr Nowojski <pi...@ververica.com> >>>> wrote: >>>> >>>>> Thanks for confirming that Yadong. I’ve created a ticket for that [1]. >>>>> >>>>> Piotrek >>>>> >>>>> [1] https://issues.apache.org/jira/browse/FLINK-16340 >>>>> >>>>> On 28 Feb 2020, at 14:32, Yadong Xie <vthink...@gmail.com> wrote: >>>>> >>>>> Hi >>>>> >>>>> 1. the old stop button was removed in flink 1.9.0 since it could not >>>>> work properly as I know >>>>> 2. if we have the feature of the stop with savepoint, we could add it >>>>> to the web UI, but it may still need some work on the rest API to support >>>>> the new feature >>>>> >>>>> >>>>> Best, >>>>> Yadong >>>>> >>>>> >>>>> Piotr Nowojski <pi...@ververica.com> 于2020年2月28日周五 下午8:49写道： >>>>> >>>>>> Hi, >>>>>> >>>>>> I’m not sure. Maybe Yadong (CC) will know more, but to the best of my >>>>>> knowledge and research: >>>>>> >>>>>> 1. In Flink 1.9 we switched from the old webUI to a new one, that >>>>>> probably explains the difference you are seeing. >>>>>> 2. The “Stop” button in the old webUI, was not working properly - >>>>>> that was not stop with savepoint, as stop with savepoint is a relatively >>>>>> new feature. >>>>>> 3. Now that we have stop with savepoint (it can be used from CLI as >>>>>> you wrote), probably we could expose this feature in the new UI as well, >>>>>> unless it’s already exposed somewhere? Yadong, do you know an answer for >>>>>> that? >>>>>> >>>>>> Piotrek >>>>>> >>>>>> On 27 Feb 2020, at 13:31, Kaymak, Tobias <tobias.kay...@ricardo.ch> >>>>>> wrote: >>>>>> >>>>>> Hello, >>>>>> >>>>>> before Flink 1.9 I was able to "Stop" a streaming pipeline - after >>>>>> clicking that button in the webinterface it performed a clean shutdown. >>>>>> Now >>>>>> with Flink 1.9 I just see the option to cancel it. >>>>>> >>>>>> However, using the commandline flink stop -d >>>>>> 266c5b38cf9d8e61a398a0bef4a1b350 still does the trick. So the >>>>>> functionality is there. >>>>>> >>>>>> Has the button been removed on purpose? >>>>>> >>>>>> Best, >>>>>> Tobias >>>>>> >>>>>> >>>>>> >>>>> >>>> >>>> -- >>>> >>>> Tobias Kaymak >>>> Data Engineer >>>> Data Intelligence >>>> >>>> tobias.kay...@ricardo.ch >>>> www.ricardo.ch >>>> Theilerstrasse 1a, 6300 Zug >>>> >>>> >>>> >>> >>
