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From Gábor Gévay <gga...@gmail.com>
Subject Re: How to ensure exactly-once semantics in output to Kafka?
Date Fri, 05 Feb 2016 12:09:55 GMT
Hello,

> I think that there is actually a fundamental latency issue with
> "exactly once sinks", no matter how you implement them in any systems:
> You can only commit once you are sure that everything went well,
> to a specific point where you are sure no replay will ever be needed.

What if the persistent buffer in the sink would be used to determine
which data elements should be emitted in case of a replay? I mean, the
sink pushes everything as soon as it arrives, and also writes
everything to the persistent buffer, and then in case of a replay it
looks into the buffer before pushing every element, and only does the
push if the buffer says that the element was not pushed before.

Best,
Gábor


2016-02-05 11:57 GMT+01:00 Stephan Ewen <sewen@apache.org>:
> Hi Niels!
>
> In general, exactly once output requires transactional cooperation from the
> target system. Kafka has that on the roadmap, we should be able to integrate
> that once it is out.
> That means output is "committed" upon completed checkpoints, which
> guarantees nothing is written multiple times.
>
> Chesnay is working on an interesting prototype as a generic solution (also
> for Kafka, while they don't have that feature):
> It buffers the data in the sink persistently (using the fault tolerance
> state backends) and pushes the results out on notification of a completed
> checkpoint.
> That gives you exactly once semantics, but involves an extra materialization
> of the data.
>
>
> I think that there is actually a fundamental latency issue with "exactly
> once sinks", no matter how you implement them in any systems:
> You can only commit once you are sure that everything went well, to a
> specific point where you are sure no replay will ever be needed.
>
> So the latency in Flink for an exactly-once output would be at least the
> checkpoint interval.
>
> I'm eager to hear your thoughts on this.
>
> Greetings,
> Stephan
>
>
> On Fri, Feb 5, 2016 at 11:17 AM, Niels Basjes <Niels@basjes.nl> wrote:
>>
>> Hi,
>>
>> It is my understanding that the exactly-once semantics regarding the input
>> from Kafka is based on the checkpointing in the source component retaining
>> the offset where it was at the checkpoint moment.
>>
>> My question is how does that work for a sink? How can I make sure that (in
>> light of failures) each message that is read from Kafka (my input) is
>> written to Kafka (my output) exactly once?
>>
>>
>> --
>> Best regards / Met vriendelijke groeten,
>>
>> Niels Basjes
>
>

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