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From pnowojski <...@git.apache.org>
Subject [GitHub] flink issue #4239: [FLINK-6988] flink-connector-kafka-0.11 with exactly-once...
Date Wed, 09 Aug 2017 21:53:22 GMT
Github user pnowojski commented on the issue:

    https://github.com/apache/flink/pull/4239
  
    Writing records in state would be very costly. It is only a "last resort" solution.
    
    > That would imply exactly-once consumers can not read past that transaction as long
as it is open
    
    Hmmm, are you sure about this thing? That would mean that Kafka doesn't support transactional
parallel writes from two different process, which would be very strange. Could you point to
a source of this information? 
    
    Resuming transactions is not a part of `KafkaProducer`'s API, however Kafka's REST API
allows to do that. However I'm aware that it wasn't an intention of the authors to do so.
Kafka Streams do not need to do that, because they achieve exactly-once semantic by using
persistent communication channels (Kafka topics), so they can easily restart each operator
on it's own by replay/rewinding every input channel (Kafka topic). This comes with a cost,
because it makes communication between operators extremely, since every message must goes
to HDDs at some point. 


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