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https://issues.apache.org/jira/browse/FLINK-4027?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15322509#comment-15322509
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Aljoscha Krettek commented on FLINK-4027:
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This is the mailing list thread about this:
http://apache-flink-user-mailing-list-archive.2336050.n4.nabble.com/Kafka-producer-sink-message-loss-td7371.html
> FlinkKafkaProducer09 sink can lose messages
> -------------------------------------------
>
> Key: FLINK-4027
> URL: https://issues.apache.org/jira/browse/FLINK-4027
> Project: Flink
> Issue Type: Bug
> Components: Kafka Connector
> Affects Versions: 1.0.3
> Reporter: Elias Levy
> Priority: Critical
>
> The FlinkKafkaProducer09 sink appears to not offer at-least-once guarantees.
> The producer is publishing messages asynchronously. A callback can record
> publishing errors, which will be raised when detected. But as far as I can
> tell, there is no barrier to wait for async errors from the sink when
> checkpointing or to track the event time of acked messages to inform the
> checkpointing process.
> If a checkpoint occurs while there are pending publish requests, and the
> requests return a failure after the checkpoint occurred, those message will
> be lost as the checkpoint will consider them processed by the sink.
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