[jira] [Commented] (KAFKA-12508) Emit-on-change tables may lose updates on error or restart in at_least_once

["Bruno Cadonna (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/KAFKA-12508?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17304894#comment-17304894 ] 

Bruno Cadonna commented on KAFKA-12508:
---------------------------------------

[~nhab] Thank you for the bug report. I could reproduce the bug and it indeed leads to data loss as you described. I will open an bug fix PR shortly.

> Emit-on-change tables may lose updates on error or restart in at_least_once
> ---------------------------------------------------------------------------
>
>                 Key: KAFKA-12508
>                 URL: https://issues.apache.org/jira/browse/KAFKA-12508
>             Project: Kafka
>          Issue Type: Bug
>          Components: streams
>    Affects Versions: 2.7.0, 2.6.1
>            Reporter: Nico Habermann
>            Priority: Major
>
> [KIP-557|https://cwiki.apache.org/confluence/display/KAFKA/KIP-557%3A+Add+emit+on+change+support+for+Kafka+Streams] added emit-on-change semantics to KTables that suppress updates for duplicate values.
> However, this may cause data loss in at_least_once topologies when records are retried from the last commit due to an error / restart / etc.
>  
> Consider the following example:
> {code:java}
> streams.table(source, materialized)
> .toStream()
> .map(mayThrow())
> .to(output){code}
>  
>  # Record A gets read
>  # Record A is stored in the table
>  # The update for record A is forwarded through the topology
>  # Map() throws (or alternatively, any restart while the forwarded update was still being processed and not yet produced to the output topic)
>  # The stream is restarted and "retries" from the last commit
>  # Record A gets read again
>  # The table will discard the update for record A because
>  ## The value is the same
>  ## The timestamp is the same
>  # Eventually the stream will commit
>  # There is absolutely no output for Record A even though we're running in at_least_once
>  
> This behaviour does not seem intentional. [The emit-on-change logic explicitly forwards records that have the same value and an older timestamp.|https://github.com/apache/kafka/blob/367eca083b44261d4e5fa8aa61b7990a8b35f8b0/streams/src/main/java/org/apache/kafka/streams/state/internals/ValueAndTimestampSerializer.java#L50]
> This logic should probably be changed to also forward updates that have an older *or equal* timestamp.



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