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https://issues.apache.org/jira/browse/KAFKA-14747?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17815424#comment-17815424
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Ayoub Omari commented on KAFKA-14747:
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I see that the ticket is open for a while. May I take it over ?
[~mjsax] I have one question, I saw that we don't test the dropped sensor count
within tests of Processor nodes (fkResponseJoin, KTableKTableJoin, etc...). Is
it because we have no way to access or mock StreamsMetrics from there ?
> FK join should record discarded subscription responses
> ------------------------------------------------------
>
> Key: KAFKA-14747
> URL: https://issues.apache.org/jira/browse/KAFKA-14747
> Project: Kafka
> Issue Type: Improvement
> Components: streams
> Reporter: Matthias J. Sax
> Assignee: Koma Zhang
> Priority: Minor
> Labels: beginner, newbie
>
> FK-joins are subject to a race condition: If the left-hand side record is
> updated, a subscription is sent to the right-hand side (including a hash
> value of the left-hand side record), and the right-hand side might send back
> join responses (also including the original hash). The left-hand side only
> processed the responses if the returned hash matches to current hash of the
> left-hand side record, because a different hash implies that the lef- hand
> side record was updated in the mean time (including sending a new
> subscription to the right hand side), and thus the data is stale and the
> response should not be processed (joining the response to the new record
> could lead to incorrect results).
> A similar thing can happen on a right-hand side update that triggers a
> response, that might be dropped if the left-hand side record was updated in
> parallel.
> While the behavior is correct, we don't record if this happens. We should
> consider to record this using the existing "dropped record" sensor or maybe
> add a new sensor.
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