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https://issues.apache.org/jira/browse/KAFKA-19460?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Luke Chen resolved KAFKA-19460.
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Fix Version/s: 4.2.0
Resolution: Fixed
> fetch result might have size < fetch.min.bytes even if data is available in
> replica
> ------------------------------------------------------------------------------------
>
> Key: KAFKA-19460
> URL: https://issues.apache.org/jira/browse/KAFKA-19460
> Project: Kafka
> Issue Type: Improvement
> Reporter: Luke Chen
> Priority: Major
> Fix For: 4.2.0
>
>
> In the doc of
> "[fetch.min.bytes|https://kafka.apache.org/documentation/#consumerconfigs_fetch.min.bytes]";,
> it said:
> ??The minimum amount of data the server should return for a fetch request. If
> insufficient data is available the request will wait for that much data to
> accumulate before answering the request.??
> It makes users believe the records returned will always greater
> fetch.min.bytes if there is sufficient data in replica. But even if the data
> is sufficient is available in the replica, there is still possible the
> returned records size < fetch.min.bytes.
>
> For example:
> # Config
> fetch.max.bytes=1500
> max.partition.fetch.bytes=1000
> fetch.min.bytes=1100
> fetch.max.wait.ms=500
> # topic foo has 2 partitions, and each partition contains 1 record with size
> 1000 bytes.
> # When a consumer fetches data from these 2 partitions, it starts from
> foo-0, and fetch 1000 bytes of data, and 500 bytes left before reaching
> fetch.max.bytes.
> # When fetching foo-1, since we only have 500 bytes available to be fetched,
> and the first batch size in foo-1 is 1000 bytes, which is greater than 500,
> so we don't fetch it.
> # In the end, the total returned size is 1000 bytes, which is less than
> fetch.min.bytes, without waiting until `fetch.max.wait.ms` expired. It's
> because we checked the total size in replicas are more than
> "fetch.min.bytes", so no wait for "fetch.max.wait.ms".
>
> I think the logic is correct. It's just we need to update the doc to make it
> clear to users. We might also need to check `replica.fetch.min.bytes` config.
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