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https://issues.apache.org/jira/browse/BEAM-8121?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16922313#comment-16922313
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Alexey Romanenko commented on BEAM-8121:
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[~TauJan] Could you or your colleagues try to compare the results of these 
simple pipelines (just read and write without other business logic you have) on 
the same amount of data:
* Only read from Kafka *without* Reshuffle
* Read from Kafka *without* Reshuffle and write into BigQuery
* Only read from Kafka *with* Reshuffle
* Read from Kafka *with* Reshuffle and write into BigQuery

Perhaps, it would help to narrow down the root cause of this issue. 


> Messages are not distributed per machines when consuming from Kafka topic 
> with 1 partition
> ------------------------------------------------------------------------------------------
>
>                 Key: BEAM-8121
>                 URL: https://issues.apache.org/jira/browse/BEAM-8121
>             Project: Beam
>          Issue Type: Bug
>          Components: io-java-kafka
>    Affects Versions: 2.14.0
>            Reporter: TJ
>            Priority: Major
>         Attachments: datalake-dataflow-cleaned.zip
>
>
> Messages are consumed from Kafka using KafkaIO. Each kafka topic contains 
> only 1 partition. (That means that messages can be consumed only by one 
> Consumer per 1 consumer group)
> When backlog of topic grows and system scales from 1 to X machines, all the 
> messages seems to be executed on  the same machine on which they are read. 
> Due to that message throughput doesn't increase comparing X machines to 1 
> machine. If one machine was reading 2K messages  per s, X machines will be 
> reading the same amount.



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