[jira] [Commented] (KAFKA-1792) change behavior of --generate to produce assignment config with fair replica distribution and minimal number of reassignments

["Neha Narkhede (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/KAFKA-1792?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14230236#comment-14230236 ] 

Neha Narkhede commented on KAFKA-1792:
--------------------------------------

[~Dmitry Pekar] Left some comments on the rb. 

At a high level, it will help to know the source of the algorithm. Is this published or well tested someplace else? If this is completely new, we'd have to do the due diligence on generating enough test cases, beyond unit tests, to verify that the algorithm achieves its objectives (fairness and minimum data movement). Would you be up for sharing such test results. The metrics for such tests would be the # of replicas per broker and the total replica movement.

> change behavior of --generate to produce assignment config with fair replica distribution and minimal number of reassignments
> -----------------------------------------------------------------------------------------------------------------------------
>
>                 Key: KAFKA-1792
>                 URL: https://issues.apache.org/jira/browse/KAFKA-1792
>             Project: Kafka
>          Issue Type: Sub-task
>          Components: tools
>            Reporter: Dmitry Pekar
>            Assignee: Dmitry Pekar
>             Fix For: 0.8.3
>
>         Attachments: KAFKA-1792.patch
>
>
> Current implementation produces fair replica distribution between specified list of brokers. Unfortunately, it doesn't take
> into account current replica assignment.
> So if we have, for instance, 3 brokers id=[0..2] and are going to add fourth broker id=3, 
> generate will create an assignment config which will redistribute replicas fairly across brokers [0..3] 
> in the same way as those partitions were created from scratch. It will not take into consideration current replica 
> assignment and accordingly will not try to minimize number of replica moves between brokers.
> As proposed by [~charmalloc] this should be improved. New output of improved --generate algorithm should suite following requirements:
> - fairness of replica distribution - every broker will have R or R+1 replicas assigned;
> - minimum of reassignments - number of replica moves between brokers will be minimal;
> Example.
> Consider following replica distribution per brokers [0..3] (we just added brokers 2 and 3):
> - broker - 0, 1, 2, 3 
> - replicas - 7, 6, 0, 0
> The new algorithm will produce following assignment:
> - broker - 0, 1, 2, 3 
> - replicas - 4, 3, 3, 3
> - moves - -3, -3, +3, +3
> It will be fair and number of moves will be 6, which is minimal for specified initial distribution.
> The scope of this issue is:
> - design an algorithm matching the above requirements;
> - implement this algorithm and unit tests;
> - test it manually using different initial assignments;



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