[jira] [Commented] (KAFKA-12495) Unbalanced connectors/tasks distribution will happen in Connect's incremental cooperative assignor

["Luke Chen (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/KAFKA-12495?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17379536#comment-17379536 ] 

Luke Chen commented on KAFKA-12495:
-----------------------------------

[~kkonstantine], could you take a look the PR? I suddenly found this bug is blocking a V3.0 blocker flaky test. Please help! Thank you.

> Unbalanced connectors/tasks distribution will happen in Connect's incremental cooperative assignor
> --------------------------------------------------------------------------------------------------
>
>                 Key: KAFKA-12495
>                 URL: https://issues.apache.org/jira/browse/KAFKA-12495
>             Project: Kafka
>          Issue Type: Bug
>            Reporter: Luke Chen
>            Assignee: Luke Chen
>            Priority: Major
>         Attachments: image-2021-03-18-15-04-57-854.png, image-2021-03-18-15-05-52-557.png, image-2021-03-18-15-07-27-103.png
>
>
> In Kafka Connect, we implement incremental cooperative rebalance algorithm based on KIP-415 ([https://cwiki.apache.org/confluence/display/KAFKA/KIP-415%3A+Incremental+Cooperative+Rebalancing+in+Kafka+Connect)|https://cwiki.apache.org/confluence/display/KAFKA/KIP-415%3A+Incremental+Cooperative+Rebalancing+in+Kafka+Connect]. However, we have a bad assumption in the algorithm implementation, which is: after revoking rebalance completed, the member(worker) count will be the same as the previous round of reblance.
>  
> Let's take a look at the example in the KIP-415:
> !image-2021-03-18-15-07-27-103.png|width=441,height=556!
> It works well for most cases. But what if W4 added after 1st rebalance completed and before 2nd rebalance started? Let's see what will happened? Let's see this example: (we'll use 10 tasks here):
>  
> {code:java}
> Initial group and assignment: W1([AC0, AT1, AT2, AT3, AT4, AT5, BC0, BT1, BT2, BT4, BT4, BT5])
> Config topic contains: AC0, AT1, AT2, AT3, AT4, AT5, BC0, BT1, BT2, BT4, BT4, BT5
> W1 is current leader
> W2 joins with assignment: []
> Rebalance is triggered
> W3 joins while rebalance is still active with assignment: []
> W1 joins with assignment: [AC0, AT1, AT2, AT3, AT4, AT5, BC0, BT1, BT2, BT4, BT4, BT5]
> W1 becomes leader
> W1 computes and sends assignments:
> W1(delay: 0, assigned: [AC0, AT1, AT2, AT3], revoked: [AT4, AT5, BC0, BT1, BT2, BT4, BT4, BT5])
> W2(delay: 0, assigned: [], revoked: [])
> W3(delay: 0, assigned: [], revoked: [])
> W1 stops revoked resources
> W1 rejoins with assignment: [AC0, AT1, AT2, AT3]
> Rebalance is triggered
> W2 joins with assignment: []
> W3 joins with assignment: []
> // one more member joined
> W4 joins with assignment: []
> W1 becomes leader
> W1 computes and sends assignments:
> // We assigned all the previous revoked Connectors/Tasks to the new member, but we didn't revoke any more C/T in this round, which cause unbalanced distribution
> W1(delay: 0, assigned: [AC0, AT1, AT2, AT3], revoked: [])
> W2(delay: 0, assigned: [AT4, AT5, BC0], revoked: [])
> W2(delay: 0, assigned: [BT1, BT2, BT4], revoked: [])
> W2(delay: 0, assigned: [BT4, BT5], revoked: [])
> {code}
> Because we didn't allow to do consecutive revoke in two consecutive rebalances (under the same leader), we will have this uneven distribution under this situation. We should allow consecutive rebalance to have another round of revocation to revoke the C/T to the other members in this case.
> expected:
> {code:java}
> Initial group and assignment: W1([AC0, AT1, AT2, AT3, AT4, AT5, BC0, BT1, BT2, BT4, BT4, BT5])
> Config topic contains: AC0, AT1, AT2, AT3, AT4, AT5, BC0, BT1, BT2, BT4, BT4, BT5
> W1 is current leader
> W2 joins with assignment: []
> Rebalance is triggered
> W3 joins while rebalance is still active with assignment: []
> W1 joins with assignment: [AC0, AT1, AT2, AT3, AT4, AT5, BC0, BT1, BT2, BT4, BT4, BT5]
> W1 becomes leader
> W1 computes and sends assignments:
> W1(delay: 0, assigned: [AC0, AT1, AT2, AT3], revoked: [AT4, AT5, BC0, BT1, BT2, BT4, BT4, BT5])
> W2(delay: 0, assigned: [], revoked: [])
> W3(delay: 0, assigned: [], revoked: [])
> W1 stops revoked resources
> W1 rejoins with assignment: [AC0, AT1, AT2, AT3]
> Rebalance is triggered
> W2 joins with assignment: []
> W3 joins with assignment: []
> // one more member joined
> W4 joins with assignment: []
> W1 becomes leader
> W1 computes and sends assignments:
> // We assigned all the previous revoked Connectors/Tasks to the new member, **and also revoke some C/T** 
> W1(delay: 0, assigned: [AC0, AT1, AT2], revoked: [AT3])
> W2(delay: 0, assigned: [AT4, AT5, BC0], revoked: [])
> W3(delay: 0, assigned: [BT1, BT2, BT4], revoked: [])
> W4(delay: 0, assigned: [BT4, BT5], revoked: [])
> // another round of rebalance to assign the new revoked C/T to the other members
> W1 rejoins with assignment: [AC0, AT1, AT2] 
> Rebalance is triggered 
> W2 joins with assignment: [AT4, AT5, BC0] 
> W3 joins with assignment: [BT1, BT2, BT4]
> W4 joins with assignment: [BT4, BT5]
> W1 becomes leader 
> W1 computes and sends assignments:
> // (final) We assigned all the previous revoked Connectors/Tasks to the members
> W1(delay: 0, assigned: [AC0, AT1, AT2], revoked: []) 
> W2(delay: 0, assigned: [AT4, AT5, BC0], revoked: []) 
> W2(delay: 0, assigned: [BT1, BT2, BT4], revoked: []) 
> W2(delay: 0, assigned: [BT4, BT5, AT3], revoked: [])
> {code}
> Note: The consumer's cooperative sticky assignor won't have this issue since we re-compute the assignment in each round.
>  
> Note2: this issue makes KAFKA-12283 test flaky.



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