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https://issues.apache.org/jira/browse/KAFKA-12495?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17575018#comment-17575018
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Sagar Rao commented on KAFKA-12495:
-----------------------------------
Thanks [~ChrisEgerton] . I would take a look. I had another idea as well which
is probably more extravagant. I was thinking we let the assignor do whatever it
does and we let it stabilise. How about we run some checks afterwards to see if
the assignments are balanced or skewed and then trigger a rebalance later on.
IIRC, kafka streams does something similar by leveraging probably the
Consumer#enforceRebalance. I can dig deep into it if needed. We can take it
even a step further by even providing users the ability to define the idea of
balanced assignments. Case in point could be running it on cloud v/s running it
on prem. This would also alleviate some of the concerns raised in this ticket
about rebalance storms(I know you think it's not a risk) or premature
successive rebalances etc.
WDYT? Does it make sense?
> 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
> Components: KafkaConnect
> Reporter: Luke Chen
> Priority: Critical
> 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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