Luke Chen created KAFKA-12495:
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Summary: 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
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].]
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 W3 left after 1st rebalance completed
and before 2nd rebalance started? Let's see what will happened (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 is down
W3 doesn't join
W1 becomes leader
W1 computes and sends assignments:
// We assigned all the previous revoked Connectors/Tasks to the new member,
which cause unbalanced distribution
W1(delay: 0, assigned: [AC0, AT1, AT2, AT3], revoked: [])
W2(delay: 0, assigned: [AT4, AT5, BC0, BT1, BT2, BT4, BT4, BT5], revoked: [])
{code}
We cannot assume the member count after keeps the same right after revocation.
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