[jira] [Created] (KAFKA-8420) Graceful handling when consumer switches from subscribe to manual assign

["Guozhang Wang (JIRA)" <ji...@apache.org>]

Guozhang Wang created KAFKA-8420:
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             Summary: Graceful handling when consumer switches from subscribe to manual assign
                 Key: KAFKA-8420
                 URL: https://issues.apache.org/jira/browse/KAFKA-8420
             Project: Kafka
          Issue Type: Improvement
          Components: consumer
            Reporter: Guozhang Wang


Today if a consumer switches between subscribe (and hence relies on group rebalance to get assignment) and manual assign, it may cause unnecessary rebalances. For example:

1. consumer.subscribe();
2. consumer.poll();     // join-group request sent, returns empty because poll timeout
3. consumer.unsubscribe();
4. consumer.assign(..);
5. consumer.poll();     // sync-group request received, and the assigned partitions does not match the current subscription-state. In this case it will tries to re-join which is not necessary.

In the worst case (i.e. leader keep sending incompatible assignment), this would case the consumer to fall into endless re-joins.

Although it is not a very common usage scenario, it still worth being better handled than the status-quo.



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