A. Sophie Blee-Goldman created KAFKA-13217:
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Summary: Reconsider skipping the LeaveGroup on close() or add an
overload that does so
Key: KAFKA-13217
URL: https://issues.apache.org/jira/browse/KAFKA-13217
Project: Kafka
Issue Type: Improvement
Components: streams
Reporter: A. Sophie Blee-Goldman
In Kafka Streams, when an instance is shut down via the close() API, we
intentionally skip sending a LeaveGroup request. This is because often the
shutdown is not due to a scaling down event but instead some transient closure,
such as during a rolling bounce. In cases where the instance is expected to
start up again shortly after, we originally wanted to avoid that member's tasks
from being redistributed across the remaining group members since this would
disturb the stable assignment and could cause unnecessary state migration and
restoration. We also hoped
to limit the disruption to just a single rebalance, rather than forcing the
group to rebalance once when the member shuts down and then again when it comes
back up. So it's really an optimization for the case in which the shutdown is
temporary.
That said, many of those optimizations are no longer necessary or at least much
less useful given recent features and improvements. For example rebalances are
now lightweight so skipping the 2nd rebalance is not as worth optimizing for,
and the new assignor will take into account the actual underlying state for
each task/partition assignment, rather than just the previous assignment, so
the assignment should be considerably more stable across bounces and rolling
restarts.
Given that, it might be time to reconsider this optimization. Alternatively, we
could introduce another form of the close() API that forces the member to leave
the group, to be used in event of actual scale down rather than a transient
bounce.
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