Dear developers of the Kafka java client,
It seems I have found a feature gap in the Kafka java client.
KAFKA-10337 and its associated pull request on Github (from 2020!) would
solve this, but it was closed without merging. We would love to see it
being reconsidered for merging. This mail has the arguments for doing so.
The javadoc of `ConsumerRebalanceListener` method `onPartitionsRevoked`
recommends you commit all offsets within the method, thereby holding up
the rebalance until those commits are done. The (perceived) feature gap
is when the user is trying to do async commits from the rebalance
listener; there is nothing available to trigger the callbacks of
completed commits. Without these callback, there is no way to know when
it is safe to return from onPartitionsRevoked. (We cannot call `poll`
because the rebalance listener is already called from inside a poll.)
Calling `commitAsync` with an empty offsets parameter seems a perfect
candidate for triggering callbacks of earlier commits. Unfortunately,
commitAsync doesn't behave that way. This is fixed by mentioned pull
request.
The pull request conversation has a comment saying that calling `commit`
with an empty offsets parameter is not something that should happen. I
found this a strange thing to say. First of all, the method does have
special handling for this situation, negating the comment outright. In
addition this special handling violates the contract of the method (as
specified in the javadoc section about ordering). Therefore, this pull
request has 2 advantages:
1. KafkaConsumer.commitAsync will be more in line with its javadoc,
2. the feature gap is gone.
Of course, it might be that I missed something and that there are other
ways to trigger the commit callbacks. I would be very happy to hear
about that because it means I do not have to wait for a release cycle.
If you agree these arguments are sound, I would be happy to make the
pull request mergable again.
Curious to your thoughts and kind regards,
Erik.
--
Erik van Oosten
e.vanoos...@grons.nl
https://day-to-day-stuff.blogspot.com
Committer of zio-kafkahttps://github.com/zio/zio-kafka
