Github user revans2 commented on the issue:
https://github.com/apache/storm/pull/1919
This is the thing with auto-commit. auto-commit removes some of the
overhead of tracking outstanding messages, but also violates a lot of the
guarantees that storm has in place.
To truly get at most once processing you need to commit the message as soon
as it is emitted. Otherwise a crash can still result in messages being
replayed. If you want at least once processing you have to ack the message
only after it is fully processed. Storm makes this work out of the box for
most spouts that commit the message when ack is called.
With auto-commit it makes it so we could replay messages even in at most
once processing and that we might not replay messages in at least once
processing. It is "I really don't care what messages I see processing".
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