Github user srdo commented on the issue:
https://github.com/apache/storm/pull/1919
Please see https://github.com/apache/storm/pull/1919#discussion_r99888764
and
https://storm.apache.org/releases/1.0.0/javadocs/org/apache/storm/Config.html#TOPOLOGY_ACKER_EXECUTORS.
If ack=0 Storm will call `ack` immediately on emit.
Yes, some tuples may be replayed if the spout crashes. What I'm saying is
you can't currently configure the spout to commit after every ack. So it's not
really at-most-once, so we shouldn't call it that. It's confusing.
Emitting with null message id is an option, but then you have to make sure
that emitted tuples leave no state behind (i.e. they aren't put in `emitted`).
It also prevents you from getting best-effort replaying unless you explicitly
check for ackers in `emitIfNotEmitted`.
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