Github user nathanmarz commented on the pull request:
https://github.com/apache/incubator-storm/pull/48#issuecomment-38454693
Trident stores the batch id with any state it stores. It then uses this
batch id to decide how to do updates. Opaque maps will update using the "prev
val" if the batch id is the same, otherwise it will update using the current
val (due to the nature of opaque semantics). Now the problem comes in when you
do *multiple updates* to the same state in the same batch. In this case, the
batch id will be the same on the second update, but you should update using the
curr val instead of the prev val.
The code was correct on updates but not on gets. If the value was updated
in the batch the current val should always be returned, otherwise the standard
opaque semantics should be done. Whether a value has had updates applied to it
or not is detected with the "CachedBatchReadsMap".
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