Apache Storm Component Guide
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Saving the Window State

One issue with windowing is that tuples cannot be acknowledged until they exit the window.

For example, consider a one-hour window that slides every minute. The tuples in the window are evaluated (passed to the bolt execute method) every minute, but tuples that arrived during the first minute are acknowledged only after one hour and one minute. If there is a system outage after one hour, Storm replays all tuples from the starting point through the sixtieth minute. The bolt’s execute method is invoked with the same set of tuples 60 times; every window is reevaluated. One way to avoid this is to track tuples that have already been evaluated, save this information in an external durable location, and use this information to trim duplicate window evaluation during recovery.

For more information about state management and how it can be used to avoid duplicate window evaluations, see Implementing State Management.