Guarantees

Storm relies on the acking mechanism to replay tuples in case of failures. It is possible that the state is committed but the worker crashes before acking the tuples. In this case the tuples are replayed causing duplicate state updates.

The StatefulBoltExecutor continues to process the tuples from a stream after it has received a checkpoint tuple on one stream while waiting for checkpoint to arrive on other input streams for saving the state. This can also cause duplicate state updates during recovery.

The state abstraction does not eliminate duplicate evaluations and currently provides only at-least once guarantee.

To provide the at-least-once guarantee, all bolts in a stateful topology are expected to anchor the tuples while emitting and ack the input tuples once it is processed. For non-stateful bolts, the anchoring and acking can be automatically managed by extending the BaseBasicBolt. Stateful bolts are expected to anchor tuples while emitting and ack the tuple after processing like in the WordCountBolt example in the State management subsection.