IBM WebSphere Adapters User Manual page 102

Callback event processing for event recovery
When there is a failure in the event processing as part of system recovery, the
adapter is able to recover the unprocessed events by implementing the
once-one-only delivery mechanism.
During real time event processing if any component of the business integration
system fails then the adapter must process the events that are not completed, and
not process the events that are completed. This ensures that once-one-only delivery
mechanism is implemented when the system recovers from a failure.
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WebSphere Adapters: WebSphere Adapter Toolkit User Guide