The At most once processing paradigm involves a mechanism to save the position of the last event received before the event is actually processed and results persisted somewhere so that, if there is a failure and the consumer restarts, the consumer will not try to read the old events again. However, since there is no guarantee that the received events were all processed, this causes potential loss of events as they are never fetched again. This results in the behavior that the events are processed at most once or not processed at all.
At most once is ideally suitable for any application that involves updating some instantaneous ticker or gauge to show current values, as well as any cumulative sum, counter, or other aggregation, provided accuracy is not mandatory or the application needs absolutely all events. Any events lost will cause incorrect results or missing results.
The sequence of operations for the consumer are as follows:
- Save offsets
- Save results
Shown in the following is an illustration of what happens if there are a failure and the consumer restarts. Since the events have not been processed but offsets are saved, the consumer will read from the saved offsets, causing a gap in events consumed. Event 0 is never processed in the following figure:
