ProcessFunction and Timers (Long Ride Alerts)(Discussion of Lab: ProcessFunction and Timers (Long Ride Alerts))
It would be interesting to test that the solution does not leak state.
A good way to write unit tests for a KeyedProcessFunction to check for state retention, etc., is to use the test harnesses described in the documentation on testing.
In fact, the reference solutions will leak state in the case where a START event is missing. They also leak in the case where the alert is generated, but then the END event does eventually arrive (after onTimer() has cleared the matching START event).
This could be addressed either by using state TTL, or by using another timer that eventually clears any remaining state. There is a tradeoff here, however: once that state has been removed, then if the matching events are not actually missing, but are instead very, very late, they will cause erroneous alerts.
This tradeoff between keeping state indefinitely versus occasionally getting things wrong when events are exceptionally late is a challenge that is inherent to stateful stream processing.