What will happen if the scheduler sends an event to Kafka but crashes before marking the record in the outbox as sent?

If a scheduler sends an event to Kafka but crashes before marking the record in the outbox as sent, then on the next startup or retry, this event may be sent again. This happens because the record in the outbox was not updated, and the system believes that the event has not been sent yet.

Such a scenario can lead to duplicate messages in Kafka if additional idempotency or deduplication mechanisms are not implemented on the consumer or producer side.

To prevent such issues, the "Outbox" pattern with transactional database updates and message sending is usually used, or mechanisms for delivery confirmation and idempotent operations are employed.