Semantika eżattament-darba waħda (EOS) tiżgura li kull messaġġ jiġi pproċessat eżattament darba waħda — l-ebda telf, l-ebda duplikati — anke meta jkun hemm fallimenti u riteniniet. Kafka takkjamet dan permezz ta' produtturi idempotenti u transazzjonijiet, għalkemm huwa kumplissat u għandu overhead.
Il-isfida
Exactly-once is HARD in distributed systems (failures, retries, duplicates are inevitable):
→ producer retries → duplicate messages; consumer reprocessing → duplicate effects
→ naive at-least-once → duplicates; at-most-once → loss
→ exactly-once requires careful mechanisms to avoid BOTH loss AND duplicates.
