Exactly-once semantics (EOS) zagotavlja, da je vsako sporočilo obdelano natanko enkrat — brez izgube, brez podvojenih sporočil — tudi ob napakah in ponovnih poskusih. Kafka to dosega z idempotentnimi proizvajalci in transakcijami, čeprav je to zapleteno in ima režijske stroške.
Izziv
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.
