Exactly-once semantics (EOS) memastikan setiap pesan diproses tepat satu kali — tanpa kehilangan, tanpa duplikat — bahkan dengan kegagalan dan retry. Kafka mencapai ini melalui idempotent producers dan transactions, meskipun kompleks dan memiliki overhead.
Tantangan
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.
