Kafka Retries and Idempotent Producers Explained: Avoid Duplicates and Ensure Reliable Delivery

In the previous article Kafka Producer Acks Explained: Replicas, ISR, and Write Guarantees, we discussed when a producer considers a write successful and how acknowledgments impact durability and availability. But even with correct acknowledgment settings, one important problem still remains: What happens when a write fails in Kafka? Or even more interesting: What happens when Kafka thinks a write failed, but it actually succeeded? This is where Kafka retries and idempotent producers become critical. The Real Problem: Uncertain Failures in Distributed Systems In distributed systems, failures are not always clear. Consider this scenario: Producer sends a message to the leader. Leader writes the message successfully. Leader sends acknowledgment. Network issue occurs → acknowledgment is lost. From Kafka’s perspective: Broker: Write succeeded Producer: Write failed Now the producer retries. 👉 The same message gets written again. This leads to duplicate messages in Kafka, even though the s