ACK

In Kafka, ACK (acknowledgment) is the producer setting (acks) that defines how much confirmation it requires before considering a write successful: acks=0 waits for no confirmation at all (fastest, riskiest), acks=1 waits for confirmation from the leader only, and acks=all waits for confirmation from every replica in the ISR (slowest, most durable). This trade-off is central to any discussion of delivery guarantees.