---
title: "Kafka acks: 0 vs 1 vs all explained"
description: "Kafka producer acks compared: acks=0, acks=1 and acks=all, what each one waits for, what each can lose, and how min.insync.replicas fits in. Simulated live."
url: https://monedula.dev/kafka-simulator/configs/acks/
---

# Kafka acks: 0 vs 1 vs all

Producer config · default all · Apache Kafka 4.3.

acks sets how many replicas must have a record before the Kafka producer counts the send as successful. acks=0 waits for nothing, acks=1 waits for the partition leader only, and acks=all (the default since Kafka 3.0) waits for every replica in the ISR. Only acks=all, with min.insync.replicas of 2 or more, survives losing the leader without losing acknowledged records.

## At a glance

- Config: `acks` (producer)
- Default: all
- Valid values: 0, 1, all (-1 means the same as all)
- Reference: [Apache Kafka documentation](https://kafka.apache.org/43/configuration/producer-configs/#producerconfigs_acks)

## Values

- `acks=0`: The producer does not wait for any response. Lowest latency, no retries on errors it never hears about, and records can be lost without anyone knowing.
- `acks=1`: The leader answers once the record is in its own log. A record that the leader acknowledged but no follower copied yet is lost if the leader fails.
- `acks=all`: The leader answers once every replica in the ISR has the record, and only if the ISR has at least min.insync.replicas members; otherwise the write fails with NOT_ENOUGH_REPLICAS.

## What it does

A produce request goes to the partition leader, and followers copy the record by fetching from it. acks decides at which point of that process the leader answers the producer. With acks=all the answer comes when the record is below the high watermark, which also means consumers can read it. Idempotence requires acks=all, so setting acks to 0 or 1 turns idempotence off unless it was enabled explicitly, in which case the producer fails with a ConfigException.

## The trade-off

Each step up costs latency: acks=1 adds the leader write, acks=all adds a follower fetch round. In return, acks=all with min.insync.replicas=2 guarantees that an acknowledged record exists on at least two brokers. The catch is availability: when the ISR shrinks below min.insync.replicas, acks=all producers are refused while acks=1 producers keep writing, with only the leader holding their records.

## Advice

- Keep acks=all for any data you cannot lose, and set min.insync.replicas=2 on topics with replication.factor=3.
- Use acks=1 or 0 only for data where losing some is acceptable, such as metrics or logs, and where the latency matters.
- Raise throughput with linger.ms, batch.size and compression rather than by lowering acks.

## See it in the simulator

- [Kafka acks=0 vs acks=1 vs acks=all on a degraded ISR](https://monedula.dev/kafka-simulator/scenarios/acks-tradeoff/) (acks trade-off), step 4: acks=all produce blocked - NotEnoughReplicas. Durability wins: acks=all refuses to ack below min.insync.replicas. [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=01.1.3&t=3)
- [Kafka acks=1 data loss: min.insync.replicas does not help](https://monedula.dev/kafka-simulator/scenarios/acks1-durability-gap/) (acks=1 durability gap), step 5: New leader at LEO 0 - record lost. A new leader is elected from the surviving ISR at LEO 0. There is nothing to recover: the only copy was on the dead leader. The high watermark is still 0 and never advances. [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=01.1.4&t=4)
- [Kafka producer acks=0: fire and forget, silent data loss](https://monedula.dev/kafka-simulator/scenarios/acks-0-fire-and-forget/) (acks=0: fire and forget), step 4: Leader died before O2 replicated · O2 lost silently. O2 was on the leader only when it was killed. A new leader is elected from a follower that never received O2, so O2 is gone - and with acks=0 the producer was never told (still ‘sent’, no error). Fire-and-forget trades durability for latency. [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=01.1.1&t=3)

## Related configs

- [Kafka min.insync.replicas explained (with acks=all)](https://monedula.dev/kafka-simulator/configs/min-insync-replicas/)
- [Kafka enable.idempotence: no duplicates on retry](https://monedula.dev/kafka-simulator/configs/enable-idempotence/)
- [Kafka unclean.leader.election.enable: the data loss risk](https://monedula.dev/kafka-simulator/configs/unclean-leader-election-enable/)