---
title: "Change Kafka min.insync.replicas at runtime, no restart"
description: "With broker-3 down the ISR is 2. Raising min.insync.replicas to 3 live, with no restart, makes Kafka reject the next acks=all write with NOT_ENOUGH_REPLICAS."
url: https://monedula.dev/kafka-simulator/scenarios/live-config-changes/
---

# Change Kafka min.insync.replicas at runtime, no restart

Scenario 07.2.1 · Live config changes. Operations · Intermediate · about 2 min · Apache Kafka 4.3.

Raising min.insync.replicas at runtime immediately changes whether the next acks=all produce can commit - no restart.

Run it: https://monedula.dev/kafka-simulator/playground/?scenario=07.2.1

## What you start with

Most topic configs are not fixed at creation - an operator can change them at runtime and the cluster reacts on the very next request. A healthy RF=3 topic commits a record, broker-3 dies and the ISR shrinks to 2 (still exactly at the min.insync.replicas floor, so the next record still commits), then a live config_change raises min.insync.replicas to 3. No broker moves, but the durability bar is now above the current ISR - so the very next acks=all produce is rejected with NOT_ENOUGH_REPLICAS.

## Step by step

1. **Topic created · min.ISR=2** (t+0.000s): A fresh RF=3 topic with all three replicas in sync. min.insync.replicas=2 sets the durability floor: an acks=all write needs at least 2 in-sync replicas to commit. See it from the CLI: `kafka-topics --bootstrap-server localhost:9092 --describe --topic orders` (bin/kafka-topics.sh in the Apache Kafka download). [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=07.2.1&t=0)
2. **O1 commits - ISR=3** (t+3.500s): p1 writes O1 with acks=all. All three replicas hold it, so the high watermark advances to 1 and the producer is acked - the healthy baseline before anything changes. [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=07.2.1&t=1)
3. **broker-3 down - ISR shrinks to 2** (t+7.000s): broker-3 is killed. Its replica drops out of the ISR, which shrinks from 3 to 2 - exactly at the min.insync.replicas floor, so the partition is still writable. [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=07.2.1&t=2)
4. **O2 still commits - ISR=2 = min.ISR** (t+8.500s): O2 is produced. The ISR (2) still meets min.insync.replicas (2), so it commits and the high watermark reaches 2. The shrunk ISR is at the edge, but not over it. [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=07.2.1&t=3)
5. **config_change: min.ISR 2 → 3** (t+10.500s): A live config_change raises min.insync.replicas from 2 to 3. No broker moves and no data changes - only the topic's durability bar. The new value applies immediately, with no restart. Make the change yourself: `kafka-configs --bootstrap-server localhost:9092 --alter --entity-type topics --entity-name orders --add-config min.insync.replicas=3`. [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=07.2.1&t=4)
6. **O3 rejected - ISR=2 < min.ISR=3** (t+12.500s): O3 is produced - and rejected with NOT_ENOUGH_REPLICAS. The ISR is still 2, but the floor is now 3, so the leader can no longer satisfy acks=all. The high watermark stays at 2: the live config change alone flipped this write from success to failure. [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=07.2.1&t=5)

## The takeaway

Topic config changes take effect immediately against live state with no restart - raising min.insync.replicas above the current ISR can start rejecting producers on the spot.

## Configs in this scenario

- [`min.insync.replicas`](https://kafka.apache.org/43/configuration/topic-configs/#topicconfigs_min.insync.replicas): The fewest in-sync replicas, leader included, that must have a write for an acks=all produce to succeed (default 1); with fewer in the ISR the leader rejects the write with NOT_ENOUGH_REPLICAS. [How min.insync.replicas works](https://monedula.dev/kafka-simulator/configs/min-insync-replicas/)
- [`acks`](https://kafka.apache.org/43/configuration/producer-configs/#producerconfigs_acks): How many replicas must have a write before the producer counts it as sent: 0 (none), 1 (the leader only) or all (every in-sync replica, the default, and required for idempotence). [How acks works](https://monedula.dev/kafka-simulator/configs/acks/)

## Further reading

- [Apache Kafka documentation](https://kafka.apache.org/43/operations/basic-kafka-operations/)

## Troubleshooting

- [NotEnoughReplicasException in Kafka: causes and fixes](https://monedula.dev/kafka-simulator/troubleshooting/not-enough-replicas/)