---
title: "Add a Kafka broker and reassign partitions onto it"
description: "A new broker-4 joins empty and stays idle until kafka-reassign-partitions moves two partitions onto it. Each new Kafka replica catches up and joins the ISR."
url: https://monedula.dev/kafka-simulator/scenarios/reassign-scale-out/
---

# Add a Kafka broker and reassign partitions onto it

Scenario 07.1.1 · Scale out + rebalance. Operations · Intermediate · about 3 min · Apache Kafka 4.3.

Add a broker, then reassign partitions onto it (kafka-reassign-partitions).

Run it: https://monedula.dev/kafka-simulator/playground/?scenario=07.1.1

## What you start with

Adding a broker gives the cluster more capacity, but Kafka never moves existing partitions onto it on its own - you have to rebalance explicitly with kafka-reassign-partitions. We produce a few records, add broker-4, then reassign two partitions onto it; each new replica starts empty, fetches from the leader, and joins the ISR once it has caught up.

## Step by step

1. **Cluster ready** (t+0.000s): Three brokers, RF 3, min.ISR 2. Leaders spread across broker-1/2/3. [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=07.1.1&t=0)
2. **Records produced** (t+6.000s): O1 committed to partition 0, O4 to partition 2 (acks=all). High-watermark 1 on both. [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=07.1.1&t=1)
3. **broker-4 added** (t+9.000s): A fourth broker joins with no partitions - idle until replicas are placed on it. [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=07.1.1&t=2)
4. **Partition 0 reassigning** (t+12.050s): kafka-reassign-partitions: broker-4 joins at offset 0 and starts catching up, not yet in ISR. The simulator drops broker-3 now; real Kafka keeps it in the replica set and ISR until broker-4 has caught up, so redundancy never dips. [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=07.1.1&t=3)
5. **Partition 0 rebalanced** (t+12.500s): broker-4 caught up to the leader and joined ISR. Leader (broker-1) unchanged. [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=07.1.1&t=4)
6. **Partition 2 reassigning** (t+16.050s): broker-4 joins and catches up, and leader broker-3 is dropped so leadership moves to broker-1. Real Kafka keeps broker-3 leading until broker-4 joins the ISR, then elects the first live in-sync replica of the new assignment. [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=07.1.1&t=5)
7. **Partition 2 rebalanced** (t+16.500s): broker-4 is in ISR on both moved partitions; load shifted off broker-3. [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=07.1.1&t=6)

## The takeaway

Scaling out is two steps: add the broker, then reassign partitions onto it - an empty broker carries no load until you deliberately rebalance data onto it.

## 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/)