---
title: "Increase the replication factor of an existing Kafka topic"
description: "RF goes from 2 to 3 through a partition reassignment. The new replica on broker-3 starts empty, back-fills the log and joins the ISR, with no Kafka downtime."
url: https://monedula.dev/kafka-simulator/scenarios/change-replication-factor/
---

# Increase the replication factor of an existing Kafka topic

Scenario 03.3.3 · Change replication factor live. Replication · Intermediate · about 2 min · Apache Kafka 4.3.

Raising RF reassigns the partition with an extra replica; the new copy back-fills the log and joins the ISR.

Run it: https://monedula.dev/kafka-simulator/playground/?scenario=03.3.3

## What you start with

A topic’s replication factor is not fixed for life - it is changed by a partition reassignment that adds or drops replicas. This partition starts at RF=2; after a few records, RF is raised to 3, assigning broker-3 as a new, initially-empty replica.

## Step by step

1. **RF=2 · two copies on brokers 1 & 2** (t+1.800s): The topic's replication factor is 2: the partition has replicas on brokers 1 and 2 (both in-sync). broker-3 holds no copy. [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=03.3.3&t=0)
2. **RF raised to 3 · broker-3 assigned and back-filling** (t+3.200s): Changing the replication factor to 3 reassigns the partition with a third replica (broker-3). It starts empty and fetches the existing records - present in the replica set, but not yet in the ISR. [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=03.3.3&t=1)
3. **broker-3 caught up · ISR {1,2,3}** (t+4.000s): broker-3 finished back-filling, caught up to the leader, and joined the ISR. The partition now has three in-sync copies. (Real Kafka performs this as a partition reassignment.) [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=03.3.3&t=2)

## The takeaway

The new replica back-fills the existing log, catches up, and joins the ISR, leaving the partition with three in-sync copies - replication factor is increased online with no downtime.

## Further reading

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