How to change the number of replicas of a Kafka topic?

To increase the number of replicas for a given topic you have to:

1. Specify the extra replicas in a custom reassignment json file

For example, you could create increase-replication-factor.json and put this content in it:

{"version":1,
  "partitions":[
     {"topic":"signals","partition":0,"replicas":[0,1,2]},
     {"topic":"signals","partition":1,"replicas":[0,1,2]},
     {"topic":"signals","partition":2,"replicas":[0,1,2]}
]}

2. Use the file with the --execute option of the kafka-reassign-partitions tool

[or kafka-reassign-partitions.sh - depending on the kafka package]

For example:

$ kafka-reassign-partitions --zookeeper localhost:2181 --reassignment-json-file increase-replication-factor.json --execute

3. Verify the replication factor with the kafka-topics tool

[or kafka-topics.sh - depending on the kafka package]

 $ kafka-topics --zookeeper localhost:2181 --topic signals --describe

Topic:signals   PartitionCount:3    ReplicationFactor:3 Configs:retention.ms=1000000000
Topic: signals  Partition: 0    Leader: 2   Replicas: 0,1,2 Isr: 2,0,1
Topic: signals  Partition: 1    Leader: 2   Replicas: 0,1,2 Isr: 2,0,1
Topic: signals  Partition: 2    Leader: 2   Replicas: 0,1,2 Isr: 2,0,1

See also: the part of the official documentation that describes how to increase the replication factor.

Edit: I was proven to be wrong - please check excellent answer from Łukasz Dumiszewski.

I'm leaving my original answer for completness for now.

I don't think you can. Normally it would be something like

./kafka-topics.sh --zookeeper localhost:2181 --alter --topic test2 --replication-factor 3

but it says

Option "[replication-factor]" can't be used with option"[alter]"

It is funny that you can change number of partitions on the fly (which is often hugely destructive action when done in runtime), but cannot increase replication factor, which should be transparent. But remember, it is 0.10, not 10.0... Please see here for enhancement request https://issues.apache.org/jira/browse/KAFKA-1543

Łukasz Dumiszewski's answer is correct but manually generating that file is a bit hard. Luckily there are some easy ways to achieve what @Łukasz Dumiszewski said.

• If you are using kafka-manager tool, from version 2.0.0.2 you can change the replication factor in Generate Partition Assignment section in a topic view. Then you should click on Reassign Partitions to apply the generated partition assignment (if you select a different replication factor, you will get a warning but you can click on Force Reassign afterward).

• If you have ruby installed you can use this helper script

• If you prefer nodejs you can generate the file with this gist too.

You can also use kafkactl for this:

# first run with --validate-only to see what kafkactl will do
kafkactl alter topic my-topic --replication-factor 2 --validate-only

# then do the replica reassignment
kafkactl alter topic my-topic --replication-factor 2

Note that the Kafka API that kafkactl is using for this is only available for Kafka ≥ 2.4.0.

Disclaimer: I am contributor to this project