How Kafka choose the follower nodes for replications?

Question

Kafka replicates the topic's partitions across the multiple machines for fault-tolerance. Each partition has a leader and one or more number of followers.

I want to know how Kafka chooses the machines that will become the followers of each topic/partition among the possible candidates?

For example, let's say there is 1 topic with 3 partitions {A,B,C} and replication factor is 3. The Kafka is running over 6 machines {1,2,...,6}.

One possible assignment is:

1 2 3 4 5 6
A B C
C A B
B C A

But the following is also possible:

1 2 3 4 5 6
A B C
  A B C
    A B C

Of course, there are tons of possible assignments.

Can anyone give me an idea how Kafka is doing this?

Thanks

Answer 1

You may look into Kafka sources on github to see how it does replica assignment by default - https://github.com/apache/kafka/blob/0.10.0/core/src/main/scala/kafka/admin/AdminUtils.scala#L47-L106