ReplicationFactor vs replicas in kafka

Question

Can we have the ReplicationFactor to 1 and no. of replicas to 3 ?

[user@master01 kafka]$ ./bin/kafka-topics.sh --describe --zookeeper localhost:2181 --topic test
Topic:test      PartitionCount:1        ReplicationFactor:1     Configs:
    Topic: test     Partition: 0    Leader: 3       Replicas: 3     Isr: 3

As I understand, if ReplicationFactor is n, Replicas can be <=n.

Can someone help me understand the following scenario? As the ReplicationFactor is set 1, where as Kafka show number of Replicas as 3.

Answer 1

ReplicationFactor tells how many replicas there are.

Replicas lists the broker id of all the replicas.

In your example, ReplicationFactor is 1 and this single replica is hosted on broker 3. It works the same way for Leader and Isr. So broker 3 is the leader for the partition and is also the in-sync replica (Isr).

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In case you had 2 as the ReplicationFactor, you could see something like:

[user@master01 kafka]$ ./bin/kafka-topics.sh --describe --zookeeper localhost:2181 --topic test
Topic:test      PartitionCount:1        ReplicationFactor:2    Configs:
Topic: test     Partition: 0    Leader: 3       Replicas: 3,1     Isr: 3,1

Now broker 3 is still the leader for the partition, but there are two replicas and two Isr (broker 3 and 1)