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With replication, each partition can have multiple replicas. The list of replicas for a partition is called the "assigned replicas". The first replica in this list is the "preferred replica". When topic/partitions are created, Kafka ensures that the "preferred replica" for the partitions across topics are equally distributed amongst the brokers in a cluster. In an ideal scenario, the leader for a given partition should be the "preferred replica". This guarantees that the leadership load across the brokers in a cluster are evenly balanced. However, over time the leadership load could get imbalanced due to broker shutdowns (caused by controlled shutdown, crashes, machine failures etc). This tool helps to restore the leadership balance between the brokers in the cluster. A summary of the steps that the tool does is shown below -

1. The tool updates the zookeeper path "/admin/preferred_replica_election" with the list of topic partitions whose leader needs to be moved to the preferred replica.
2. The controller listens to the path above. When a data change update is triggered, the controller reads the list of topic partitions from zookeeper.
3. For each topic partition, the controller gets the preferred replica (the first replica in the assigned replicas list). If the preferred replica is not already the leader and it is present in the isr, the controller issues a request to the broker that owns the preferred replica to become the leader for the partition.

Note that the tool only updates the zookeeper path and exits. The controller moves the leader for a partition to the preferred replica asynchronously.

How to use the tool?