Balancing leadership
Whenever a broker stops or crashes leadership for that broker's partitions transfers to other replicas. This means that by default when the broker is restarted it will only be a follower for all its partitions, meaning it will not be used for client reads and writes.
To avoid this imbalance, Kafka has a notion of preferred replicas. If the list of replicas for a partition is 1,5,9 then node 1 is preferred as the leader to either node 5 or 9 because it is earlier in the replica list. You can have the Kafka cluster try to restore leadership to the restored replicas by running the command:
> bin/kafka-preferred-replica-election.sh --zookeeper zk_host:port/chroot
Since running this command can be tedious you can also configure Kafka to do this automatically by setting the following configuration:
auto.leader.rebalance.enable=true
配置：
auto.leader.rebalance.enable 默认值：false
如果设为true，复制控制器会周期性的自动尝试，为所有的broker的每个partition平衡leadership，为更优先(preferred)的replica分配leadership。
leader.imbalance.per.broker.percentage 默认值：10
每个broker允许的不平衡的leader的百分比。如果每个broker超过了这个百分比，复制控制器会重新平衡leadership。
leader.imbalance.check.interval.seconds 默认值：300
检测leader不平衡的时间间隔。