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.
任何时候当一台broker服务停掉或者崩溃后，位于本机上的partition的leader身份会赋予到其他的副本上面。这意味着默认情况下当一台broker服务重启后它上面的所有的partition都是follower身份，不会用于服务外部客户端的读写操作（所以会造成不均衡）
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:
为了避免上述情况，kafka发明了一个prefered replicas的概念。如果某一个副本的副本分布id为1，5，9，那么相比5和9的节点,节点1的由于在副本队列里更靠前就成为了preferer的partition。可以在集群上运行下列命令将各副本的leader分配恢复到服务崩溃之前的状况。
> 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:
由于手动运行这个命令会很枯燥，也可以在配置文件里设置如下属性打开kafka自动balance选举的功能
auto.leader.rebalance.enable=true