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一、困难点

建立topic的时候，可以通过指定参数 --replication-factor 设置备份数量。但是，一旦完成建立topic，则无法通过kafka-topic.sh 或者 命令修改replica数量。

二、解决办法

​ 实际上，我们可以考虑一种 “另类” 的办法：可以利用 kafka-reassign-partitions.sh 命令对所有分区进行重新分布，在做分区重新分布的时候，通过增加每个分区的replica备份数量来达到目的。

​ 本文将介绍如何利用 kafka-reassign-partitions.sh 命令增加topic的备份数量。

注意：以下命令使用到的topic名称、zookeeper的ip和port，需要读者替换成为实际集群的参数。

（假设kafka集群有4个broker，id分别为：1001,1002,1003,1004）

2.1、获取当前topic的所有分区分布在broker的情况

[root@tbds bin]# ./kafka-topics.sh --zookeeper 172.16.32.13:2181 --topic ranger_audits --describe
Topic:ranger_audits     PartitionCount:10       ReplicationFactor:1     Configs:Topic: ranger_audits    Partition: 0    Leader: 1001    Replicas: 1001  Isr: 1001Topic: ranger_audits    Partition: 1    Leader: 1002    Replicas: 1002  Isr: 1002Topic: ranger_audits    Partition: 2    Leader: 1001    Replicas: 1001  Isr: 1001Topic: ranger_audits    Partition: 3    Leader: 1002    Replicas: 1002  Isr: 1002Topic: ranger_audits    Partition: 4    Leader: 1001    Replicas: 1001  Isr: 1001Topic: ranger_audits    Partition: 5    Leader: 1002    Replicas: 1002  Isr: 1002Topic: ranger_audits    Partition: 6    Leader: 1001    Replicas: 1001  Isr: 1001Topic: ranger_audits    Partition: 7    Leader: 1002    Replicas: 1002  Isr: 1002Topic: ranger_audits    Partition: 8    Leader: 1001    Replicas: 1001  Isr: 1001Topic: ranger_audits    Partition: 9    Leader: 1002    Replicas: 1002  Isr: 1002

可以看出，ranger_audits 这个topic有10个分区，每个分区只有一个feplica备份，分布在1001和1002两台broker上面。

下面我们需要将ranger_audits 的每个分区数据都增加到2个replica备份，且分布到4个broker上面。

2.2、创建增加replica备份数量的配置文件

（注意：尽量保持topic的原有每个分区的主备份不变化。因此，配置文件的每个分区的第一个broker保持不变。）

[root@tbds bin]# vim ../config/increase-replication-factor.json
{"version":1,
"partitions":[
{"topic":"ranger_audits","partition":0,"replicas":[1001,1003]},
{"topic":"ranger_audits","partition":1,"replicas":[1002,1004]},
{"topic":"ranger_audits","partition":2,"replicas":[1001,1003]},
{"topic":"ranger_audits","partition":3,"replicas":[1002,1004]},
{"topic":"ranger_audits","partition":4,"replicas":[1001,1003]},
{"topic":"ranger_audits","partition":5,"replicas":[1002,1004]},
{"topic":"ranger_audits","partition":6,"replicas":[1001,1003]},
{"topic":"ranger_audits","partition":7,"replicas":[1002,1004]},
{"topic":"ranger_audits","partition":8,"replicas":[1001,1003]},
{"topic":"ranger_audits","partition":9,"replicas":[1002,1004]}
]}

上面的配置文件说明，我们将topic的每个分区都增加了一个replica，且保持每个分区原有的主备份所在broker不变化，将每个分区新增的replica备份数据放到到1003和1004两个broker上面。

2.3、开始执行增加分区

[root@tbds bin]# ./kafka-reassign-partitions.sh -zookeeper 172.16.32.13:2181 --reassignment-json-file ../config/increase-replication-factor.json --execute
Current partition replica assignment
{"version":1,"partitions":[{"topic":"ranger_audits","partition":3,"replicas":[1002]},{"topic":"ranger_audits","partition":9,"replicas":[1002]},{"topic":"ranger_audits","partition":8,"replicas":[1001]},{"topic":"ranger_audits","partition":1,"replicas":[1002]},{"topic":"ranger_audits","partition":4,"replicas":[1001]},{"topic":"ranger_audits","partition":2,"replicas":[1001]},{"topic":"ranger_audits","partition":5,"replicas":[1002]},{"topic":"ranger_audits","partition":0,"replicas":[1001]},{"topic":"ranger_audits","partition":6,"replicas":[1001]},{"topic":"ranger_audits","partition":7,"replicas":[1002]}]}Save this to use as the --reassignment-json-file option during rollback
Successfully started reassignment of partitions
{"version":1,"partitions":[{"topic":"ranger_audits","partition":0,"replicas":[1001,1003]},{"topic":"ranger_audits","partition":8,"replicas":[1001,1003]},{"topic":"ranger_audits","partition":5,"replicas":[1002,1004]},{"topic":"ranger_audits","partition":2,"replicas":[1001,1003]},{"topic":"ranger_audits","partition":9,"replicas":[1002,1004]},{"topic":"ranger_audits","partition":1,"replicas":[1002,1004]},{"topic":"ranger_audits","partition":3,"replicas":[1002,1004]},{"topic":"ranger_audits","partition":4,"replicas":[1001,1003]},{"topic":"ranger_audits","partition":7,"replicas":[1002,1004]},{"topic":"ranger_audits","partition":6,"replicas":[1001,1003]}]}

2.4、查看执行进度

[root@tbds bin]# ./kafka-reassign-partitions.sh -zookeeper 172.16.32.13:2181 --reassignment-json-file ../config/increase-replication-factor.json --verify
Status of partition reassignment:
Reassignment of partition [ranger_audits,0] completed successfully
Reassignment of partition [ranger_audits,8] completed successfully
Reassignment of partition [ranger_audits,5] completed successfully
Reassignment of partition [ranger_audits,2] completed successfully
Reassignment of partition [ranger_audits,9] completed successfully
Reassignment of partition [ranger_audits,1] completed successfully
Reassignment of partition [ranger_audits,3] completed successfully
Reassignment of partition [ranger_audits,4] completed successfully
Reassignment of partition [ranger_audits,7] completed successfully
Reassignment of partition [ranger_audits,6] completed successfully

上面显示增加分区操作成功

2.5、再次查看topic的情况

[root@tbds bin]# ./kafka-topics.sh --zookeeper 172.16.32.13:2181 --topic ranger_audits --describe
Topic:ranger_audits     PartitionCount:10       ReplicationFactor:2     Configs:Topic: ranger_audits    Partition: 0    Leader: 1001    Replicas: 1001,1003     Isr: 1001,1003Topic: ranger_audits    Partition: 1    Leader: 1002    Replicas: 1002,1004     Isr: 1002,1004Topic: ranger_audits    Partition: 2    Leader: 1001    Replicas: 1001,1003     Isr: 1001,1003Topic: ranger_audits    Partition: 3    Leader: 1002    Replicas: 1002,1004     Isr: 1002,1004Topic: ranger_audits    Partition: 4    Leader: 1001    Replicas: 1001,1003     Isr: 1001,1003Topic: ranger_audits    Partition: 5    Leader: 1002    Replicas: 1002,1004     Isr: 1002,1004Topic: ranger_audits    Partition: 6    Leader: 1001    Replicas: 1001,1003     Isr: 1001,1003Topic: ranger_audits    Partition: 7    Leader: 1002    Replicas: 1002,1004     Isr: 1002,1004Topic: ranger_audits    Partition: 8    Leader: 1001    Replicas: 1001,1003     Isr: 1001,1003Topic: ranger_audits    Partition: 9    Leader: 1002    Replicas: 1002,1004     Isr: 1002,1004

从上面可以看出，备份数量增加成功

三、进一步思考

​ 利用上述介绍的办法，除了可以用来增加topic的备份数量之外，还能够处理以下几个场景：

1、对topic的所有分区数据进行整体迁移。怎么理解呢？假如集群有N个broker，后来新扩容M个broker。由于新扩容的broker磁盘都是空的，原有的broker磁盘占用都很满。那么我们可以利用上述方法，将存储在原有N个broker的某些topic整体搬迁到新扩容的M个broker，进而实现kafka集群的整体数据均衡。

​ 具体使用方法就是：通过编写2.2章节的配置文件，将topic的所有分区都配置到新的M个broker上面去，再执行excute，即可完成topic的所有分区数据整体迁移到新扩容的M个broker节点。

*2、broker坏掉的情况。*导致某些topic的某些分区的replica数量减少，可以利用kafka-reassign-partitions.sh增加replica；

*3、kafka 某些broker磁盘占用很满，某些磁盘占用又很少。*可以利用kafka-reassign-partitions.sh迁移某些topic的分区数据到磁盘占用少的broker，实现数据均衡；

*4、kafka集群扩容。*需要把原来broker的topic数据整体迁移到新的broker，合理利用新扩容的broker，实现负载均衡。

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