最近我们生产环境的kafka集群有增加节点的需求，然而kafka在新增节点后并不会像elasticsearch那样感知到新节点加入后自动将数据reblance到新集群中，因此这个过程需要我们手动分配。一番折腾之后，实现了增加kafka集群节点并将原有数据均匀分配到扩容后的集群。下面结合一个例子谈一下整个过程。

一、环境说明

1.集群状况

假定当前的cluster中只有（101，102，103）三个kafka节点，有一个名为think_tank的topic，该topic有2个replica，均匀分布在三个节点上.

2.目的

我们要做的是在cluster中新增两个节点（记为104，105）后，将的数据均匀分到新集群中的5个节点上。

二、操作步骤

新增kafka节点的部署不是本文重点，就不在此赘述。

其实官方文档的这一小节关于集群扩容讲解很详细：Expanding your cluster ,整个过程需要分为三个步骤：获取kafka给出的建议分配方案、按照给出的分配方案执行分配、查看分配的进度以及状态。这三个步骤对应了kafka脚本提供的三个partition reassigment工具。

--generate: In this mode, given a list of topics and a list of brokers, the tool generates a candidate reassignment to move all partitions of the specified topics to the new brokers. This option merely provides a convenient way to generate a partition reassignment plan given a list of topics and target brokers.

--execute: In this mode, the tool kicks off the reassignment of partitions based on the user provided reassignment plan. (using the --reassignment-json-file option). This can either be a custom reassignment plan hand crafted by the admin or provided by using the --generate option

--verify: In this mode, the tool verifies the status of the reassignment for all partitions listed during the last --execute. The status can be either of successfully completed, failed or in progress

结合例子具体说明：

1、生成重新分配topic的方案

脚本的参数是以json文件的形式传入的，首先要新建一个json文件并设置需要分配哪些topic，think_tank-to-move.json:

{
    "topics":[
        {
            "topic":"think_tank"
        },
        {
            "topic":"这里可以同时指定多个..."
        }
    ],
    "version":1
}

使用/bin目录中提供的kafka-reassign-partitions.sh的脚本请求获取生成分配方案：

./bin/kafka-reassign-partitions.sh --zookeeper your_zk_address:2181 --topics-to-move-json-file think_tank-to-move.json --broker-list "101,102,103,104,105" --generate

--broker-lsit 的参数 "101,102,103,104,105"是指集群中每个broker的id，由于我们是需要将所有topic均匀分配到扩完结点的5台机器上，所以要指定。同理，当业务改变为将原来的所有数据从旧节点（01,102,103）迁移到新节点（104，105）实现数据平滑迁移，这时的参数应"104，105".

脚本执行后返回的结果如下：

Current partition replica assignment
{"version":1,"partitions":[{"topic":"think_tank","partition":2,"replicas":[101,102]},{"topic":"think_tank","partition":4,"replicas":[103,102]},{"topic":"think_tank","partition":3,"replicas":[102,101]},{"topic":"think_tank","partition":0,"replicas":[102,103]},{"topic":"think_tank","partition":1,"replicas":[103,101]}]}

Proposed partition reassignment configuration
{"version":1,"partitions":[{"topic":"think_tank","partition":2,"replicas":[103,101]},{"topic":"think_tank","partition":4,"replicas":[105,103]},{"topic":"think_tank","partition":3,"replicas":[104,102]},{"topic":"think_tank","partition":0,"replicas":[101,104]},{"topic":"think_tank","partition":1,"replicas":[102,105]}]}

可以看出当前正在运行的方案中，think_tank的replica都是分布在101，102，103这3个节点，新给出的建议方案中replica均匀分布在扩容后的5个节点中。

2.执行分配方案

将上一个步骤中生成的建议方案复制到新建的think_tank_reassignment.json中：

{"version":1,"partitions":[{"topic":"think_tank","partition":2,"replicas":[103,101]},{"topic":"think_tank","partition":4,"replicas":[105,103]},{"topic":"think_tank","partition":3,"replicas":[104,102]},{"topic":"think_tank","partition":0,"replicas":[101,104]},{"topic":"think_tank","partition":1,"replicas":[102,105]}]}

使用脚本执行：

 ./bin/kafka-reassign-partitions.sh --zookeeper your_zk_address:2181 --reassignment-json-file think_tank_reassignment.json --execute

脚本执行，返回内容：

Current partition replica assignment

{"version":1,"partitions":[{"topic":"think_tank","partition":2,"replicas":[101,102]},{"topic":"think_tank","partition":4,"replicas":[103,102]},{"topic":"think_tank","partition":3,"replicas":[102,101]},{"topic":"think_tank","partition":0,"replicas":[102,103]},{"topic":"think_tank","partition":1,"replicas":[103,101]}]}

Save this to use as the --reassignment-json-file option during rollback
Successfully started reassignment of partitions.

如上，成功开始执行分配数据，同时提示你如果有需要将之前的分配方案备份便于回滚到原方案。

3.查看配过程进

查看脚本的方法如下，注意这次的json文件要和执行步骤中的json是同一个文件：

 ./bin/kafka-reassign-partitions.sh --zookeeper your_zk_address:2181 --reassignment-json-file think_tank_reassignment.json --verify

返回结果：

Reassignment of partition [think_tank,2] completed successfully
Reassignment of partition [think_tank,1] completed successfully
Reassignment of partition [think_tank,3] is still in progress
Reassignment of partition [think_tank,4] completed successfully
Reassignment of partition [think_tank,0] is still in progress

is still in progress表示还在处理中，全部迁移成功后每个partition都会显示 completed successfully.注意如果topic数据量大，这个过程可能会时间长一些，不要轻易重启节点！可能会导致数据不一致！！！

三、其它

这个partion reassignment工具同样可以按需手动地将某个特定的topic指定到特定的broker上，所要做的就是按照步骤一给定的格式关联partition到borker即可，如，将think_tank的partition0指定到101、102两节点上：

{
    "version":1,
    "partitions":[
        {
            "topic":"think_tank",
            "partition":0,
            "replicas":[
                101,
                105
            ]
        }
    ]
}

另外，如果有增加replica的个数的需求，同样可以使用这个脚本，可以翻一下官网文档。

One more thing

一点儿感触，在确定问题所在后，官方的文档应该作为我们优先考虑的一个重要资料源，网上的资料由于时间较早、版本不同的原因，解决方式可能需要细微的改动才能达到目的，这些坑在官方的一手资料上其实是可以规避的。

欢迎拍砖，欢迎交流~

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