Kafka中topic的每个分区可以设置多个副本。如果副本数为1，当该分区副本的leader节点宕机后，会导致该分区不可用。故需要设置多副本来保证可用性。
实际项目中，存在项目初期创建了副本数为1的topic，但是后期又需要扩大副本数的场景。通常不能直接删除topic重建，可以通过如下操作实现。
创建副本为1的topic
kafka-topics --zookeeper mdw:2181/kafka --create --replication-factor 1 --partitions 3 --topic test_topic
查看topic信息
kafka-topics --describe --zookeeper mdw:2181/kafka --topic test_topic
输出：
Topic:test_topic PartitionCount:3 ReplicationFactor:1 Configs:
Topic: test_topic Partition: 0 Leader: 364 Replicas: 364 Isr: 364
Topic: test_topic Partition: 1 Leader: 365 Replicas: 365 Isr: 365
Topic: test_topic Partition: 2 Leader: 366 Replicas: 366 Isr: 366
可以看出3个分区，各自都是只有一个副本。当Leader对应节点挂掉后，分区就不可用了。
使用官方自带的kafka-reassign-partitions.sh脚本实现。该脚本用来移动分区的副本位置。除了可以实现增加副本，还可以实现将分区的副本移动到指定机器上。该脚本的help见附录。
以下步骤实现将test_topic的各个分区增加为三副本。
制定分区及副本分配策略
创建json文件，写入如下内容：
填写说明：topic为topic名称，partition为分区号，replicas为broker id的数组
{
"version": 1,
"partitions": [
{
"topic": "test_topic",
"partition": 0,
"replicas": [
364,
365,
366
]
},
{
"topic": "test_topic",
"partition": 1,
"replicas": [
364,
365,
366
]
},
{
"topic": "test_topic",
"partition": 2,
"replicas": [
364,
365,
366
]
}
]
}
执行扩副本操作
本例中创建的json文件为test_topic.json
执行如下命令：
kafka-reassign-partitions --zookeeper mdw:2181/kafka --reassignment-json-file test_topic.json --execute
输出内容：
Current partition replica assignment
{"version":1,"partitions":[{"topic":"test_topic","partition":2,"replicas":[366],"log_dirs":["any"]},{"topic":"test_topic","partition":1,"replicas":[365],"log_dirs":["any"]},{"topic":"test_topic","partition":0,"replicas":[364],"log_dirs":["any"]}]}
Save this to use as the --reassignment-json-file option during rollback
Successfully started reassignment of partitions.
这是个异步操作，执行命令后开始执行扩副本。
输出内容中有之前的副本策略，可以保存下来，用于回滚。
查看执行进度
kafka-reassign-partitions --zookeeper mdw:2181/kafka --reassignment-json-file test_topic.json --verify
出现如下内容说明执行完成
Status of partition reassignment:
Reassignment of partition test_topic-0 completed successfully
Reassignment of partition test_topic-1 completed successfully
Reassignment of partition test_topic-2 completed successfully
验证是否成功
kafka-topics --describe --zookeeper mdw:2181/kafka --topic test_topic
输出：
Topic:test_topic PartitionCount:3 ReplicationFactor:3 Configs:
Topic: test_topic Partition: 0 Leader: 364 Replicas: 364,365,366 Isr: 364,366,365
Topic: test_topic Partition: 1 Leader: 365 Replicas: 364,365,366 Isr: 365,366,364
Topic: test_topic Partition: 2 Leader: 366 Replicas: 364,365,366 Isr: 366,365,364
从输出中可以看出每个分区的副本都变成了3个。
# kafka-reassign-partitions
This tool helps to moves topic partitions between replicas.
Option Description
------ -----------
--bootstrap-server bootstrapping. REQUIRED if an
absolute path of the log directory
is specified for any replica in the
reassignment json file
--broker-list The list of brokers to which the
partitions need to be reassigned in
the form "0,1,2". This is required
if --topics-to-move-json-file is
used to generate reassignment
configuration
--command-config passed to Admin Client.
--disable-rack-aware Disable rack aware replica assignment
--execute Kick off the reassignment as specified
by the --reassignment-json-file
option.
--generate Generate a candidate partition
reassignment configuration. Note
that this only generates a candidate
assignment, it does not execute it.
--help Print usage information.
--reassignment-json-file reassignment configurationThe format
to use is -
{"partitions":
[{"topic": "foo",
"partition": 1,
"replicas": [1,2,3],
"log_dirs": ["dir1","dir2","dir3"]
}],
"version":1
}
Note that "log_dirs" is optional. When
it is specified, its length must
equal the length of the replicas
list. The value in this list can be
either "any" or the absolution path
of the log directory on the broker.
If absolute log directory path is
specified, the replica will be moved
to the specified log directory on
the broker.
--replica-alter-log-dirs-throttle The movement of replicas between log
directories on the same broker will
be throttled to this value
(bytes/sec). Rerunning with this
option, whilst a rebalance is in
progress, will alter the throttle
value. The throttle rate should be
at least 1 KB/s. (default: -1)
--throttle The movement of partitions between
brokers will be throttled to this
value (bytes/sec). Rerunning with
this option, whilst a rebalance is
in progress, will alter the throttle
value. The throttle rate should be
at least 1 KB/s. (default: -1)
--timeout The maximum time in ms allowed to wait
for partition reassignment execution
to be successfully initiated
(default: 10000)
--topics-to-move-json-file to move the partitions of the
specified topics to the list of
brokers specified by the --broker-
list option. The format to use is -
{"topics":
[{"topic": "foo"},{"topic": "foo1"}],
"version":1
}
--verify Verify if the reassignment completed
as specified by the --reassignment-
json-file option. If there is a
throttle engaged for the replicas
specified, and the rebalance has
completed, the throttle will be
removed
--zookeeper REQUIRED: The connection string for
the zookeeper connection in the form
host:port. Multiple URLS can be
given to allow fail-over.