實際專案中我們可能在建立topic時沒有設定好正確的replication-factor，導致kafka叢集雖然是高可用的，但是該topic在有broker宕機時，可能發生無法使用的情況。topic一旦使用又不能輕易刪除重建，因此動態增加副本因子就成為最終的選擇。

原因分析：

假設我們有3個kafka broker分別brokerA、brokerB、brokerC.

1. 當我們建立的topic有3個分割槽partition時並且replication-factor為1，基本上一個broker上一個分割槽。擋一個broker宕機了，該topic就無法使用了，因為三個分割槽只有兩個能用，
2. 當我們建立的topic有3個分割槽partition時並且replication-factor為2時，可能分割槽資料分佈情況是
   brokerA， partiton0，partiton1，
   brokerB， partiton1，partiton2
   brokerC， partiton2，partiton0，
   每個分割槽有一個副本，當其中一個broker宕機了，kafka叢集還能完整湊出該topic的三個分割槽，例如當brokerA宕機了，可以通過brokerB和brokerC組合出topic的三個分割槽。

如何動態給已經建立的topic新增replication-factor？
可能很多人想使用kafka-topics.sh指令碼，那麼事情情況如何了？

[[email protected] bin]# ./kafka-topics.sh --alter --topic  yqtopic01  --zookeeper localhost:2181    --replication-factor 3
Option "[replication-factor]" can't be used with option"[alter]"
Option                                   Description
------                                   -----------
--alter                                  Alter the number of partitions,
                                           replica assignment, and/or
                                           configuration for
 
 the topic.
--config <String: name=value>            A topic configuration override for the

截圖

可以看出kafka-topics.sh不能用來增加副本因子replication-factor。實際應該使用kafka bin目錄下面的kafka-reassign-partitions.sh。
a, 首先我們配置topic的副本，儲存為json檔案（）
例如， 我們想把yqtopic01的部分設定為3，（我的kafka叢集有3個broker，id分別為0,1,2）， json檔名稱為increase-replication-factor.json

{“version”:1,
“partitions”:[
{“topic”:“yqtopic01”,“partition”:0,“replicas”:[0,1,2]},
{“topic”:“yqtopic01”,“partition”:1,“replicas”:[0,1,2]},
{“topic”:“yqtopic01”,“partition”:2,“replicas”:[0,1,2]}
]}
b, 然後執行指令碼
./kafka-reassign-partitions.sh -zookeeper 127.0.0.1:2181 --reassignment-json-file increase-replication-factor.json --execute

kafka-reassign-partitions.sh執行截圖

我們可以通過執行
kafka-topics.sh --describe --zookeeper localhost:2181 --topic yqtopic01檢視現在該topic的副本因子。

總結
所有文件官方文件最權威。https://kafka.apache.org/documentation/#basic_ops_increase_replication_factor

摘錄如下：
Increasing replication factor
Increasing the replication factor of an existing partition is easy. Just specify the extra replicas in the custom reassignment json file and use it with the --execute option to increase the replication factor of the specified partitions.

For instance, the following example increases the replication factor of partition 0 of topic foo from 1 to 3. Before increasing the replication factor, the partition’s only replica existed on broker 5. As part of increasing the replication factor, we will add more replicas on brokers 6 and 7.

The first step is to hand craft the custom reassignment plan in a json file:

> cat increase-replication-factor.json
{"version":1,
"partitions":[{"topic":"foo","partition":0,"replicas":[5,6,7]}]}

Then, use the json file with the --execute option to start the reassignment process:

	
> bin/kafka-reassign-partitions.sh --zookeeper localhost:2181 --reassignment-json-file increase-replication-factor.json --execute
Current partition replica assignment
 
{"version":1,
"partitions":[{"topic":"foo","partition":0,"replicas":[5]}]}

 
Save this to use as the --reassignment-json-file option during rollback
Successfully started reassignment of partitions
{"version":1,
"partitions":[{"topic":"foo","partition":0,"replicas":[5,6,7]}]}

The --verify option can be used with the tool to check the status of the partition reassignment. Note that the same increase-replication-factor.json (used with the --execute option) should be used with the --verify option:

	
> bin/kafka-reassign-partitions.sh --zookeeper localhost:2181 --reassignment-json-file increase-replication-factor.json --verify
Status of partition reassignment:
Reassignment of partition [foo,0] completed successfully

You can also verify the increase in replication factor with the kafka-topics tool:

	
> bin/kafka-topics.sh --zookeeper localhost:2181 --topic foo --describe
Topic:foo   PartitionCount:1    ReplicationFactor:3 Configs:
  Topic: foo    Partition: 0    Leader: 5   Replicas: 5,6,7 Isr: 5,6,7