Kafka消费者组重平衡（二）

文章目录

- 概要
- 重平衡通知机制
- 消费组组状态
- 消费端重平衡流程
- Broker端重平衡流程

概要

上一篇Kafka消费者组重平衡主要介绍了重平衡相关的概念，本篇主要梳理重平衡发生的流程。

为了更好地观察，数据准备如下：
kafka版本：kafka_2.13-3.2.1
控制台创建topic (2个分区1个副本):
bin/kafka-topics.sh --create --bootstrap-server node1:9092 --replication-factor 1 --partitions 2 --topic test-rebalance

本地启动两个SpringBoot项目实例，代码如下

@KafkaListener(topics = "test-rebalance", groupId = "test-group")
public void rebalanceConsumer(ConsumerRecord<String, String> recordInfo) {int partition = recordInfo.partition();System.out.println("partition:" + partition + " value:" + recordInfo.value());
}

重平衡通知机制

Kafka Java 消费者需要定期地发送心跳请求（Heartbeat Request）到 Broker 端的协调者，以表明它还存活着。在 Kafka 0.10.1.0 版本之前，发送心跳请求是在消费者主线程完成的，也就是调用 KafkaConsumer.poll 方法的那个线程。
这样的设计存在弊端，一旦消息处理消耗了过长的时间，心跳请求将无法及时发到协调者那里，导致协调者“错误地”认为该消费者已“死”。自 0.10.1.0 版本开始，Kafka引入了一个单独的心跳线程来专门执行心跳请求发送，避免了这个问题。

重平衡的通知机制正是通过心跳线程来完成的。当协调者决定开启新一轮重平衡后，它会将“REBALANCE_IN_PROGRESS”封装进心跳请求的响应中，发还给消费者实例。当消费者实例发现心跳响应中包含了“REBALANCE_IN_PROGRESS”，就能立马知道重平衡又开始了，这就是重平衡的通知机制。

消费组组状态

Kafka 为消费者组定义了 5 种状态，它们分别是：Empty、Dead、PreparingRebalance、CompletingRebalance 和 Stable。那么，这 5 种状态的含义如下：

状态	含义
Empty	组内没有任何成员
Dead	组内没有任何成员，但组的元数据已经在协调者端删除
PreparingRebalance	消费组组准备开启重平衡，此时所有成员都要重新请求加入消费者组
CompletingRebalance	消费者组下所有成员已经加入，各个成员正在等待分配方案
Stable	消费者组的稳定状态，该状态表明重平衡已经完成，组内各成员都能够正常消费数据了

以下是各个状态的流转：

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一个消费者组最开始是 Empty 状态，当重平衡过程开启后，它会被置于PreparingRebalance 状态等待成员加入，之后变更到CompletingRebalance 状态等待分配方案，最后流转到 Stable 状态完成重平衡。当有新成员加入或已有成员退出时，消费者组的状态从 Stable 直接跳到PreparingRebalance 状态，此时，所有现存成员就必须重新申请加入组。

创建一个topic并逐步启动两个消费者实例：
服务端日志：

INFO [GroupCoordinator 0]: Dynamic member with unknown member id joins group test-group in Empty state. Created a new member id consumer-1-a6c11c9f-ec26-4e66-adeb-832f699f1247 and request the member to rejoin with this id. (kafka.coordinator.group.GroupCoordinator)
INFO [GroupCoordinator 0]: Preparing to rebalance group test-group in state PreparingRebalance with old generation 3 (__consumer_offsets-12) (reason: Adding new member consumer-1-a6c11c9f-ec26-4e66-adeb-832f699f1247 with group instance id None; client reason: not provided) (kafka.coordinator.group.GroupCoordinator)
INFO [GroupCoordinator 0]: Stabilized group test-group generation 4 (__consumer_offsets-12) with 1 members (kafka.coordinator.group.GroupCoordinator)
INFO [GroupCoordinator 0]: Assignment received from leader consumer-1-a6c11c9f-ec26-4e66-adeb-832f699f1247 for group test-group for generation 4. The group has 1 members, 0 of which are static. (kafka.coordinator.group.GroupCoordinator)

客户端一启动日志

o.a.k.c.c.internals.AbstractCoordinator  : [Consumer clientId=consumer-1, groupId=test-group] (Re-)joining group
o.a.k.c.c.internals.AbstractCoordinator  : [Consumer clientId=consumer-1, groupId=test-group] (Re-)joining group
o.a.k.c.c.internals.AbstractCoordinator  : [Consumer clientId=consumer-1, groupId=test-group] Successfully joined group with generation 4
o.a.k.c.c.internals.ConsumerCoordinator  : [Consumer clientId=consumer-1, groupId=test-group] Setting newly assigned partitions: test-rebalance-1, test-rebalance-0
o.a.k.c.c.internals.ConsumerCoordinator  : [Consumer clientId=consumer-1, groupId=test-group] Setting offset for partition test-rebalance-1 to the committed offset FetchPosition{offset=0, offsetEpoch=Optional.empty, currentLeader=LeaderAndEpoch{leader=node1:9092 (id: 0 rack: null), epoch=0}}
o.a.k.c.c.internals.ConsumerCoordinator  : [Consumer clientId=consumer-1, groupId=test-group] Setting offset for partition test-rebalance-0 to the committed offset FetchPosition{offset=0, offsetEpoch=Optional.empty, currentLeader=LeaderAndEpoch{leader=node1:9092 (id: 0 rack: null), epoch=0}}
o.s.k.l.KafkaMessageListenerContainer    : test-group: partitions assigned: [test-rebalance-1, test-rebalance-0]

有新成员加入后broker日志

INFO [GroupCoordinator 0]: Dynamic member with unknown member id joins group test-group in Stable state. Created a new member id consumer-1-97eba1d4-7f5c-4d78-b979-ba6ab9c82395 and request the member to rejoin with this id. (kafka.coordinator.group.GroupCoordinator)
INFO [GroupCoordinator 0]: Preparing to rebalance group test-group in state PreparingRebalance with old generation 4 (__consumer_offsets-12) (reason: Adding new member consumer-1-97eba1d4-7f5c-4d78-b979-ba6ab9c82395 with group instance id None; client reason: not provided) (kafka.coordinator.group.GroupCoordinator)
INFO [GroupCoordinator 0]: Stabilized group test-group generation 5 (__consumer_offsets-12) with 2 members (kafka.coordinator.group.GroupCoordinator)
INFO [GroupCoordinator 0]: Assignment received from leader consumer-1-a6c11c9f-ec26-4e66-adeb-832f699f1247 for group test-group for generation 5. The group has 2 members, 0 of which are static. (kafka.coordinator.group.GroupCoordinator)

接下来启动客户端二

客户端一日志

 o.a.k.c.c.internals.AbstractCoordinator  : [Consumer clientId=consumer-1, groupId=test-group] (Re-)joining group
o.a.k.c.c.internals.AbstractCoordinator  : [Consumer clientId=consumer-1, groupId=test-group] (Re-)joining group
o.a.k.c.c.internals.AbstractCoordinator  : [Consumer clientId=consumer-1, groupId=test-group] Successfully joined group with generation 5
o.a.k.c.c.internals.ConsumerCoordinator  : [Consumer clientId=consumer-1, groupId=test-group] Setting newly assigned partitions: test-rebalance-0
o.a.k.c.c.internals.ConsumerCoordinator  : [Consumer clientId=consumer-1, groupId=test-group] Setting offset for partition test-rebalance-0 to the committed offset FetchPosition{offset=0, offsetEpoch=Optional.empty, currentLeader=LeaderAndEpoch{leader=node1:9092 (id: 0 rack: null), epoch=0}}
o.s.k.l.KafkaMessageListenerContainer    : test-group: partitions assigned: [test-rebalance-0]

客户端二日志


o.a.k.c.c.internals.AbstractCoordinator  : [Consumer clientId=consumer-1, groupId=test-group] Attempt to heartbeat failed since group is rebalancing
o.a.k.c.c.internals.ConsumerCoordinator  : [Consumer clientId=consumer-1, groupId=test-group] Revoking previously assigned partitions [test-rebalance-1, test-rebalance-0]
o.s.k.l.KafkaMessageListenerContainer    : test-group: partitions revoked: [test-rebalance-1, test-rebalance-0]
o.a.k.c.c.internals.AbstractCoordinator  : [Consumer clientId=consumer-1, groupId=test-group] (Re-)joining group
o.a.k.c.c.internals.AbstractCoordinator  : [Consumer clientId=consumer-1, groupId=test-group] Successfully joined group with generation 5
o.a.k.c.c.internals.ConsumerCoordinator  : [Consumer clientId=consumer-1, groupId=test-group] Setting newly assigned partitions: test-rebalance-1
o.a.k.c.c.internals.ConsumerCoordinator  : [Consumer clientId=consumer-1, groupId=test-group] Setting offset for partition test-rebalance-1 to the committed offset FetchPosition{offset=0, offsetEpoch=Optional.empty, o.s.k.l.KafkaMessageListenerContainer    : test-group: partitions assigned: [test-rebalance-1]

当所有成员都退出组后，消费者组状态变更为 Empty。Kafka 定期自动删除过期位移的条件就是，组要处于 Empty 状态。因此，如果你的消费者组停掉了很长时间（超过 7 天），那么 Kafka 很可能就把该组的位移数据删除了。

消费端重平衡流程

重平衡的完整流程需要消费者端和协调者组件(什么是协调者)共同参与才能完成。下面先梳理消费者端的重平衡流程。主要分为3各阶段。

第一阶段：确定组协调器
关于如何确定组协调器，参考Kafka消费者重平衡（一）

第二阶段：JoinGroup

在此阶段的消费者会向Group Coordinator 发送 JoinGroupRequest 请求，并处理响应。然后GroupCoordinator 从一个consumer group中选择第一个（通常情况下）加入group的consumer作为leader(消费组协调器)，把consumer group情况发送给这个leader，接着这个leader会负责制定分区方案。

在这一步中，领导者向协调者发送 SyncGroup 请求，将刚刚做出的分配方案发给协调者。值得注意的是，其他成员也会向协调者发送 SyncGroup 请求，只不过请求体中并没有实际的内容。这一步的主要目的是让协调者接收分配方案，然后统一以 SyncGroup 响应的方式分发给所有成员，这样组内所有成员就都知道自己该消费哪些分区了。

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