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Kafka源码分析－序列7 －Consumer －coordinator协议与heartbeat实现原理_我们实现hearbeator

作者：2023面试高手 | 2024-05-27 02:01:05

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我们实现hearbeator

单线程的consumer

在前面我们讲过，KafkaProducer是线程安全的，同时其内部还有一个Sender，开了一个后台线程，不断从队列中取消息进行发送。 
而consumer，是一个纯粹的单线程程序，后面所讲的所有机制，包括coordinator，rebalance, heartbeat等，都是在这个单线程的poll函数里面完成的。也因此，在consumer的代码内部，没有锁的出现。

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//客户端线程</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span> (<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">true</span>) {
         ConsumerRecords<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><</span><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">String</span>, <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">String</span><span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">></span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">records</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> consumer<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>poll(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">100</span>); 
         。。。
     }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

何为coordinator?

去Zookeeper依赖

在0.9以前的client api中，consumer是要依赖Zookeeper的。因为同一个consumer group中的所有consumer需要进行协同，进行下面所讲的rebalance。

但是因为zookeeper的“herd”与“split brain”，导致一个group里面，不同的consumer拥有了同一个partition，进而会引起消息的消费错乱。为此，在0.9中，不再用zookeeper，而是Kafka集群本身来进行consumer之间的同步。下面引自kafka设计的原文：

https://cwiki.apache.org/confluence/display/KAFKA/Kafka+0.9+Consumer+Rewrite+Design#Kafka0.9ConsumerRewriteDesign-Failuredetectionprotocol

<code class="hljs oxygene has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">The current version <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">of</span> the high level consumer suffers <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">from</span> herd <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">and</span> split brain problems, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">where</span> multiple consumers <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">in</span> a <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">group</span> run a distributed algorithm <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">to</span> agree <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">on</span> the same partition ownership decision. Due <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">to</span> different view <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">of</span> the zookeeper data, they run <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">into</span> conflicts that makes the rebalancing attempt fail. But there <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">is</span> no way <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> a consumer <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">to</span> verify <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> a rebalancing operation completed successfully <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">on</span> the entire <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">group</span>. This also leads <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">to</span> some potential bugs <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">in</span> the rebalancing logic, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> example, https:<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//issues.apache.org/jira/browse/KAFKA-242</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

为什么在一个group内部，1个parition只能被1个consumer拥有？

我们知道，对于属于不同consumer group的consumers，可以消费同1个partition，从而实现Pub/Sub模式。

但是在一个group内部，是不允许多个consumer消费同一个partition的，这也就意味着，对于1个topic，1个group来说， 其partition数目 >= consumer个数。

比如对于1个topic，有4个partition，那么在一个group内部，最多只能有4个consumer。你加入更多的consumer，它们也不会分配到partition。

那为什么要做这个限制呢？原因在下面这篇文章中，有详细阐述： 
http://stackoverflow.com/questions/25896109/in-apache-kafka-why-cant-there-be-more-consumer-instances-than-partitions

简单来说，一个是因为这样做就没办法保证同1个partition消息的时序；另1方面，Kafka的服务器，是每个topic的每个partition的每个consumer group对应1个offset，即(topic, partition, consumer_group_id) – offset。如果多个consumer并行消费同1个partition，那offset的confirm就会出问题。

知道了这个前提，下面我们就来分析partition的分配问题。

coordinator协议 / partition分配问题

问题的提出： 
给定一个topic，有4个partition: p0, p1, p2, p3， 一个group有3个consumer: c0, c1, c2。那么，如果按范围分配策略，分配结果是： 
c0: p0, c1: p1, c2: p2, p3 
如果按轮询分配策略： 
c0: p1, p3, c1: p1, c2: p2

那这整个分配过程是如何进行的呢？见下图所示： 
这里写图片描述

3步分配过程

步骤1：对于每1个consumer group，Kafka集群为其从broker集群中选择一个broker作为其coordinator。因此，第1步就是找到这个coordinator。

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> Map<TopicPartition, List<ConsumerRecord<K, V>>> <span class="hljs-title" style="box-sizing: border-box;">pollOnce</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> timeout) {
        coordinator.ensureCoordinatorKnown();  <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//poll函数的第1行，就是寻找coordinator。如果没找到，就会一直阻塞在这里</span>
        ...
 }

    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">ensureCoordinatorKnown</span>() {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span> (coordinatorUnknown()) {
            RequestFuture<Void> future = sendGroupMetadataRequest();
            client.poll(future);

            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (future.failed()) {
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (future.isRetriable())
                    client.awaitMetadataUpdate();
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span>
                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">throw</span> future.exception();
            }
        }
    }

    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> RequestFuture<Void> <span class="hljs-title" style="box-sizing: border-box;">sendGroupMetadataRequest</span>() {
        Node node = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.client.leastLoadedNode();
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (node == <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) {
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> RequestFuture.noBrokersAvailable();
        } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {
            GroupCoordinatorRequest metadataRequest = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> GroupCoordinatorRequest(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.groupId);  <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//向集群中负载最小的node，发送请求，询问这个group id对应的coordinator是谁</span>
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> client.send(node, ApiKeys.GROUP_COORDINATOR, metadataRequest)
                    .compose(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> RequestFutureAdapter<ClientResponse, Void>() {
                        <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
                        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onSuccess</span>(ClientResponse response, RequestFuture<Void> future) {
                            handleGroupMetadataResponse(response, future);
                        }
                    });
        }
    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li></ul>

步骤2：找到coordinator之后，发送JoinGroup请求

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> Map<TopicPartition, List<ConsumerRecord<K, V>>> <span class="hljs-title" style="box-sizing: border-box;">pollOnce</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> timeout) {
        coordinator.ensureCoordinatorKnown();  <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//步骤1：寻找coordinator</span>

        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (subscriptions.partitionsAutoAssigned())
            coordinator.ensurePartitionAssignment(); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//步骤2+3： JoinGroup + SyncGroup</span>

    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">ensureActiveGroup</span>() {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (!needRejoin())
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span>;

        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (needsJoinPrepare) {
            onJoinPrepare(generation, memberId);
            needsJoinPrepare = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">false</span>;
        }

        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span> (needRejoin()) {
            ensureCoordinatorKnown();

           <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (client.pendingRequestCount(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.coordinator) > <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) {
                client.awaitPendingRequests(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.coordinator);
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">continue</span>;
            }

            RequestFuture<ByteBuffer> future = performGroupJoin();
            client.poll(future);

            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (future.succeeded()) {
                onJoinComplete(generation, memberId, protocol, future.value());
                needsJoinPrepare = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>;
                heartbeatTask.reset();
            } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {
                RuntimeException exception = future.exception();
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (exception <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">instanceof</span> UnknownMemberIdException ||
                        exception <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">instanceof</span> RebalanceInProgressException ||
                        exception <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">instanceof</span> IllegalGenerationException)
                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">continue</span>;
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (!future.isRetriable())
                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">throw</span> exception;
                time.sleep(retryBackoffMs);
            }
        }
    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li></ul>

步骤3：JoinGroup返回之后，发送SyncGroup，得到自己所分配到的partition

<code class="hljs axapta has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> RequestFuture<ByteBuffer> performGroupJoin() {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (coordinatorUnknown())
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> RequestFuture.coordinatorNotAvailable();

        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// send a join group request to the coordinator</span>
        log.debug(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"(Re-)joining group {}"</span>, groupId);
        JoinGroupRequest request = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> JoinGroupRequest(
                groupId,
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.sessionTimeoutMs,
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.memberId,
                protocolType(),
                metadata());

        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// create the request for the coordinator</span>
        log.debug(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Issuing request ({}: {}) to coordinator {}"</span>, ApiKeys.JOIN_GROUP, request, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.coordinator.id());
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">client</span>.send(coordinator, ApiKeys.JOIN_GROUP, request)
                .compose(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> JoinGroupResponseHandler());
    }


    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">JoinGroupResponseHandler</span> <span class="hljs-inheritance" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">extends</span></span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">CoordinatorResponseHandler</span><<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">JoinGroupResponse</span>, <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">ByteBuffer</span>> {</span>

        @Override
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> JoinGroupResponse parse(ClientResponse response) {
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> JoinGroupResponse(response.responseBody());
        }

        @Override
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> handle(JoinGroupResponse joinResponse, RequestFuture<ByteBuffer> future) {
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// process the response</span>
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">short</span> errorCode = joinResponse.errorCode();
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (errorCode == Errors.NONE.code()) {
                log.debug(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Joined group: {}"</span>, joinResponse.toStruct());
                AbstractCoordinator.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.memberId = joinResponse.memberId();
                AbstractCoordinator.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.generation = joinResponse.generationId();
                AbstractCoordinator.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.rejoinNeeded = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">false</span>;
                AbstractCoordinator.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.protocol = joinResponse.groupProtocol();
                sensors.joinLatency.record(response.requestLatencyMs());
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (joinResponse.isLeader()) {
                    onJoinLeader(joinResponse).chain(future); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//关键点：在JoinGroup返回之后，竟跟着发送SyncGroup消息</span>
                } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {
                    onJoinFollower().chain(future);
                }
            } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (errorCode == Errors.GROUP_LOAD_IN_PROGRESS.code()) {
               。。。
            }
        }
    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li></ul>

注意，在上面3步中，有一个关键点： partition的分配策略和分配结果其实是由client决定的，而不是由coordinator决定的。什么意思呢？在第2步，所有consumer都往coordinator发送JoinGroup消息之后，coordinator会指定其中一个consumer作为leader，其他consumer作为follower。

然后由这个leader进行partition分配。然后在第3步，leader通过SyncGroup消息，把分配结果发给coordinator，其他consumer也发送SyncGroup消息，获得这个分配结果。

为什么要在consumer中选一个leader出来，进行分配，而不是由coordinator直接分配呢？关于这个, Kafka的官方文档有详细的分析。其中一个重要原因是为了灵活性：如果让server分配，一旦需要新的分配策略，server集群要重新部署，这对于已经在线上运行的集群来说，代价是很大的；而让client分配，server集群就不需要重新部署了。

rebalance机制

所谓rebalance，就是在某些条件下，partition要在consumer中重新分配。那哪些条件下，会触发rebalance呢？ 
条件1：有新的consumer加入 
条件2：旧的consumer挂了 
条件3：coordinator挂了，集群选举出新的coordinator 
条件4：topic的partition新加 
条件5：consumer调用unsubscrible()，取消topic的订阅

当consumers检测到要rebalance时，所有consumer都会重走上面的流程，进行步骤2 + 步骤3： JoinGroup + SyncGroup。

可问题是： 当一个consumer挂了，或者有新的consumer加入，其他consumers怎么知道要进行rebalance呢？ 答案就是下面的heartbeat。

heartbeat的实现原理

每一个consumer都会定期的往coordinator发送heartbeat消息，一旦coordinator返回了某个特定的error code：ILLEGAL_GENERATION, 就说明之前的group无效了（解散了），要重新进行JoinGroup + SyncGroup操作。

那这个定期发送如何实现呢？一个直观的想法就是开一个后台线程，定时发送heartbeat消息，但维护一个后台线程，很显然会增大实现的复杂性。上面也说了， consumer是单线程程序。在这里是通过DelayedQueue来实现的。

DelayedQueue与HeartBeatTask

其基本思路是把HeartBeatRequest放入一个DelayedQueue中，然后在while循环的poll中，每次从DelayedQueue中把请求拿出来发送出去（只有时间到了，Task才能从Queue中拿出来）。

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">HeartbeatTask</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">implements</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">DelayedTask</span> {</span>

        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">boolean</span> requestInFlight = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">false</span>;  <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//关键变量：判断当前是否有发送出去的HeartBeatRequest，其Response还没回来</span>

        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//reset本质就是发送函数</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">reset</span>() {      
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> now = time.milliseconds();
            heartbeat.resetSessionTimeout(now);
            client.unschedule(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>);

            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (!requestInFlight)
                client.schedule(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>, now);  <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//如果没有requestInFlight，则把HeartBeatTask放入DelayedQueue中</span>
        }

        <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">run</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> now) {
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (generation < <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> || needRejoin() || coordinatorUnknown()) {
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span>;
            }

            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (heartbeat.sessionTimeoutExpired(now)) {
                coordinatorDead();
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span>;
            }

            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (!heartbeat.shouldHeartbeat(now)) {
                client.schedule(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>, now + heartbeat.timeToNextHeartbeat(now));
            } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {
                heartbeat.sentHeartbeat(now);
                requestInFlight = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>;

                RequestFuture<Void> future = sendHeartbeatRequest();
                future.addListener(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> RequestFutureListener<Void>() {
                    <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onSuccess</span>(Void value) {
                        requestInFlight = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">false</span>;
                        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> now = time.milliseconds();
                        heartbeat.receiveHeartbeat(now);
                        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> nextHeartbeatTime = now + heartbeat.timeToNextHeartbeat(now);
                        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//放入delayedQueue</span>
                        client.schedule(HeartbeatTask.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>, nextHeartbeatTime);
                    }

         <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//hearbeat返回之后，无论response成功，还是失败，把下1个heartbeat放入delayedQueue，从而形成循环间隔发送</span>
                    <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onFailure</span>(RuntimeException e) {
                        requestInFlight = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">false</span>;
                        client.schedule(HeartbeatTask.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>, time.milliseconds() + retryBackoffMs);
                    }
                });
            }
        }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li><li style="box-sizing: border-box; padding: 0px 5px;">50</li><li style="box-sizing: border-box; padding: 0px 5px;">51</li><li style="box-sizing: border-box; padding: 0px 5px;">52</li></ul>

rebalance检测

首先一点说明：个人认为这里的网络框架，封装的有点冗余：sendHeartbeatRequest既有callback机制（CompleteHandler），又为其Future加了Listener机制（上面的代码）。

也就是在heartbeat的completeHandler中，完成了rebalance的检测：从下面代码可以看出，对于以下的response error code，都会触发rebalance:

  1.  * GROUP_COORDINATOR_NOT_AVAILABLE (15)
  2.  * NOT_COORDINATOR_FOR_GROUP (16)
  3.  * ILLEGAL_GENERATION (22)
  4.  * UNKNOWN_MEMBER_ID (25)
  5.  * REBALANCE_IN_PROGRESS (27)
  6.  * GROUP_AUTHORIZATION_FAILED (30)
<code class="hljs avrasm has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">    public RequestFuture<Void> sendHeartbeatRequest() {
        HeartbeatRequest req = new HeartbeatRequest(this<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.groupId</span>, this<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.generation</span>, this<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.memberId</span>)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
        return client<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.send</span>(coordinator, ApiKeys<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.HEARTBEAT</span>, req)
                <span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.compose</span>(new HeartbeatCompletionHandler())<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
    }

    private class HeartbeatCompletionHandler extends CoordinatorResponseHandler<HeartbeatResponse, Void> {
        @Override
        public HeartbeatResponse parse(ClientResponse response) {
            return new HeartbeatResponse(response<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.responseBody</span>())<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
        }

        @Override
        public void handle(HeartbeatResponse heartbeatResponse, RequestFuture<Void> future) {
            sensors<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.heartbeatLatency</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.record</span>(response<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.requestLatencyMs</span>())<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
            short errorCode = heartbeatResponse<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.errorCode</span>()<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
            if (errorCode == Errors<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.NONE</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.code</span>()) {
                log<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.debug</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Received successful heartbeat response."</span>)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
                future<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.complete</span>(null)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
            } else if (errorCode == Errors<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.GROUP</span>_COORDINATOR_NOT_AVAILABLE<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.code</span>()
                    || errorCode == Errors<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.NOT</span>_COORDINATOR_FOR_GROUP<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.code</span>()) {
                log<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.info</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Attempt to heart beat failed since coordinator is either not started or not valid, marking it as dead."</span>)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
                coordinatorDead()<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
                future<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.raise</span>(Errors<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.forCode</span>(errorCode))<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
            } else if (errorCode == Errors<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.REBALANCE</span>_IN_PROGRESS<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.code</span>()) {
                log<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.info</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Attempt to heart beat failed since the group is rebalancing, try to re-join group."</span>)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
                AbstractCoordinator<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.this</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.rejoinNeeded</span> = true<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
                future<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.raise</span>(Errors<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.REBALANCE</span>_IN_PROGRESS)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
            } else if (errorCode == Errors<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.ILLEGAL</span>_GENERATION<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.code</span>()) {
                log<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.info</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Attempt to heart beat failed since generation id is not legal, try to re-join group."</span>)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
                AbstractCoordinator<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.this</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.rejoinNeeded</span> = true<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
                future<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.raise</span>(Errors<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.ILLEGAL</span>_GENERATION)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
            } else if (errorCode == Errors<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.UNKNOWN</span>_MEMBER_ID<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.code</span>()) {
                log<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.info</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Attempt to heart beat failed since member id is not valid, reset it and try to re-join group."</span>)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
                memberId = JoinGroupRequest<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.UNKNOWN</span>_MEMBER_ID<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
                AbstractCoordinator<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.this</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.rejoinNeeded</span> = true<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
                future<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.raise</span>(Errors<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.UNKNOWN</span>_MEMBER_ID)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
            } else if (errorCode == Errors<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.GROUP</span>_AUTHORIZATION_FAILED<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.code</span>()) {
                future<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.raise</span>(new GroupAuthorizationException(groupId))<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
            } else {
                future<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.raise</span>(new KafkaException(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Unexpected errorCode in heartbeat response: "</span>
                        + Errors<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.forCode</span>(errorCode)<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.exception</span>()<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.getMessage</span>()))<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
            }
        }
    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li></ul>

关键点：这里所谓的触发，其实就是把rejoinNeeded置为了true。然后在下一次poll循环的时候，检测到rejoinNeeded为true，就会重走上面的步骤2 + 步骤3

failover

对于这整个体系来说，consumer可能挂掉，coordinator也可能挂掉。因此双方需要互相检测，对方是否挂了。

检测方法同样是上面的heartbeat：当consumer发现heartbeat返回超时，或者coordinator很久没有收到heartbeat，就会认为对方挂了。

当然，这会有“误操作”，比如consumer处理消息很慢（因为是单线程），导致下1次heartbeat迟迟没有发出去。这个时候coordinator就会认为该consumer挂了，会主动断开连接。从而触发1次rebalance。

consumer认为coordinator挂了

就会从上面的步骤1开始，重新discovery coordinator，然后JoinGroup + SyncGroup

coordinator认为consumer挂了

从上面的步骤2开始，通知其他所有剩下的consumers，进行JoinGroup + SyncGroup

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