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本文所讲的Leader是指集群中的Controller，而不是各个Partition的Leader。
为什么要有Leader？
在Kafka早期版本，对于分区和副本的状态的管理依赖于zookeeper的Watcher和队列：每一个broker都会在zookeeper注册Watcher，所以zookeeper就会出现大量的Watcher, 如果宕机的broker上的partition很多比较多，会造成多个Watcher触发，造成集群内大规模调整；每一个replica都要去再次zookeeper上注册监视器，当集群规模很大的时候，zookeeper负担很重。这种设计很容易出现脑裂和羊群效应以及zookeeper集群过载。
新的版本中该变了这种设计，使用KafkaController，只有KafkaController，Leader会向zookeeper上注册Watcher，其他broker几乎不用监听zookeeper的状态变化。
Kafka集群中多个broker，有一个会被选举为controller leader，负责管理整个集群中分区和副本的状态，比如partition的leader 副本故障，由controller 负责为该partition重新选举新的leader 副本；当检测到ISR列表发生变化，有controller通知集群中所有broker更新其MetadataCache信息；或者增加某个topic分区的时候也会由controller管理分区的重新分配工作
Kafka集群Leader选举原理
我们知道Zookeeper集群中也有选举机制，是通过Paxos算法，通过不同节点向其他节点发送信息来投票选举出leader，但是Kafka的leader的选举就没有这么复杂了。
Kafka的Leader选举是通过在zookeeper上创建/controller临时节点来实现leader选举，并在该节点中写入当前broker的信息
{"version":1,"brokerid":1,"timestamp":"1512018424988"}
利用Zookeeper的强一致性特性，一个节点只能被一个客户端创建成功，创建成功的broker即为leader，即先到先得原则，leader也就是集群中的controller，负责集群中所有大小事务。
当leader和zookeeper失去连接时，临时节点会删除，而其他broker会监听该节点的变化，当节点删除时，其他broker会收到事件通知，重新发起leader选举。
KafkaController
KafkaController初始化ZookeeperLeaderElector对象，为ZookeeperLeaderElector设置两个回调方法，onControllerFailover和onControllerResignation onControllerFailover在选举leader成功后会回调，在onControllerFailover中进行leader依赖的模块初始化，包括向zookeeper上/controller_epoch节点上记录leader的选举次数，这个epoch数值在处理分布式脑裂的场景中很有用。 而onControllerResignation在当前broker不再成为leader（即当前leader退位后）时会回调。 KafkaController在启动后注册zookeeper的会话超时监听器，并尝试选举leader。
class KafkaController {
private val controllerElector = new ZookeeperLeaderElector(controllerContext, ZkUtils.ControllerPath, onControllerFailover, onControllerResignation, config.brokerId)
def startup() = {
inLock(controllerContext.controllerLock) {
info("Controller starting up")
//注册Session过期监听器
registerSessionExpirationListener()
isRunning = true
//每次启动时，尝试选举leader
controllerElector.startup
info("Controller startup complete")
}
}
private def registerSessionExpirationListener() = {
zkUtils.zkClient.subscribeStateChanges(new SessionExpirationListener())
}
}
SessionExpirationListener
当broker和zookeeper重新建立连接后，SessionExpirationListener中的handleNewSession会被调用，这时先关闭之前的leader相关模块，然后重新尝试选举成为leader。
class SessionExpirationListener() extends IZkStateListener with Logging {
this.logIdent = "[SessionExpirationListener on " + config.brokerId + "], "
@throws(classOf[Exception])
def handleStateChanged(state: KeeperState) {
// do nothing, since zkclient will do reconnect for us.
}
/**
* Called after the zookeeper session has expired and a new session has been created. You would have to re-create
* any ephemeral nodes here.
*
* @throws Exception
* On any error.
*/
@throws(classOf[Exception])
def handleNewSession() {
info("ZK expired; shut down all controller components and try to re-elect")
//和Zookeeper重新建立连接后，此方法会被调用
inLock(controllerContext.controllerLock) {
//先注销一些已经注册的监听器，关闭资源
onControllerResignation()
//重新尝试选举成controller
controllerElector.elect
}
}
override def handleSessionEstablishmentError(error: Throwable): Unit = {
//no-op handleSessionEstablishmentError in KafkaHealthCheck should handle this error in its handleSessionEstablishmentError
}
}
ZookeeperLeaderElector
ZookeeperLeaderElector类实现leader选举的功能，但是它并不负责处理broker和zookeeper的会话超时（连接超时）的情况，而是认为调用者应该在会话恢复（连接重新建立）时进行重新选举。
class ZookeeperLeaderElector(controllerContext: ControllerContext,
electionPath: String,
onBecomingLeader: () => Unit,
onResigningAsLeader: () => Unit,
brokerId: Int)
extends LeaderElector with Logging {
var leaderId = -1
// create the election path in ZK, if one does not exist
val index = electionPath.lastIndexOf("/")
if (index > 0)
controllerContext.zkUtils.makeSurePersistentPathExists(electionPath.substring(0, index))
val leaderChangeListener = new LeaderChangeListener
def startup {
inLock(controllerContext.controllerLock) {
// 添加/controller节点的IZkDataListener监听器
controllerContext.zkUtils.zkClient.subscribeDataChanges(electionPath, leaderChangeListener)
// 选举
elect
}
}
}
ZookeeperLeaderElector的startup方法中调用elect方法选举leader
有下面几种情况会调用elect方法
- broker启动时，第一次调用
- 上一次创建节点成功，但是可能在等Zookeeper响应的时候，连接中断，resign方法中删除/controller节点后，触发了leaderChangeListener的handleDataDeleted
- 上一次创建节点未成功，但是可能在等Zookeeper响应的时候，连接中断，而再次进入elect方法时，已有别的broker创建controller节点成功，成为了leader
- 上一次创建节点成功，但是onBecomingLeader抛出了异常，而再次进入
所以elect方法中先获取/controller节点信息，判断是否已经存在，然后再尝试选举leader
private def getControllerID(): Int = {
controllerContext.zkUtils.readDataMaybeNull(electionPath)._1 match {
case Some(controller) => KafkaController.parseControllerId(controller)
case None => -1
}
}
def elect: Boolean = {
val timestamp = SystemTime.milliseconds.toString
val electString = Json.encode(Map("version" -> 1, "brokerid" -> brokerId, "timestamp" -> timestamp))
//先尝试获取/controller节点信息
leaderId = getControllerID
/*
* We can get here during the initial startup and the handleDeleted ZK callback. Because of the potential race condition,
* it's possible that the controller has already been elected when we get here. This check will prevent the following
* createEphemeralPath method from getting into an infinite loop if this broker is already the controller.
*/
// 有下面几种情况会调用elect方法
// 1.broker启动时，第一次调用
// 2.上一次创建节点成功，但是可能在等Zookeeper响应的时候，连接中断，resign方法中删除/controller节点后，触发了leaderChangeListener的handleDataDeleted
// 3.上一次创建节点未成功，但是可能在等Zookeeper响应的时候，连接中断，而再次进入elect方法时，已有别的broker创建controller节点成功，成为了leader
// 4.上一次创建节点成功，但是onBecomingLeader抛出了异常，而再次进入
// 所以先获取节点信息，判断是否已经存在
if(leaderId != -1) {
debug("Broker %d has been elected as leader, so stopping the election process.".format(leaderId))
return amILeader
}
try {
val zkCheckedEphemeral = new ZKCheckedEphemeral(electionPath,
electString,
controllerContext.zkUtils.zkConnection.getZookeeper,
JaasUtils.isZkSecurityEnabled())
//创建/controller节点，并写入controller信息，brokerid, version, timestamp
zkCheckedEphemeral.create()
info(brokerId + " successfully elected as leader")
leaderId = brokerId
//写入成功，成为Leader，回调
onBecomingLeader()
} catch {
case e: ZkNodeExistsException =>
// If someone else has written the path, then
leaderId = getControllerID
//写入失败，节点已经存在，说明已有其他broker创建成功
if (leaderId != -1)
debug("Broker %d was elected as leader instead of broker %d".format(leaderId, brokerId))
else
warn("A leader has been elected but just resigned, this will result in another round of election")
case e2: Throwable =>
error("Error while electing or becoming leader on broker %d".format(brokerId), e2)
//这里有可能是创建节点时，和zookeeper断开了连接，也有可能是onBecomingLeader的回调方法里出了异常
//onBecomingLeader方法里，一般是初始化leader的相关的模块，如果初始化失败，则调用resign方法先删除/controller节点
//当/controller节点被删除时，会触发leaderChangeListener的handleDataDeleted，会重新尝试选举成Leader，更重要的是也让其他broker有机会成为leader，避免某一个broker的onBecomingLeader一直失败造成整个集群一直处于“群龙无首”的尴尬局面
resign()
}
amILeader
}
def close = {
leaderId = -1
}
def amILeader : Boolean = leaderId == brokerId
def resign() = {
leaderId = -1
// 删除/controller节点
controllerContext.zkUtils.deletePath(electionPath)
}
在创建/controller节点时，若收到的异常是ZkNodeExistsException，则说明其他broker已经成为了leader。
而若是onBecomingLeader的回调方法里出了异常，一般是初始化leader的相关的模块出了问题，如果初始化失败，则调用resign方法先删除/controller节点。
当/controller节点被删除时，会触发leaderChangeListener的handleDataDeleted，会重新尝试选举成Leader。
更重要的是也让其他broker有机会成为leader，避免某一个broker的onBecomingLeader一直失败造成整个集群一直处于“群龙无首”的尴尬局面。
LeaderChangeListener
在startup方法中，注册了/controller节点的IZkDataListener监听器即LeaderChangeListener。
若节点数据有变化时，则有可能别的broker成为了leader，则调用onResigningAsLeader方法。
若节点被删除，则是leader已经出了故障下线了，如果当前broker之前是leader，则调用onResigningAsLeader方法，然后重新尝试选举成为leader。
class LeaderChangeListener extends IZkDataListener with Logging {
/**
* Called when the leader information stored in zookeeper has changed. Record the new leader in memory
* @throws Exception On any error.
*/
@throws(classOf[Exception])
def handleDataChange(dataPath: String, data: Object) {
inLock(controllerContext.controllerLock) {
val amILeaderBeforeDataChange = amILeader
leaderId = KafkaController.parseControllerId(data.toString)
info("New leader is %d".format(leaderId))
// The old leader needs to resign leadership if it is no longer the leader
if (amILeaderBeforeDataChange && !amILeader)
//如果之前是Leader，而现在不是Leader
onResigningAsLeader()
}
}
/**
* Called when the leader information stored in zookeeper has been delete. Try to elect as the leader
* @throws Exception
* On any error.
*/
@throws(classOf[Exception])
def handleDataDeleted(dataPath: String) {
inLock(controllerContext.controllerLock) {
debug("%s leader change listener fired for path %s to handle data deleted: trying to elect as a leader"
.format(brokerId, dataPath))
if(amILeader)
//如果之前是Leader
onResigningAsLeader()
//重新尝试选举成Leader
elect
}
}
}
onBecomingLeader方法对应KafkaController里的onControllerFailover方法，当成为新的leader后，要初始化leader所依赖的功能模块
onResigningAsLeader方法对应KafkaController里的onControllerResignation方法，当leader退位后，要关闭leader所依赖的功能模块
Leader选举流程图
整个leader选举的过程的流程图为
[参考资料] http://blog.csdn.net/zhanglh046/article/details/72821995