Apache Kafka源码分析 - PartitionStateMachine

startup

在onControllerFailover中被调用，

initializePartitionState

private def initializePartitionState() {
for((topicPartition, replicaAssignment) <- controllerContext.partitionReplicaAssignment) { // 取出所有的partitions
// check if leader and isr path exists for partition. If not, then it is in NEW state
controllerContext.partitionLeadershipInfo.get(topicPartition) match {
case Some(currentLeaderIsrAndEpoch) =>
// else, check if the leader for partition is alive. If yes, it is in Online state, else it is in Offline state
controllerContext.liveBrokerIds.contains(currentLeaderIsrAndEpoch.leaderAndIsr.leader) match {
case true => // leader is alive
partitionState.put(topicPartition, OnlinePartition)
case false =>
partitionState.put(topicPartition, OfflinePartition)
}
case None =>
partitionState.put(topicPartition, NewPartition)
}
}
}

这里注意offlinePartition和newPartition的区别，

如果controllerContext.partitionLeadershipInfo中没有这个partition的leader信息，那么说明是newPartition

如果有leader，但leader所在broker不是alive的，那么就是offlinePartition

当然，如果leader所在broker是alive的，那么就是onlinePartition

 

triggerOnlinePartitionStateChange

试图将所有offline和new partition的状态变成online

def triggerOnlinePartitionStateChange() {
try {
brokerRequestBatch.newBatch()
// try to move all partitions in NewPartition or OfflinePartition state to OnlinePartition state except partitions
// that belong to topics to be deleted
for((topicAndPartition, partitionState) <- partitionState
if(!controller.deleteTopicManager.isTopicQueuedUpForDeletion(topicAndPartition.topic))) {
if(partitionState.equals(OfflinePartition) || partitionState.equals(NewPartition))
handleStateChange(topicAndPartition.topic, topicAndPartition.partition, OnlinePartition, controller.offlinePartitionSelector,
(new CallbackBuilder).build)
}
brokerRequestBatch.sendRequestsToBrokers(controller.epoch, controllerContext.correlationId.getAndIncrement)
} catch {
case e: Throwable => error("Error while moving some partitions to the online state", e)
// TODO: It is not enough to bail out and log an error, it is important to trigger leader election for those partitions
}
}

这里看到，brokerRequestBatch，这个经常出现，ControllerBrokerRequestBatch

这个类封装了leaderAndIsrRequestMap，stopReplicaRequestMap，updateMetadataRequestMap

用来记录和cache，handleStateChange中产生的这些request

最终用sendRequestsToBrokers，将这些requests，批量的发出去

handleStateChange的逻辑后面单独看，这里看看controller.offlinePartitionSelector，这个selector实现如何为一个newPartition或offlinePartition选一个leader

代码挺长的，注释讲的挺清楚的，就不贴代码了

首先如果ISR里面有活的broker，那没有好说的，直接用它作为新的leader

如果没有，这里需要看一下是否容忍unclean leader election，其实就是是否可以容忍丢数据，如果可以

那么就看看这AR里面有没有活的broker，如果有就以它为leader，但这个既然不在ISR里面，说明这个replica是不同步的，所以一定有data loss

如果AR里面也没有活的broker，那只能是elect失败了

/**
* Select the new leader, new isr and receiving replicas (for the LeaderAndIsrRequest):
* 1. If at least one broker from the isr is alive, it picks a broker from the live isr as the new leader and the live
*    isr as the new isr.
* 2. Else, if unclean leader election for the topic is disabled, it throws a NoReplicaOnlineException.
* 3. Else, it picks some alive broker from the assigned replica list as the new leader and the new isr.
* 4. If no broker in the assigned replica list is alive, it throws a NoReplicaOnlineException
* Replicas to receive LeaderAndIsr request = live assigned replicas
* Once the leader is successfully registered in zookeeper, it updates the allLeaders cache
*/

registerListeners

在onControllerFailover中被调用，

这里负责注册一下listener到zk，deleteTopicListener先不管

先看看TopicChangeListener，当topics发生变化时，我们做什么处理？

 

registerTopicChangeListener

private def registerTopicChangeListener() = {
zkClient.subscribeChildChanges(ZkUtils.BrokerTopicsPath, topicChangeListener) //"/brokers/topics"
}

Listen这个目录， /brokers/topics，如果发生变化，触发topicChangeListener

 

TopicChangeListener

/**
* This is the zookeeper listener that triggers all the state transitions for a partition
*/
class TopicChangeListener extends IZkChildListener with Logging {
this.logIdent = "[TopicChangeListener on Controller " + controller.config.brokerId + "]: "

@throws(classOf[Exception])
def handleChildChange(parentPath : String, children : java.util.List[String]) {
inLock(controllerContext.controllerLock) {
if (hasStarted.get) {
try {
val currentChildren = {
import JavaConversions._
debug("Topic change listener fired for path %s with children %s".format(parentPath, children.mkString(",")))
(children: Buffer[String]).toSet
}
val newTopics = currentChildren -- controllerContext.allTopics //context里面没记录，但zk有的，就是新topic
val deletedTopics = controllerContext.allTopics -- currentChildren //反之，就被删除的topic
controllerContext.allTopics = currentChildren //更新context

val addedPartitionReplicaAssignment = ZkUtils.getReplicaAssignmentForTopics(zkClient, newTopics.toSeq) //从zk取出新topic的assignment情况
controllerContext.partitionReplicaAssignment = controllerContext.partitionReplicaAssignment.filter(p =>  //从context中的assignment情况中删掉deletedtopic的
!deletedTopics.contains(p._1.topic))
controllerContext.partitionReplicaAssignment.++=(addedPartitionReplicaAssignment) //把新的topic的assignment加入context
info("New topics: [%s], deleted topics: [%s], new partition replica assignment [%s]".format(newTopics,
deletedTopics, addedPartitionReplicaAssignment))
if(newTopics.size > 0)
controller.onNewTopicCreation(newTopics, addedPartitionReplicaAssignment.keySet.toSet) //最终调用KafkaController.onNewTopicCreation
} catch {
case e: Throwable => error("Error while handling new topic", e )
}
}
}
}
}

 

onNewTopicCreation

def onNewTopicCreation(topics: Set[String], newPartitions: Set[TopicAndPartition]) {
info("New topic creation callback for %s".format(newPartitions.mkString(",")))
// subscribe to partition changes
topics.foreach(topic => partitionStateMachine.registerPartitionChangeListener(topic)) //添加partition change listener
onNewPartitionCreation(newPartitions) //partition和replica的状态变化
}

def registerPartitionChangeListener(topic: String) = {
addPartitionsListener.put(topic, new AddPartitionsListener(topic))
zkClient.subscribeDataChanges(ZkUtils.getTopicPath(topic), addPartitionsListener(topic)) ///brokers/topics/topic-name
}

 

AddPartitionsListener

和topic listener很想，就是从zk读出partition情况，和当前context里面的比较，找出新的partitions，调用

controller.onNewPartitionCreation(partitionsToBeAdded.keySet.toSet)

可见无论是TopicChangeListener还是AddPartitionsListener，最终都是调用到onNewPartitionCreation，毕竟topic是个逻辑概念

onNewPartitionCreation

def onNewPartitionCreation(newPartitions: Set[TopicAndPartition]) {
info("New partition creation callback for %s".format(newPartitions.mkString(",")))
partitionStateMachine.handleStateChanges(newPartitions, NewPartition)
replicaStateMachine.handleStateChanges(controllerContext.replicasForPartition(newPartitions), NewReplica)
partitionStateMachine.handleStateChanges(newPartitions, OnlinePartition, offlinePartitionSelector)
replicaStateMachine.handleStateChanges(controllerContext.replicasForPartition(newPartitions), OnlineReplica)
}

很简单，只是首先将所有新的partition和相应的replica的状态设为new，然后再设为online

 

handleStateChange

这是状态机的主逻辑，

private def handleStateChange(topic: String, partition: Int, targetState: PartitionState,
leaderSelector: PartitionLeaderSelector,
callbacks: Callbacks) {
val topicAndPartition = TopicAndPartition(topic, partition)
val currState = partitionState.getOrElseUpdate(topicAndPartition, NonExistentPartition) // 取得当前状态
try {
targetState match {
case NewPartition =>
// pre: partition did not exist before this
assertValidPreviousStates(topicAndPartition, List(NonExistentPartition), NewPartition)
assignReplicasToPartitions(topic, partition) // 从zk取得AR，并更新controllerContext.partitionReplicaAssignment
partitionState.put(topicAndPartition, NewPartition)
// post: partition has been assigned replicas
case OnlinePartition =>
assertValidPreviousStates(topicAndPartition, List(NewPartition, OnlinePartition, OfflinePartition), OnlinePartition)
partitionState(topicAndPartition) match {
case NewPartition =>
// initialize leader and isr path for new partition
initializeLeaderAndIsrForPartition(topicAndPartition)
case OfflinePartition =>
electLeaderForPartition(topic, partition, leaderSelector)
case OnlinePartition => // invoked when the leader needs to be re-elected
electLeaderForPartition(topic, partition, leaderSelector)
case _ => // should never come here since illegal previous states are checked above
}
partitionState.put(topicAndPartition, OnlinePartition)
val leader = controllerContext.partitionLeadershipInfo(topicAndPartition).leaderAndIsr.leader
stateChangeLogger.trace("Controller %d epoch %d changed partition %s from %s to %s with leader %d"
.format(controllerId, controller.epoch, topicAndPartition, currState, targetState, leader))
// post: partition has a leader
case OfflinePartition =>
// pre: partition should be in New or Online state
assertValidPreviousStates(topicAndPartition, List(NewPartition, OnlinePartition, OfflinePartition), OfflinePartition)
// should be called when the leader for a partition is no longer alive
stateChangeLogger.trace("Controller %d epoch %d changed partition %s state from %s to %s"
.format(controllerId, controller.epoch, topicAndPartition, currState, targetState))
partitionState.put(topicAndPartition, OfflinePartition)
// post: partition has no alive leader
case NonExistentPartition =>
// pre: partition should be in Offline state
assertValidPreviousStates(topicAndPartition, List(OfflinePartition), NonExistentPartition)
stateChangeLogger.trace("Controller %d epoch %d changed partition %s state from %s to %s"
.format(controllerId, controller.epoch, topicAndPartition, currState, targetState))
partitionState.put(topicAndPartition, NonExistentPartition)
// post: partition state is deleted from all brokers and zookeeper
}
} catch {
case t: Throwable =>
stateChangeLogger.error("Controller %d epoch %d initiated state change for partition %s from %s to %s failed"
.format(controllerId, controller.epoch, topicAndPartition, currState, targetState), t)
}
}

可以看到，对于转变到OfflinePartition，NonExistentPartition，只是单纯的设置state

而转变到NewPartition，除了设置state，也就多了步初始化AR

只有转变到OnlinePartition的时候比较复杂些，

其中从NewPartition--》OnlinePartition，需要做些初始化的工作，所以调用initializeLeaderAndIsrForPartition

initializeLeaderAndIsrForPartition

NewPartition是在zk中，没有leaderAndISR path的，所以初始化需要创建path，创建后，就再也不能回到New的状态，只能到offline

其中逻辑除了创建zk path，就是进行leader elect，这里的elect逻辑是写死的，初始化的时候，一定是prefered selector，即选取live AR的head

/**
* Invoked on the NewPartition->OnlinePartition state change. When a partition is in the New state, it does not have
* a leader and isr path in zookeeper. Once the partition moves to the OnlinePartition state, it's leader and isr
* path gets initialized and it never goes back to the NewPartition state. From here, it can only go to the
* OfflinePartition state.
* @param topicAndPartition   The topic/partition whose leader and isr path is to be initialized
*/
private def initializeLeaderAndIsrForPartition(topicAndPartition: TopicAndPartition) {
val replicaAssignment = controllerContext.partitionReplicaAssignment(topicAndPartition)
val liveAssignedReplicas = replicaAssignment.filter(r => controllerContext.liveBrokerIds.contains(r)) // 找出AR中活着的replica
liveAssignedReplicas.size match {
case 0 => // 没有活的，那肯定无法转成online的
val failMsg = ("encountered error during state change of partition %s from New to Online, assigned replicas are [%s], " +
"live brokers are [%s]. No assigned replica is alive.")
.format(topicAndPartition, replicaAssignment.mkString(","), controllerContext.liveBrokerIds)
stateChangeLogger.error("Controller %d epoch %d ".format(controllerId, controller.epoch) + failMsg)
throw new StateChangeFailedException(failMsg)
case _ =>
debug("Live assigned replicas for partition %s are: [%s]".format(topicAndPartition, liveAssignedReplicas))
// make the first replica in the list of assigned replicas, the leader
val leader = liveAssignedReplicas.head  // 取出第一个活的replica，作为leader replica
val leaderIsrAndControllerEpoch = new LeaderIsrAndControllerEpoch(new LeaderAndIsr(leader, liveAssignedReplicas.toList), // 封装出LeaderIsrAndControllerEpoch
controller.epoch)
debug("Initializing leader and isr for partition %s to %s".format(topicAndPartition, leaderIsrAndControllerEpoch))
try {
ZkUtils.createPersistentPath(controllerContext.zkClient,  // 创建zk的LeaderAndIsrPath，关键的初始步骤
ZkUtils.getTopicPartitionLeaderAndIsrPath(topicAndPartition.topic, topicAndPartition.partition),
ZkUtils.leaderAndIsrZkData(leaderIsrAndControllerEpoch.leaderAndIsr, controller.epoch))
// NOTE: the above write can fail only if the current controller lost its zk session and the new controller
// took over and initialized this partition. This can happen if the current controller went into a long
// GC pause
controllerContext.partitionLeadershipInfo.put(topicAndPartition, leaderIsrAndControllerEpoch) // 更新context中的partitionLeadershipInfo
brokerRequestBatch.addLeaderAndIsrRequestForBrokers(liveAssignedReplicas, topicAndPartition.topic, // 添加LeaderAndIsrRequest到requestbatch
topicAndPartition.partition, leaderIsrAndControllerEpoch, replicaAssignment)
} catch {
case e: ZkNodeExistsException =>
// read the controller epoch
val leaderIsrAndEpoch = ReplicationUtils.getLeaderIsrAndEpochForPartition(zkClient, topicAndPartition.topic,
topicAndPartition.partition).get
val failMsg = ("encountered error while changing partition %s's state from New to Online since LeaderAndIsr path already " +
"exists with value %s and controller epoch %d")
.format(topicAndPartition, leaderIsrAndEpoch.leaderAndIsr.toString(), leaderIsrAndEpoch.controllerEpoch)
stateChangeLogger.error("Controller %d epoch %d ".format(controllerId, controller.epoch) + failMsg)
throw new StateChangeFailedException(failMsg)
}
}
}

 

OfflinePartition或OnlinePartition –》OnlinePartition

这个相对简单，只需要重新选举一下leader

electLeaderForPartition

def electLeaderForPartition(topic: String, partition: Int, leaderSelector: PartitionLeaderSelector) {
val topicAndPartition = TopicAndPartition(topic, partition)
try {
var zookeeperPathUpdateSucceeded: Boolean = false
var newLeaderAndIsr: LeaderAndIsr = null
var replicasForThisPartition: Seq[Int] = Seq.empty[Int]
while(!zookeeperPathUpdateSucceeded) { // while，只有更新zk成功，或发生异常才会跳出，这样写是不是有点危险
val currentLeaderIsrAndEpoch = getLeaderIsrAndEpochOrThrowException(topic, partition) // 去zk获取leaderAndIsr信息，如果取不到，抛异常，因为offline或online都应该在zk上有数据的
val currentLeaderAndIsr = currentLeaderIsrAndEpoch.leaderAndIsr
val controllerEpoch = currentLeaderIsrAndEpoch.controllerEpoch
if (controllerEpoch > controller.epoch) { // 判断leaderAndISR如果已经被其他更新epoch的controller改过，那就说明当前controller已经过期了，抛异常
val failMsg = ("aborted leader election for partition [%s,%d] since the LeaderAndIsr path was " +
"already written by another controller. This probably means that the current controller %d went through " +
"a soft failure and another controller was elected with epoch %d.")
.format(topic, partition, controllerId, controllerEpoch)
stateChangeLogger.error("Controller %d epoch %d ".format(controllerId, controller.epoch) + failMsg)
throw new StateChangeFailedException(failMsg)
}
// elect new leader or throw exception
val (leaderAndIsr, replicas) = leaderSelector.selectLeader(topicAndPartition, currentLeaderAndIsr) // 调用Selector来选取leader，不同的Selector会有不同的选取逻辑
val (updateSucceeded, newVersion) = ReplicationUtils.updateLeaderAndIsr(zkClient, topic, partition, // 在zk上更新leaderAndISR
leaderAndIsr, controller.epoch, currentLeaderAndIsr.zkVersion)
newLeaderAndIsr = leaderAndIsr
newLeaderAndIsr.zkVersion = newVersion
zookeeperPathUpdateSucceeded = updateSucceeded
replicasForThisPartition = replicas
}
val newLeaderIsrAndControllerEpoch = new LeaderIsrAndControllerEpoch(newLeaderAndIsr, controller.epoch)
// update the leader cache
controllerContext.partitionLeadershipInfo.put(TopicAndPartition(topic, partition), newLeaderIsrAndControllerEpoch)
stateChangeLogger.trace("Controller %d epoch %d elected leader %d for Offline partition %s"
.format(controllerId, controller.epoch, newLeaderAndIsr.leader, topicAndPartition))
val replicas = controllerContext.partitionReplicaAssignment(TopicAndPartition(topic, partition))
// store new leader and isr info in cache
brokerRequestBatch.addLeaderAndIsrRequestForBrokers(replicasForThisPartition, topic, partition,
newLeaderIsrAndControllerEpoch, replicas)
} catch {
case lenne: LeaderElectionNotNeededException => // swallow
case nroe: NoReplicaOnlineException => throw nroe
case sce: Throwable =>
val failMsg = "encountered error while electing leader for partition %s due to: %s.".format(topicAndPartition, sce.getMessage)
stateChangeLogger.error("Controller %d epoch %d ".format(controllerId, controller.epoch) + failMsg)
throw new StateChangeFailedException(failMsg, sce)
}
debug("After leader election, leader cache is updated to %s".format(controllerContext.partitionLeadershipInfo.map(l => (l._1, l._2))))
}