深入学习Kafka：PartitionLeaderSelector源码分析

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PartitionLeaderSelector主要是为分区选举出leader broker，该trait只定义了一个方法selectLeader，接收一个TopicAndPartition对象和一个LeaderAndIsr对象。

TopicAndPartition表示要选leader的分区，而第二个参数表示zookeeper中保存的该分区的当前leader和ISR记录。该方法会返回一个元组包括了选举出来的leader和ISR以及需要接收LeaderAndISr请求的一组副本。

trait PartitionLeaderSelector {
def selectLeader(topicAndPartition: TopicAndPartition, currentLeaderAndIsr: LeaderAndIsr): (LeaderAndIsr, Seq[Int])
}

PartitionLeaderSelector的实现类主要有

1. NoOpLeaderSelector

2. OfflinePartitionLeaderSelector

3. ReassignedPartitionLeaderSelector

4. PreferredReplicaPartitionLeaderSelector

5. ControlledShutdownLeaderSelector

NoOpLeaderSelector

NoOpLeaderSelector就是啥也不做的LeaderSelector。

class NoOpLeaderSelector(controllerContext: ControllerContext) extends PartitionLeaderSelector with Logging {

this.logIdent = "[NoOpLeaderSelector]: "

def selectLeader(topicAndPartition: TopicAndPartition, currentLeaderAndIsr: LeaderAndIsr): (LeaderAndIsr, Seq[Int]) = {
warn("I should never have been asked to perform leader election, returning the current LeaderAndIsr and replica assignment.")
(currentLeaderAndIsr, controllerContext.partitionReplicaAssignment(topicAndPartition))
}
}

ControlledShutdownLeaderSelector

当controller收到shutdown命令后，触发新的分区主副本选举

先找出已分配的副本集合（assignedReplicas），然后过滤出仍存活的副本集合（liveAssignedReplicas），在该列表中选取第一个broker作为该分区的主副本

class ControlledShutdownLeaderSelector(controllerContext: ControllerContext)
extends PartitionLeaderSelector
with Logging {

this.logIdent = "[ControlledShutdownLeaderSelector]: "

def selectLeader(topicAndPartition: TopicAndPartition, currentLeaderAndIsr: LeaderAndIsr): (LeaderAndIsr, Seq[Int]) = {
val currentLeaderEpoch = currentLeaderAndIsr.leaderEpoch
val currentLeaderIsrZkPathVersion = currentLeaderAndIsr.zkVersion

val currentLeader = currentLeaderAndIsr.leader

//已分配的Replica
val assignedReplicas = controllerContext.partitionReplicaAssignment(topicAndPartition)
val liveOrShuttingDownBrokerIds = controllerContext.liveOrShuttingDownBrokerIds
//仍存活的Replica
val liveAssignedReplicas = assignedReplicas.filter(r => liveOrShuttingDownBrokerIds.contains(r))

//当前的ISR列表中滤掉挂掉的broker
val newIsr = currentLeaderAndIsr.isr.filter(brokerId => !controllerContext.shuttingDownBrokerIds.contains(brokerId))
//存活的ISR列表中，选出第一个broker作为该分区的Leader（主副本）
liveAssignedReplicas.filter(newIsr.contains).headOption match {
case Some(newLeader) =>
//如果存在，则将当前LeaderEpoch计数器加1，对应的Zookeeper节点的版本号也加1
debug("Partition %s : current leader = %d, new leader = %d".format(topicAndPartition, currentLeader, newLeader))
(LeaderAndIsr(newLeader, currentLeaderEpoch + 1, newIsr, currentLeaderIsrZkPathVersion + 1), liveAssignedReplicas)
case None =>
//不存在则报错StateChangeFailedException
throw new StateChangeFailedException(("No other replicas in ISR %s for %s besides" +
" shutting down brokers %s").format(currentLeaderAndIsr.isr.mkString(","), topicAndPartition, controllerContext.shuttingDownBrokerIds.mkString(",")))
}
}
}

PreferredReplicaPartitionLeaderSelector

当controller收到分区主副本重新优化分配命令后，触发新的分区主副本优化，即将AR里的第一个取出，作为优化后的主副本

class PreferredReplicaPartitionLeaderSelector(controllerContext: ControllerContext) extends PartitionLeaderSelector
with Logging {
this.logIdent = "[PreferredReplicaPartitionLeaderSelector]: "

def selectLeader(topicAndPartition: TopicAndPartition, currentLeaderAndIsr: LeaderAndIsr): (LeaderAndIsr, Seq[Int]) = {
//已分配的Replica
val assignedReplicas = controllerContext.partitionReplicaAssignment(topicAndPartition)
//已分配的Replica列表中第一个即为最优的副本
val preferredReplica = assignedReplicas.head
// check if preferred replica is the current leader
//检查是否当前分区主副本已经是最优的副本，则报错LeaderElectionNotNeededException
val currentLeader = controllerContext.partitionLeadershipInfo(topicAndPartition).leaderAndIsr.leader
if (currentLeader == preferredReplica) {
throw new LeaderElectionNotNeededException("Preferred replica %d is already the current leader for partition %s"
.format(preferredReplica, topicAndPartition))
} else {
info("Current leader %d for partition %s is not the preferred replica.".format(currentLeader, topicAndPartition) +
" Trigerring preferred replica leader election")
// check if preferred replica is not the current leader and is alive and in the isr
if (controllerContext.liveBrokerIds.contains(preferredReplica) && currentLeaderAndIsr.isr.contains(preferredReplica)) {
//如果当前的最优主副本存活，返回将其设为最优主副本，则将当前LeaderEpoch计数器加1，对应的Zookeeper节点的版本号也加1
(new LeaderAndIsr(preferredReplica, currentLeaderAndIsr.leaderEpoch + 1, currentLeaderAndIsr.isr,
currentLeaderAndIsr.zkVersion + 1), assignedReplicas)
} else {
//如果当前的最优主副本挂掉了，则报错StateChangeFailedException
throw new StateChangeFailedException("Preferred replica %d for partition ".format(preferredReplica) +
"%s is either not alive or not in the isr. Current leader and ISR: [%s]".format(topicAndPartition, currentLeaderAndIsr))
}
}
}
}

ReassignedPartitionLeaderSelector

在某个topic重新分配分区的时候，触发新的主副本选举，将存活的ISR中的第一个副本选举成为主副本（leader）

class ReassignedPartitionLeaderSelector(controllerContext: ControllerContext) extends PartitionLeaderSelector with Logging {
this.logIdent = "[ReassignedPartitionLeaderSelector]: "

/**
* The reassigned replicas are already in the ISR when selectLeader is called.
*/
def selectLeader(topicAndPartition: TopicAndPartition, currentLeaderAndIsr: LeaderAndIsr): (LeaderAndIsr, Seq[Int]) = {
//重新分配的ISR副本集
val reassignedInSyncReplicas = controllerContext.partitionsBeingReassigned(topicAndPartition).newReplicas
val currentLeaderEpoch = currentLeaderAndIsr.leaderEpoch
val currentLeaderIsrZkPathVersion = currentLeaderAndIsr.zkVersion
//过滤出仍存活的重新分配的ISR副本集
val aliveReassignedInSyncReplicas = reassignedInSyncReplicas.filter(r => controllerContext.liveBrokerIds.contains(r) &&
currentLeaderAndIsr.isr.contains(r))
//选取ISR中的第一个为主副本
val newLeaderOpt = aliveReassignedInSyncReplicas.headOption
newLeaderOpt match {
//返回ISR中的第一个为主副本，则将当前LeaderEpoch计数器加1，对应的Zookeeper节点的版本号也加1
case Some(newLeader) => (new LeaderAndIsr(newLeader, currentLeaderEpoch + 1, currentLeaderAndIsr.isr,
currentLeaderIsrZkPathVersion + 1), reassignedInSyncReplicas)
case None =>
//如果没有存活的ISR，则报错NoReplicaOnlineException，选举失败
reassignedInSyncReplicas.size match {
case 0 =>
throw new NoReplicaOnlineException("List of reassigned replicas for partition " +
" %s is empty. Current leader and ISR: [%s]".format(topicAndPartition, currentLeaderAndIsr))
case _ =>
throw new NoReplicaOnlineException("None of the reassigned replicas for partition " +
"%s are in-sync with the leader. Current leader and ISR: [%s]".format(topicAndPartition, currentLeaderAndIsr))
}
}
}
}

OfflinePartitionLeaderSelector

选出新的Leader，新的ISR，步骤如下：

1. 如果至少有一个broker在ISR列表中，并且存活，则将其选为leader，ISR中存活的为新的ISR

2. 如果ISR列表为空，且unclean.leader.election.enable=false，则报错NoReplicaOnlineException

3. 如果unclean.leader.election.enable=true，即意味着可以选举不在ISR列表中的broker为Leader，即在AR列表中选出Leader，但是这样会引起数据不一致

4. 若AR列表也为空，则报错NoReplicaOnlineException

class OfflinePartitionLeaderSelector(controllerContext: ControllerContext, config: KafkaConfig)
extends PartitionLeaderSelector with Logging {
this.logIdent = "[OfflinePartitionLeaderSelector]: "

def selectLeader(topicAndPartition: TopicAndPartition, currentLeaderAndIsr: LeaderAndIsr): (LeaderAndIsr, Seq[Int]) = {
controllerContext.partitionReplicaAssignment.get(topicAndPartition) match {
case Some(assignedReplicas) =>
val liveAssignedReplicas = assignedReplicas.filter(r => controllerContext.liveBrokerIds.contains(r))
val liveBrokersInIsr = currentLeaderAndIsr.isr.filter(r => controllerContext.liveBrokerIds.contains(r))
val currentLeaderEpoch = currentLeaderAndIsr.leaderEpoch
val currentLeaderIsrZkPathVersion = currentLeaderAndIsr.zkVersion
val newLeaderAndIsr = liveBrokersInIsr.isEmpty match {
case true =>
// Prior to electing an unclean (i.e. non-ISR) leader, ensure that doing so is not disallowed by the configuration
// for unclean leader election.
if (!LogConfig.fromProps(config.originals, AdminUtils.fetchEntityConfig(controllerContext.zkUtils,
ConfigType.Topic, topicAndPartition.topic)).uncleanLeaderElectionEnable) {
throw new NoReplicaOnlineException(("No broker in ISR for partition " +
"%s is alive. Live brokers are: [%s],".format(topicAndPartition, controllerContext.liveBrokerIds)) +
" ISR brokers are: [%s]".format(currentLeaderAndIsr.isr.mkString(",")))
}

debug("No broker in ISR is alive for %s. Pick the leader from the alive assigned replicas: %s"
.format(topicAndPartition, liveAssignedReplicas.mkString(",")))
liveAssignedReplicas.isEmpty match {
case true =>
//若AR列表也为空，则报错NoReplicaOnlineException
throw new NoReplicaOnlineException(("No replica for partition " +
"%s is alive. Live brokers are: [%s],".format(topicAndPartition, controllerContext.liveBrokerIds)) +
" Assigned replicas are: [%s]".format(assignedReplicas))
case false =>
//如果unclean.leader.election.enable=true，即意味着可以选举不在ISR列表中的broker为Leader，即在AR列表中，选出Leader
ControllerStats.uncleanLeaderElectionRate.mark()
val newLeader = liveAssignedReplicas.head
warn("No broker in ISR is alive for %s. Elect leader %d from live brokers %s. There's potential data loss."
.format(topicAndPartition, newLeader, liveAssignedReplicas.mkString(",")))
new LeaderAndIsr(newLeader, currentLeaderEpoch + 1, List(newLeader), currentLeaderIsrZkPathVersion + 1)
}
case false =>
//如果至少有一个broker在ISR列表中，并且存活，则将其选为leader，ISR中存活的为新的ISR
val liveReplicasInIsr = liveAssignedReplicas.filter(r => liveBrokersInIsr.contains(r))
val newLeader = liveReplicasInIsr.head
debug("Some broker in ISR is alive for %s. Select %d from ISR %s to be the leader."
.format(topicAndPartition, newLeader, liveBrokersInIsr.mkString(",")))
new LeaderAndIsr(newLeader, currentLeaderEpoch + 1, liveBrokersInIsr.toList, currentLeaderIsrZkPathVersion + 1)
}
info("Selected new leader and ISR %s for offline partition %s".format(newLeaderAndIsr.toString(), topicAndPartition))
(newLeaderAndIsr, liveAssignedReplicas)
case None =>
throw new NoReplicaOnlineException("Partition %s doesn't have replicas assigned to it".format(topicAndPartition))
}
}
}

[参考资料]

https://yq.aliyun.com/articles/15283

http://blog.csdn.net/zhanglh046/article/details/72822066