kafka之三----Controller职责之主题删除

Controller 注册TopicDeletionListener监听器，监听ZK的/admin/delete_topics节点，定义了 TopicDeletionHandler，用它来实现对删除主题的监听，用来实际执行删除Topic的动作。

class TopicDeletionHandler(eventManager: ControllerEventManager) extends ZNodeChildChangeHandler {
  // ZooKeeper节点：/admin/delete_topics
  override val path: String = DeleteTopicsZNode.path
  // 向事件队列写入TopicDeletion事件
  override def handleChildChange(): Unit = eventManager.put(TopicDeletion)
}

删除流程：

1. 删除主题都是在 /admin/delete_topics 节点下创建名为待删除主题名的子节点，比如/admin/delete_topics/xxx 节点
2. 一旦监听到该节点被创建，TopicDeletionHandler 的 handleChildChange 方法就会被触发
3. Controller 会向事件队列写入 TopicDeletion 事件
4. 由EventExecuteThread执行processTopicDeletion方法

private def processTopicDeletion(): Unit = {
  if (!isActive) return
  // 从ZooKeeper中获取待删除主题列表
  var topicsToBeDeleted = zkClient.getTopicDeletions.toSet
  debug(s"Delete topics listener fired for topics ${topicsToBeDeleted.mkString(",")} to be deleted")
  // 找出不存在的主题列表
  val nonExistentTopics = topicsToBeDeleted -- controllerContext.allTopics
  if (nonExistentTopics.nonEmpty) {
    warn(s"Ignoring request to delete non-existing topics ${nonExistentTopics.mkString(",")}")
    zkClient.deleteTopicDeletions(nonExistentTopics.toSeq, controllerContext.epochZkVersion)
  }
  topicsToBeDeleted --= nonExistentTopics
  // 如果delete.topic.enable参数设置成true
  if (config.deleteTopicEnable) {
    if (topicsToBeDeleted.nonEmpty) {
      info(s"Starting topic deletion for topics ${topicsToBeDeleted.mkString(",")}")
      topicsToBeDeleted.foreach { topic =>
        val partitionReassignmentInProgress = controllerContext.partitionsBeingReassigned.map(_.topic).contains(topic)
        if (partitionReassignmentInProgress)
          topicDeletionManager.markTopicIneligibleForDeletion(
            Set(topic), reason = "topic reassignment in progress")
      }
      // 将待删除主题插入到删除等待集合交由TopicDeletionManager处理
      topicDeletionManager.enqueueTopicsForDeletion(topicsToBeDeleted)
    }
  } else { // 不允许删除主题
    info(s"Removing $topicsToBeDeleted since delete topic is disabled")
    // 清除ZooKeeper下/admin/delete_topics下的子节点
    zkClient.deleteTopicDeletions(topicsToBeDeleted.toSeq, controllerContext.epochZkVersion)
  }
}

5. 方法内首页获取ZooKeeper 的 /admin/delete_topics 下子节点列表（待删除主题列表）
6. 与元数据缓存中的主题列表比较，找出不存在的主题列表，删除 /admin/delete_topics 下对应的子节点
7. 对于已存在的主题列表，检查 Broker 端参数 delete.topic.enable 的值
8. 如果该参数为 false，即不允许删除主题，删除 /admin/delete_topics 下对应的子节点
9. 如果该参数为 true，遍历待删除主题列表，将那些正在执行分区迁移的主题暂时设置成“不可删除”状态；把待删除主题列表中的主题交由 TopicDeletionManager，由它执行真正的删除逻辑

class TopicDeletionManager(config: KafkaConfig, // KafkaConfig类，保存Broker端参数
                           controllerContext: ControllerContext, // 集群元数据
                           replicaStateMachine: ReplicaStateMachine, // 副本状态机，用于设置副本状态
                           partitionStateMachine: PartitionStateMachine, // 分区状态机，用于设置分区状态
                           client: DeletionClient // 负责实现删除主题以及后续的动作，比如更新元数据等。
                           // 这个接口里定义了 4 个方法，分别是 deleteTopic、deleteTopicDeletions、mutePartitionModifications 和 sendMetadataUpdate。
                           // ControllerDeletionClient 实现 DeletionClient 接口的类，分别实现了刚刚说到的那 4 个方法。
                           ) extends Logging {
  this.logIdent = s"[Topic Deletion Manager ${config.brokerId}] "
  // 是否允许删除主题
  val isDeleteTopicEnabled: Boolean = config.deleteTopicEnable
}

class ControllerDeletionClient(controller: KafkaController, zkClient: KafkaZkClient) extends DeletionClient {
  // 删除给定主题,用于删除主题在 ZooKeeper 上的所有“痕迹”
  // 分别调用 KafkaZkClient 的 3 个方法去删除 ZooKeeper 下 /brokers/topics/节点、/config/topics/节点和 /admin/delete_topics/节点。
  override def deleteTopic(topic: String, epochZkVersion: Int): Unit = {
    // 删除/brokers/topics/节点
    zkClient.deleteTopicZNode(topic, epochZkVersion)
    // 删除/config/topics/节点
    zkClient.deleteTopicConfigs(Seq(topic), epochZkVersion)
    // 删除/admin/delete_topics/节点
    zkClient.deleteTopicDeletions(Seq(topic), epochZkVersion)
  }
  // 删除/admin/delete_topics下的给定topic子节点
  override def deleteTopicDeletions(topics: Seq[String], epochZkVersion: Int): Unit = {
    zkClient.deleteTopicDeletions(topics, epochZkVersion)
  }
  // 取消/brokers/topics/节点数据变更的监听
  // 具体实现原理其实就是取消 /brokers/topics/节点数据变更的监听。
  override def mutePartitionModifications(topic: String): Unit = {
    controller.unregisterPartitionModificationsHandlers(Seq(topic))
  }
  // 向集群Broker发送指定分区的元数据更新请求
  // 给集群所有 Broker 发送更新请求，告诉它们不要再为已删除主题的分区提供服务
  override def sendMetadataUpdate(partitions: Set[TopicPartition]): Unit = {
    // 给集群所有Broker发送UpdateMetadataRequest
    // 通知它们给定partitions的状态变化
    controller.sendUpdateMetadataRequest(controller.controllerContext.liveOrShuttingDownBrokerIds.toSeq, partitions)
  }
}

10. 删除给定主题,用于删除主题在 ZooKeeper 上的所有“痕迹；删除 ZooKeeper 下 /brokers/topics/节点、/config/topics/节点和 /admin/delete_topics/节点
11. 取消/brokers/topics/节点数据变更的监听
12. 向集群Broker发送UpdateMetadataRequest元数据更新请求，通知它们给定partitions的状态变化，告诉它们不要再为已删除主题的分区提供服务
13. 若主题因为某些事件可能一时无法完成删除，比如主题分区正在进行副本重分配等，则调用 resumeDeletions 重启删除操作

  private def resumeDeletions(): Unit = {
    // 从元数据缓存中获取要删除的主题列表
    val topicsQueuedForDeletion = Set.empty[String] ++ controllerContext.topicsToBeDeleted
    if (topicsQueuedForDeletion.nonEmpty)
      info(s"Handling deletion for topics ${topicsQueuedForDeletion.mkString(",")}")
 
    topicsQueuedForDeletion.foreach { topic =>
      // if all replicas are marked as deleted successfully, then topic deletion is done
 
      if (controllerContext.areAllReplicasInState(topic, ReplicaDeletionSuccessful)) {
        // 如果该主题所有副本已经是ReplicaDeletionSuccessful状态
        // 即该主题已经被删除
        // clear up all state for this topic from controller cache and zookeeper
        // 调用completeDeleteTopic方法完成后续操作即可
        completeDeleteTopic(topic)
        info(s"Deletion of topic $topic successfully completed")
      } else if (!controllerContext.isAnyReplicaInState(topic, ReplicaDeletionStarted)) {
        // 如果主题删除尚未开始并且主题当前无法执行删除的话
        // if you come here, then no replica is in TopicDeletionStarted and all replicas are not in
        // TopicDeletionSuccessful. That means, that either given topic haven't initiated deletion
        // or there is at least one failed replica (which means topic deletion should be retried).
        if (controllerContext.isAnyReplicaInState(topic, ReplicaDeletionIneligible)) {
          // 把该主题加到待重试主题列表中用于后续重试
          retryDeletionForIneligibleReplicas(topic)
        }
      }
 
      // Try delete topic if it is eligible for deletion.
      if (isTopicEligibleForDeletion(topic)) {
        // 如果该主题能够被删除
        info(s"Deletion of topic $topic (re)started")
        // topic deletion will be kicked off
        onTopicDeletion(Set(topic))
      }
    }
  }

14. 如果该主题所有副本已经是被删除状态，调用completeDeleteTopic方法完成后续操作，清除元数据和zk状态；反之加到重试列表后续进行重试

  private def completeDeleteTopic(topic: String): Unit = {
    // deregister partition change listener on the deleted topic. This is to prevent the partition change listener
    // firing before the new topic listener when a deleted topic gets auto created
    // 第1步：注销分区变更监听器，防止删除过程中因分区数据变更
    // 导致监听器被触发，引起状态不一致
    client.mutePartitionModifications(topic)
    // 第2步：获取该主题下处于ReplicaDeletionSuccessful状态的所有副本对象，
    // 即所有已经被成功删除的副本对象
    val replicasForDeletedTopic = controllerContext.replicasInState(topic, ReplicaDeletionSuccessful)
    // controller will remove this replica from the state machine as well as its partition assignment cache
    // 第3步：利用副本状态机将这些副本对象转换成NonExistentReplica状态。
    // 等同于在状态机中删除这些副本
    replicaStateMachine.handleStateChanges(replicasForDeletedTopic.toSeq, NonExistentReplica)
    // 第4步：更新元数据缓存中的待删除主题列表和已开始删除的主题列表
    // 因为主题已经成功删除了，没有必要出现在这两个列表中了
    controllerContext.topicsToBeDeleted -= topic
    controllerContext.topicsWithDeletionStarted -= topic
    // 第5步：移除ZooKeeper上关于该主题的一切“痕迹”
    client.deleteTopic(topic, controllerContext.epochZkVersion)
    // 第6步：移除元数据缓存中关于该主题的一切“痕迹”
    controllerContext.removeTopic(topic)
  }

15. 找出给定待删除主题列表中那些尚未开启删除操作的所有主题， 开启主题删除操作；进行分区删除操作，将物理磁盘文件删除

  private def onTopicDeletion(topics: Set[String]): Unit = {
    info(s"Topic deletion callback for ${topics.mkString(",")}")
    // send update metadata so that brokers stop serving data for topics to be deleted
    val partitions = topics.flatMap(controllerContext.partitionsForTopic)
    // 找出给定待删除主题列表中那些尚未开启删除操作的所有主题
    val unseenTopicsForDeletion = topics -- controllerContext.topicsWithDeletionStarted
    if (unseenTopicsForDeletion.nonEmpty) {
      // 获取到这些主题的所有分区对象
      val unseenPartitionsForDeletion = unseenTopicsForDeletion.flatMap(controllerContext.partitionsForTopic)
      // 将这些分区的状态依次调整成OfflinePartition和NonExistentPartition
      // 等同于将这些分区从分区状态机中删除
      partitionStateMachine.handleStateChanges(unseenPartitionsForDeletion.toSeq, OfflinePartition)
      partitionStateMachine.handleStateChanges(unseenPartitionsForDeletion.toSeq, NonExistentPartition)
      // adding of unseenTopicsForDeletion to topics with deletion started must be done after the partition
      // state changes to make sure the offlinePartitionCount metric is properly updated
      // 把这些主题加到“已开启删除操作”主题列表中
      controllerContext.beginTopicDeletion(unseenTopicsForDeletion)
    }
    // 给集群所有Broker发送元数据更新请求，告诉它们不要再为这些主题处理数据了
    client.sendMetadataUpdate(partitions)
    topics.foreach { topic =>
      // 分区删除操作会执行底层的物理磁盘文件删除动作
      onPartitionDeletion(controllerContext.partitionsForTopic(topic))
    }
  }