abstract class RDD[T: ClassTag](
  //@transient 注解表示将字段标记为瞬态的
    @transient private var _sc: SparkContext,
  // Seq是序列，元素有插入的先后顺序，可以有重复的元素。
    @transient private var deps: Seq[Dependency[_]]
  ) extends Serializable with Logging {

  
  if (classOf[RDD[_]].isAssignableFrom(elementClassTag.runtimeClass)) {
  user programs that
   
  }

//这里应该是声明sparkContext对象后才能使用RDD的调用
  private def sc: SparkContext = {
    if (_sc == null) {
      throw new SparkException(
        "RDD transformations and actions can only be invoked by the driver, not inside of other " +
        "transformations; for example, rdd1.map(x => rdd2.values.count() * x) is invalid because " +
        "the values transformation and count action cannot be performed inside of the rdd1.map " +
        "transformation. For more information, see SPARK-5063.")
    }
    _sc
  }

//构建一个RDD应该是一对一的关系，比如子RDD对应唯一的父RDD
  def this(@transient oneParent: RDD[_]) =
    this(oneParent.context , List(new OneToOneDependency(oneParent)))



  private[spark] def conf: SparkConf = _conf


//sparkconf的设置
def getConf: SparkConf = conf.clone()

//获取相应的配置信息
def jars: Seq[String] = _jars
def files: Seq[String] = _files
def master: String = _conf.get("spark.master")
def appName: String = _conf.get("spark.app.name")


private[spark] def isEventLogEnabled: Boolean = _conf.getBoolean("spark.eventLog.enabled", false)
  private[spark] def eventLogDir: Option[URI] = _eventLogDir
  private[spark] def eventLogCodec: Option[String] = _eventLogCodec

//临时文件夹的名称为spark+随机时间戳
  val externalBlockStoreFolderName = "spark-" + randomUUID.toString()

//判断是否为local模式
def isLocal: Boolean = (master == "local" || master.startsWith("local["))

 //用于触发事件的监听
 private[spark] val listenerBus = new LiveListenerBus

// 该方法可用于测试用
  private[spark] def createSparkEnv(
      conf: SparkConf,
      isLocal: Boolean,
      listenerBus: LiveListenerBus): SparkEnv = {
    SparkEnv.createDriverEnv(conf, isLocal, listenerBus, SparkContext.numDriverCores(master))
  }

//加载env配置文件
  private[spark] def env: SparkEnv = _env

  private[spark] val addedFiles = HashMap[String, Long]()
  private[spark] val addedJars = HashMap[String, Long]()

//监听所有调用persist的RDD
  private[spark] val persistentRdds = new TimeStampedWeakValueHashMap[Int, RDD[_]]

//重用配置hadoop Configuration
  def hadoopConfiguration: Configuration = _hadoopConfiguration

//用于设置executorMemory的内存数量
  private[spark] def executorMemory: Int = _executorMemory

  // 将环境参数传递给exeuctor
  private[spark] val executorEnvs = HashMap[String, String]()

  // 设置正在使用SparkContext的用户
  val sparkUser = Utils.getCurrentUserName()


//设置提交的appliaction的唯一标识。就是当提交给yarn或local模式时，申请资源的applaction名称
  def applicationId: String = _applicationId
  def applicationAttemptId: Option[String] = _applicationAttemptId

  def metricsSystem: MetricsSystem = if (_env != null) _env.metricsSystem else null

  private[spark] def eventLogger: Option[EventLoggingListener] = _eventLogger

  private[spark] def executorAllocationManager: Option[ExecutorAllocationManager] =
    _executorAllocationManager

  private[spark] def cleaner: Option[ContextCleaner] = _cleaner

  private[spark] var checkpointDir: Option[String] = None


// 用户可以使用本地变量来传递消息
  protected[spark] val localProperties = new InheritableThreadLocal[Properties] {
    override protected def childValue(parent: Properties): Properties = {
//clone一下，防止父变量改变从而影响子变量
semantics (SPARK-10563).
      if (conf.get("spark.localProperties.clone", "false").toBoolean) {
        SerializationUtils.clone(parent).asInstanceOf[Properties]
      } else {
        new Properties(parent)
      }
    }
    override protected def initialValue(): Properties = new Properties()
  }

 private def warnSparkMem(value: String): String = {
    logWarning("Using SPARK_MEM to set amount of memory to use per executor process is " +
      "deprecated, please use spark.executor.memory instead.")
    value
  }


//设置log级别，包括ALL, DEBUG, ERROR, FATAL, INFO, OFF, TRACE, WARN
def setLogLevel(logLevel: String) {
    val validLevels = Seq("ALL", "DEBUG", "ERROR", "FATAL", "INFO", "OFF", "TRACE", "WARN")
    if (!validLevels.contains(logLevel)) {
      throw new IllegalArgumentException(
        s"Supplied level $logLevel did not match one of: ${validLevels.mkString(",")}")
    }
    Utils.setLogLevel(org.apache.log4j.Level.toLevel(logLevel))
  }

//不同模式的配置参数
    if (!_conf.contains("spark.master")) {
      throw new SparkException("A master URL must be set in your configuration")
    }
    if (!_conf.contains("spark.app.name")) {
      throw new SparkException("An application name must be set in your configuration")
    }

    // System property spark.yarn.app.id must be set if user code ran by AM on a YARN cluster
    // yarn-standalone is deprecated, but still supported
    if ((master == "yarn-cluster" || master == "yarn-standalone") &&
        !_conf.contains("spark.yarn.app.id")) {
      throw new SparkException("Detected yarn-cluster mode, but isn't running on a cluster. " +
        "Deployment to YARN is not supported directly by SparkContext. Please use spark-submit.")
    }

    _conf.setIfMissing("spark.driver.host", Utils.localHostName())
    _conf.setIfMissing("spark.driver.port", "0")


 _conf.set("spark.executor.id", SparkContext.DRIVER_IDENTIFIER)

    _jars = _conf.getOption("spark.jars").map(_.split(",")).map(_.filter(_.size != 0)).toSeq.flatten
    _files = _conf.getOption("spark.files").map(_.split(",")).map(_.filter(_.size != 0))
      .toSeq.flatten

    _eventLogDir =
      if (isEventLogEnabled) {
        val unresolvedDir = conf.get("spark.eventLog.dir", EventLoggingListener.DEFAULT_LOG_DIR)
          .stripSuffix("/")
        Some(Utils.resolveURI(unresolvedDir))
      } else {
        None
      }

    _eventLogCodec = {
      val compress = _conf.getBoolean("spark.eventLog.compress", false)
      if (compress && isEventLogEnabled) {
        Some(CompressionCodec.getCodecName(_conf)).map(CompressionCodec.getShortName)
      } else {
        None
      }
    }
//jobProgressListener应该在创建sparkEnv之前，因为当创建sparkEnv时，一些信息将会被发送到jobProgressListener，否则就会丢失啦。
    _jobProgressListener = new JobProgressListener(_conf)
    listenerBus.addListener(jobProgressListener)

_env = createSparkEnv(_conf, isLocal, listenerBus)
    SparkEnv.set(_env)

    _metadataCleaner = new MetadataCleaner(MetadataCleanerType.SPARK_CONTEXT, this.cleanup, _conf)

    _statusTracker = new SparkStatusTracker(this)

    _progressBar =
      if (_conf.getBoolean("spark.ui.showConsoleProgress", true) && !log.isInfoEnabled) {
        Some(new ConsoleProgressBar(this))
      } else {
        None
      }

    _ui =
      if (conf.getBoolean("spark.ui.enabled", true)) {
        Some(SparkUI.createLiveUI(this, _conf, listenerBus, _jobProgressListener,
          _env.securityManager, appName, startTime = startTime))
      } else {
        None
      }

    if (jars != null) {
      jars.foreach(addJar)
    }

    if (files != null) {
      files.foreach(addFile)
    }

//获取启动app设置的参数变量，如果没有则获取配置文件中的
    _executorMemory = _conf.getOption("spark.executor.memory")
      .orElse(Option(System.getenv("SPARK_EXECUTOR_MEMORY")))
      .orElse(Option(System.getenv("SPARK_MEM"))
      .map(warnSparkMem))
      .map(Utils.memoryStringToMb)
      .getOrElse(1024)

//500这里在创建HeartbeatReceiver 之前先创建createTaskScheduler，因为每个Executor在构造函数中检索HeartbeatReceiver
    _heartbeatReceiver = env.rpcEnv.setupEndpoint(
      HeartbeatReceiver.ENDPOINT_NAME, new HeartbeatReceiver(this))