Spark1.6的RPC

Spark RPC的核心是RpcEnv、RpcEndpoint、RpcEndpointRef，底层可以选择是基于Akka或Netty，可以扩展。

/**
 * An RPC environment. [[RpcEndpoint]]s need to register itself with a name to [[RpcEnv]] to
 * receives messages. Then [[RpcEnv]] will process messages sent from [[RpcEndpointRef]] or remote
 * nodes, and deliver them to corresponding [[RpcEndpoint]]s. For uncaught exceptions caught by
 * [[RpcEnv]], [[RpcEnv]] will use [[RpcCallContext.sendFailure]] to send exceptions back to the
 * sender, or logging them if no such sender or `NotSerializableException`.
 *
 * [[RpcEnv]] also provides some methods to retrieve [[RpcEndpointRef]]s given name or uri.
 */
private[spark] abstract class RpcEnv(conf: SparkConf) {

RpcEnv类似于Akka中的ActorSystem，所有的RpcEndpoint都需要把自身注册到RpcEnv实例对象中（注册时会指定注册的名称，这样就可以通过名称来查找RpcEndpoint的RpcEndpointRef引用，然后进行通信）。

/**
 * An end point for the RPC that defines what functions to trigger given a message.
 *
 * It is guaranteed that `onStart`, `receive` and `onStop` will be called in sequence.
 *
 * The life-cycle of an endpoint is:
 *
 * constructor -> onStart -> receive* -> onStop
 *
 * Note: `receive` can be called concurrently. If you want `receive` to be thread-safe, please use
 * [[ThreadSafeRpcEndpoint]]
 *
 * If any error is thrown from one of [[RpcEndpoint]] methods except `onError`, `onError` will be
 * invoked with the cause. If `onError` throws an error, [[RpcEnv]] will ignore it.
 */
private[spark] trait RpcEndpoint {

  /**
   * The [[RpcEnv]] that this [[RpcEndpoint]] is registered to.
   */
  val rpcEnv: RpcEnv

  /**
   * The [[RpcEndpointRef]] of this [[RpcEndpoint]]. `self` will become valid when `onStart` is
   * called. And `self` will become `null` when `onStop` is called.
   *
   * Note: Because before `onStart`, [[RpcEndpoint]] has not yet been registered and there is not
   * valid [[RpcEndpointRef]] for it. So don't call `self` before `onStart` is called.
   */
  final def self: RpcEndpointRef = {
    require(rpcEnv != null, "rpcEnv has not been initialized")
    rpcEnv.endpointRef(this)
  }

  /**
   * Process messages from [[RpcEndpointRef.send]] or [[RpcCallContext.reply)]]. If receiving a
   * unmatched message, [[SparkException]] will be thrown and sent to `onError`.
   */
  def receive: PartialFunction[Any, Unit] = {
    case _ => throw new SparkException(self + " does not implement 'receive'")
  }

  /**
   * Process messages from [[RpcEndpointRef.ask]]. If receiving a unmatched message,
   * [[SparkException]] will be thrown and sent to `onError`.
   */
  def receiveAndReply(context: RpcCallContext): PartialFunction[Any, Unit] = {
    case _ => context.sendFailure(new SparkException(self + " won't reply anything"))
  }

  /**
   * Invoked when any exception is thrown during handling messages.
   */
  def onError(cause: Throwable): Unit = {
    // By default, throw e and let RpcEnv handle it
    throw cause
  }

  /**
   * Invoked when `remoteAddress` is connected to the current node.
   */
  def onConnected(remoteAddress: RpcAddress): Unit = {
    // By default, do nothing.
  }

  /**
   * Invoked when `remoteAddress` is lost.
   */
  def onDisconnected(remoteAddress: RpcAddress): Unit = {
    // By default, do nothing.
  }

  /**
   * Invoked when some network error happens in the connection between the current node and
   * `remoteAddress`.
   */
  def onNetworkError(cause: Throwable, remoteAddress: RpcAddress): Unit = {
    // By default, do nothing.
  }

  /**
   * Invoked before [[RpcEndpoint]] starts to handle any message.
   */
  def onStart(): Unit = {
    // By default, do nothing.
  }

  /**
   * Invoked when [[RpcEndpoint]] is stopping. `self` will be `null` in this method and you cannot
   * use it to send or ask messages.
   */
  def onStop(): Unit = {
    // By default, do nothing.
  }

  /**
   * A convenient method to stop [[RpcEndpoint]].
   */
  final def stop(): Unit = {
    val _self = self
    if (_self != null) {
      rpcEnv.stop(_self)
    }
  }
}

RpcEndpoint的注释已经很明白的说明了其作用，RpcEndpoint的生命周期是 constructor -> onStart -> receive* -> onStop。需要注意receive与receiveAndReply的区别。receiveAndReply中会通过RpcCallContext中的reply方法来回复消息给发送者。

/**
 * A reference for a remote [[RpcEndpoint]]. [[RpcEndpointRef]] is thread-safe.
 */
private[spark] abstract class RpcEndpointRef(conf: SparkConf)
  extends Serializable with Logging {

  private[this] val maxRetries = RpcUtils.numRetries(conf)
  private[this] val retryWaitMs = RpcUtils.retryWaitMs(conf)
  private[this] val defaultAskTimeout = RpcUtils.askRpcTimeout(conf)

  /**
   * return the address for the [[RpcEndpointRef]]
   */
  def address: RpcAddress

  def name: String

  
  def send(message: Any): Unit

  def ask[T: ClassTag](message: Any, timeout: RpcTimeout): Future[T]

  def ask[T: ClassTag](message: Any): Future[T] = ask(message, defaultAskTimeout)

  def askWithRetry[T: ClassTag](message: Any): T = askWithRetry(message, defaultAskTimeout)

  def askWithRetry[T: ClassTag](message: Any, timeout: RpcTimeout): T = {
    // TODO: Consider removing multiple attempts
    var attempts = 0
    var lastException: Exception = null
    while (attempts < maxRetries) {
      attempts += 1
      try {
        val future = ask[T](message, timeout)
        val result = timeout.awaitResult(future)
        if (result == null) {
          throw new SparkException("RpcEndpoint returned null")
        }
        return result
      } catch {
        case ie: InterruptedException => throw ie
        case e: Exception =>
          lastException = e
          logWarning(s"Error sending message [message = $message] in $attempts attempts", e)
      }

      if (attempts < maxRetries) {
        Thread.sleep(retryWaitMs)
      }
    }

    throw new SparkException(
      s"Error sending message [message = $message]", lastException)
  }
}

注释说明RpcEndpointRef是远端RpcEndpoint的一个引用，当想给远端RpcEndpoint发送消息时，可以通过该RpcEndpoint之前注册在RpcEnv的RpcEndpointRef给远端RpcEndpoint发送消息.RpcEndpointRef通过address方法获知RpcAddress,包括hostname和port信息。