path: root/core/src/main/scala/spark/SparkContext.scala

package spark

import java.io._
import java.net.URI
import java.util.Properties
import java.util.concurrent.ConcurrentHashMap
import java.util.concurrent.atomic.AtomicInteger

import scala.collection.JavaConversions._
import scala.collection.Map
import scala.collection.generic.Growable
import scala.collection.mutable.HashMap
import scala.collection.JavaConversions._
import scala.util.DynamicVariable
import scala.collection.mutable.{ConcurrentMap, HashMap}

import akka.actor.Actor._

import org.apache.hadoop.conf.Configuration
import org.apache.hadoop.fs.Path
import org.apache.hadoop.io.ArrayWritable
import org.apache.hadoop.io.BooleanWritable
import org.apache.hadoop.io.BytesWritable
import org.apache.hadoop.io.DoubleWritable
import org.apache.hadoop.io.FloatWritable
import org.apache.hadoop.io.IntWritable
import org.apache.hadoop.io.LongWritable
import org.apache.hadoop.io.NullWritable
import org.apache.hadoop.io.Text
import org.apache.hadoop.io.Writable
import org.apache.hadoop.mapred.FileInputFormat
import org.apache.hadoop.mapred.InputFormat
import org.apache.hadoop.mapred.JobConf
import org.apache.hadoop.mapred.SequenceFileInputFormat
import org.apache.hadoop.mapred.TextInputFormat
import org.apache.hadoop.mapreduce.{InputFormat => NewInputFormat}
import org.apache.hadoop.mapreduce.{Job => NewHadoopJob}
import org.apache.hadoop.mapreduce.lib.input.{FileInputFormat => NewFileInputFormat}
import org.apache.hadoop.security.UserGroupInformation

import org.apache.mesos.MesosNativeLibrary

import spark.deploy.{LocalSparkCluster, SparkHadoopUtil}
import spark.partial.{ApproximateEvaluator, PartialResult}
import spark.rdd.{CheckpointRDD, HadoopRDD, NewHadoopRDD, UnionRDD, ParallelCollectionRDD}
import spark.scheduler.{DAGScheduler, ResultTask, ShuffleMapTask, SparkListener, SplitInfo, Stage, StageInfo, TaskScheduler}
import spark.scheduler.cluster.{StandaloneSchedulerBackend, SparkDeploySchedulerBackend, ClusterScheduler}
import spark.scheduler.local.LocalScheduler
import spark.scheduler.mesos.{CoarseMesosSchedulerBackend, MesosSchedulerBackend}
import spark.storage.{StorageStatus, StorageUtils, RDDInfo}
import spark.util.{MetadataCleaner, TimeStampedHashMap}
import ui.{SparkUI}

/**
 * Main entry point for Spark functionality. A SparkContext represents the connection to a Spark
 * cluster, and can be used to create RDDs, accumulators and broadcast variables on that cluster.
 *
 * @param master Cluster URL to connect to (e.g. mesos://host:port, spark://host:port, local[4]).
 * @param appName A name for your application, to display on the cluster web UI.
 * @param sparkHome Location where Spark is installed on cluster nodes.
 * @param jars Collection of JARs to send to the cluster. These can be paths on the local file
 *             system or HDFS, HTTP, HTTPS, or FTP URLs.
 * @param environment Environment variables to set on worker nodes.
 */
class SparkContext(
    val master: String,
    val appName: String,
    val sparkHome: String = null,
    val jars: Seq[String] = Nil,
    val environment: Map[String, String] = Map(),
    // This is used only by yarn for now, but should be relevant to other cluster types (mesos, etc) too.
    // This is typically generated from InputFormatInfo.computePreferredLocations .. host, set of data-local splits on host
    val preferredNodeLocationData: scala.collection.Map[String, scala.collection.Set[SplitInfo]] = scala.collection.immutable.Map())
  extends Logging {

  // Ensure logging is initialized before we spawn any threads
  initLogging()

  // Set Spark driver host and port system properties
  if (System.getProperty("spark.driver.host") == null) {
    System.setProperty("spark.driver.host", Utils.localHostName())
  }
  if (System.getProperty("spark.driver.port") == null) {
    System.setProperty("spark.driver.port", "0")
  }

  private val isLocal = (master == "local" || master.startsWith("local["))

  // Create the Spark execution environment (cache, map output tracker, etc)
  private[spark] val env = SparkEnv.createFromSystemProperties(
    "<driver>",
    System.getProperty("spark.driver.host"),
    System.getProperty("spark.driver.port").toInt,
    true,
    isLocal)
  SparkEnv.set(env)

  // Used to store a URL for each static file/jar together with the file's local timestamp
  private[spark] val addedFiles = HashMap[String, Long]()
  private[spark] val addedJars = HashMap[String, Long]()

  // Keeps track of all persisted RDDs
  private[spark] val persistentRdds = new TimeStampedHashMap[Int, RDD[_]]
  private[spark] val metadataCleaner = new MetadataCleaner("SparkContext", this.cleanup)

  // Initalize the Spark UI
  private[spark] val ui = new SparkUI(this)
  ui.bind()

  // Add each JAR given through the constructor
  if (jars != null) {
    jars.foreach { addJar(_) }
  }

  // Environment variables to pass to our executors
  private[spark] val executorEnvs = HashMap[String, String]()
  // Note: SPARK_MEM is included for Mesos, but overwritten for standalone mode in ExecutorRunner
  for (key <- Seq("SPARK_CLASSPATH", "SPARK_LIBRARY_PATH", "SPARK_JAVA_OPTS", "SPARK_TESTING")) {
    val value = System.getenv(key)
    if (value != null) {
      executorEnvs(key) = value
    }
  }
  // Since memory can be set with a system property too, use that
  executorEnvs("SPARK_MEM") = SparkContext.executorMemoryRequested + "m"
  if (environment != null) {
    executorEnvs ++= environment
  }

  // Create and start the scheduler
  private var taskScheduler: TaskScheduler = {
    // Regular expression used for local[N] master format
    val LOCAL_N_REGEX = """local\[([0-9]+)\]""".r
    // Regular expression for local[N, maxRetries], used in tests with failing tasks
    val LOCAL_N_FAILURES_REGEX = """local\[([0-9]+)\s*,\s*([0-9]+)\]""".r
    // Regular expression for simulating a Spark cluster of [N, cores, memory] locally
    val LOCAL_CLUSTER_REGEX = """local-cluster\[\s*([0-9]+)\s*,\s*([0-9]+)\s*,\s*([0-9]+)\s*]""".r
    // Regular expression for connecting to Spark deploy clusters
    val SPARK_REGEX = """(spark://.*)""".r
    //Regular expression for connection to Mesos cluster
    val MESOS_REGEX = """(mesos://.*)""".r

    master match {
      case "local" =>
        new LocalScheduler(1, 0, this)

      case LOCAL_N_REGEX(threads) =>
        new LocalScheduler(threads.toInt, 0, this)

      case LOCAL_N_FAILURES_REGEX(threads, maxFailures) =>
        new LocalScheduler(threads.toInt, maxFailures.toInt, this)

      case SPARK_REGEX(sparkUrl) =>
        val scheduler = new ClusterScheduler(this)
        val backend = new SparkDeploySchedulerBackend(scheduler, this, sparkUrl, appName)
        scheduler.initialize(backend)
        scheduler

      case LOCAL_CLUSTER_REGEX(numSlaves, coresPerSlave, memoryPerSlave) =>
        // Check to make sure memory requested <= memoryPerSlave. Otherwise Spark will just hang.
        val memoryPerSlaveInt = memoryPerSlave.toInt
        if (SparkContext.executorMemoryRequested > memoryPerSlaveInt) {
          throw new SparkException(
            "Asked to launch cluster with %d MB RAM / worker but requested %d MB/worker".format(
              memoryPerSlaveInt, SparkContext.executorMemoryRequested))
        }

        val scheduler = new ClusterScheduler(this)
        val localCluster = new LocalSparkCluster(
          numSlaves.toInt, coresPerSlave.toInt, memoryPerSlaveInt)
        val sparkUrl = localCluster.start()
        val backend = new SparkDeploySchedulerBackend(scheduler, this, sparkUrl, appName)
        scheduler.initialize(backend)
        backend.shutdownCallback = (backend: SparkDeploySchedulerBackend) => {
          localCluster.stop()
        }
        scheduler

      case "yarn-standalone" =>
        val scheduler = try {
          val clazz = Class.forName("spark.scheduler.cluster.YarnClusterScheduler")
          val cons = clazz.getConstructor(classOf[SparkContext])
          cons.newInstance(this).asInstanceOf[ClusterScheduler]
        } catch {
          // TODO: Enumerate the exact reasons why it can fail
          // But irrespective of it, it means we cannot proceed !
          case th: Throwable => {
            throw new SparkException("YARN mode not available ?", th)
          }
        }
        val backend = new StandaloneSchedulerBackend(scheduler, this.env.actorSystem)
        scheduler.initialize(backend)
        scheduler

      case _ =>
        if (MESOS_REGEX.findFirstIn(master).isEmpty) {
          logWarning("Master %s does not match expected format, parsing as Mesos URL".format(master))
        }
        MesosNativeLibrary.load()
        val scheduler = new ClusterScheduler(this)
        val coarseGrained = System.getProperty("spark.mesos.coarse", "false").toBoolean
        val masterWithoutProtocol = master.replaceFirst("^mesos://", "")  // Strip initial mesos://
        val backend = if (coarseGrained) {
          new CoarseMesosSchedulerBackend(scheduler, this, masterWithoutProtocol, appName)
        } else {
          new MesosSchedulerBackend(scheduler, this, masterWithoutProtocol, appName)
        }
        scheduler.initialize(backend)
        scheduler
    }
  }
  taskScheduler.start()

  @volatile private var dagScheduler = new DAGScheduler(taskScheduler)
  dagScheduler.start()

  ui.start()

  /** A default Hadoop Configuration for the Hadoop code (e.g. file systems) that we reuse. */
  val hadoopConfiguration = {
    val conf = SparkHadoopUtil.newConfiguration()
    // Explicitly check for S3 environment variables
    if (System.getenv("AWS_ACCESS_KEY_ID") != null && System.getenv("AWS_SECRET_ACCESS_KEY") != null) {
      conf.set("fs.s3.awsAccessKeyId", System.getenv("AWS_ACCESS_KEY_ID"))
      conf.set("fs.s3n.awsAccessKeyId", System.getenv("AWS_ACCESS_KEY_ID"))
      conf.set("fs.s3.awsSecretAccessKey", System.getenv("AWS_SECRET_ACCESS_KEY"))
      conf.set("fs.s3n.awsSecretAccessKey", System.getenv("AWS_SECRET_ACCESS_KEY"))
    }
    // Copy any "spark.hadoop.foo=bar" system properties into conf as "foo=bar"
    for (key <- System.getProperties.toMap[String, String].keys if key.startsWith("spark.hadoop.")) {
      conf.set(key.substring("spark.hadoop.".length), System.getProperty(key))
    }
    val bufferSize = System.getProperty("spark.buffer.size", "65536")
    conf.set("io.file.buffer.size", bufferSize)
    conf
  }

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

  // Thread Local variable that can be used by users to pass information down the stack
  private val localProperties = new DynamicVariable[Properties](null)

  def initLocalProperties() {
      localProperties.value = new Properties()
  }

  def addLocalProperties(key: String, value: String) {
    if(localProperties.value == null) {
      localProperties.value = new Properties()
    }
    localProperties.value.setProperty(key,value)
  }
  // Post init
  taskScheduler.postStartHook()

  // Methods for creating RDDs

  /** Distribute a local Scala collection to form an RDD. */
  def parallelize[T: ClassManifest](seq: Seq[T], numSlices: Int = defaultParallelism): RDD[T] = {
    new ParallelCollectionRDD[T](this, seq, numSlices, Map[Int, Seq[String]]())
  }

  /** Distribute a local Scala collection to form an RDD. */
  def makeRDD[T: ClassManifest](seq: Seq[T], numSlices: Int = defaultParallelism): RDD[T] = {
    parallelize(seq, numSlices)
  }

  /** Distribute a local Scala collection to form an RDD, with one or more
    * location preferences (hostnames of Spark nodes) for each object.
    * Create a new partition for each collection item. */
   def makeRDD[T: ClassManifest](seq: Seq[(T, Seq[String])]): RDD[T] = {
    val indexToPrefs = seq.zipWithIndex.map(t => (t._2, t._1._2)).toMap
    new ParallelCollectionRDD[T](this, seq.map(_._1), seq.size, indexToPrefs)
  }

  /**
   * Read a text file from HDFS, a local file system (available on all nodes), or any
   * Hadoop-supported file system URI, and return it as an RDD of Strings.
   */
  def textFile(path: String, minSplits: Int = defaultMinSplits): RDD[String] = {
    hadoopFile(path, classOf[TextInputFormat], classOf[LongWritable], classOf[Text], minSplits)
      .map(pair => pair._2.toString)
  }

  /**
   * Get an RDD for a Hadoop-readable dataset from a Hadoop JobConf giving its InputFormat and any
   * other necessary info (e.g. file name for a filesystem-based dataset, table name for HyperTable,
   * etc).
   */
  def hadoopRDD[K, V](
      conf: JobConf,
      inputFormatClass: Class[_ <: InputFormat[K, V]],
      keyClass: Class[K],
      valueClass: Class[V],
      minSplits: Int = defaultMinSplits
      ): RDD[(K, V)] = {
    new HadoopRDD(this, conf, inputFormatClass, keyClass, valueClass, minSplits)
  }

  /** Get an RDD for a Hadoop file with an arbitrary InputFormat */
  def hadoopFile[K, V](
      path: String,
      inputFormatClass: Class[_ <: InputFormat[K, V]],
      keyClass: Class[K],
      valueClass: Class[V],
      minSplits: Int = defaultMinSplits
      ) : RDD[(K, V)] = {
    val conf = new JobConf(hadoopConfiguration)
    FileInputFormat.setInputPaths(conf, path)
    new HadoopRDD(this, conf, inputFormatClass, keyClass, valueClass, minSplits)
  }

  /**
   * Smarter version of hadoopFile() that uses class manifests to figure out the classes of keys,
   * values and the InputFormat so that users don't need to pass them directly. Instead, callers
   * can just write, for example,
   * {{{
   * val file = sparkContext.hadoopFile[LongWritable, Text, TextInputFormat](path, minSplits)
   * }}}
   */
  def hadoopFile[K, V, F <: InputFormat[K, V]](path: String, minSplits: Int)
      (implicit km: ClassManifest[K], vm: ClassManifest[V], fm: ClassManifest[F])
      : RDD[(K, V)] = {
    hadoopFile(path,
        fm.erasure.asInstanceOf[Class[F]],
        km.erasure.asInstanceOf[Class[K]],
        vm.erasure.asInstanceOf[Class[V]],
        minSplits)
  }

  /**
   * Smarter version of hadoopFile() that uses class manifests to figure out the classes of keys,
   * values and the InputFormat so that users don't need to pass them directly. Instead, callers
   * can just write, for example,
   * {{{
   * val file = sparkContext.hadoopFile[LongWritable, Text, TextInputFormat](path)
   * }}}
   */
  def hadoopFile[K, V, F <: InputFormat[K, V]](path: String)
      (implicit km: ClassManifest[K], vm: ClassManifest[V], fm: ClassManifest[F]): RDD[(K, V)] =
    hadoopFile[K, V, F](path, defaultMinSplits)

  /** Get an RDD for a Hadoop file with an arbitrary new API InputFormat. */
  def newAPIHadoopFile[K, V, F <: NewInputFormat[K, V]](path: String)
      (implicit km: ClassManifest[K], vm: ClassManifest[V], fm: ClassManifest[F]): RDD[(K, V)] = {
    newAPIHadoopFile(
        path,
        fm.erasure.asInstanceOf[Class[F]],
        km.erasure.asInstanceOf[Class[K]],
        vm.erasure.asInstanceOf[Class[V]])
  }

  /**
   * Get an RDD for a given Hadoop file with an arbitrary new API InputFormat
   * and extra configuration options to pass to the input format.
   */
  def newAPIHadoopFile[K, V, F <: NewInputFormat[K, V]](
      path: String,
      fClass: Class[F],
      kClass: Class[K],
      vClass: Class[V],
      conf: Configuration = hadoopConfiguration): RDD[(K, V)] = {
    val job = new NewHadoopJob(conf)
    NewFileInputFormat.addInputPath(job, new Path(path))
    val updatedConf = job.getConfiguration
    new NewHadoopRDD(this, fClass, kClass, vClass, updatedConf)
  }

  /**
   * Get an RDD for a given Hadoop file with an arbitrary new API InputFormat
   * and extra configuration options to pass to the input format.
   */
  def newAPIHadoopRDD[K, V, F <: NewInputFormat[K, V]](
      conf: Configuration = hadoopConfiguration,
      fClass: Class[F],
      kClass: Class[K],
      vClass: Class[V]): RDD[(K, V)] = {
    new NewHadoopRDD(this, fClass, kClass, vClass, conf)
  }

  /** Get an RDD for a Hadoop SequenceFile with given key and value types. */
  def sequenceFile[K, V](path: String,
      keyClass: Class[K],
      valueClass: Class[V],
      minSplits: Int
      ): RDD[(K, V)] = {
    val inputFormatClass = classOf[SequenceFileInputFormat[K, V]]
    hadoopFile(path, inputFormatClass, keyClass, valueClass, minSplits)
  }

  /** Get an RDD for a Hadoop SequenceFile with given key and value types. */
  def sequenceFile[K, V](path: String, keyClass: Class[K], valueClass: Class[V]): RDD[(K, V)] =
    sequenceFile(path, keyClass, valueClass, defaultMinSplits)

  /**
   * Version of sequenceFile() for types implicitly convertible to Writables through a
   * WritableConverter. For example, to access a SequenceFile where the keys are Text and the
   * values are IntWritable, you could simply write
   * {{{
   * sparkContext.sequenceFile[String, Int](path, ...)
   * }}}
   *
   * WritableConverters are provided in a somewhat strange way (by an implicit function) to support
   * both subclasses of Writable and types for which we define a converter (e.g. Int to
   * IntWritable). The most natural thing would've been to have implicit objects for the
   * converters, but then we couldn't have an object for every subclass of Writable (you can't
   * have a parameterized singleton object). We use functions instead to create a new converter
   * for the appropriate type. In addition, we pass the converter a ClassManifest of its type to
   * allow it to figure out the Writable class to use in the subclass case.
   */
   def sequenceFile[K, V](path: String, minSplits: Int = defaultMinSplits)
      (implicit km: ClassManifest[K], vm: ClassManifest[V],
          kcf: () => WritableConverter[K], vcf: () => WritableConverter[V])
      : RDD[(K, V)] = {
    val kc = kcf()
    val vc = vcf()
    val format = classOf[SequenceFileInputFormat[Writable, Writable]]
    val writables = hadoopFile(path, format,
        kc.writableClass(km).asInstanceOf[Class[Writable]],
        vc.writableClass(vm).asInstanceOf[Class[Writable]], minSplits)
    writables.map{case (k,v) => (kc.convert(k), vc.convert(v))}
  }

  /**
   * Load an RDD saved as a SequenceFile containing serialized objects, with NullWritable keys and
   * BytesWritable values that contain a serialized partition. This is still an experimental storage
   * format and may not be supported exactly as is in future Spark releases. It will also be pretty
   * slow if you use the default serializer (Java serialization), though the nice thing about it is
   * that there's very little effort required to save arbitrary objects.
   */
  def objectFile[T: ClassManifest](
      path: String,
      minSplits: Int = defaultMinSplits
      ): RDD[T] = {
    sequenceFile(path, classOf[NullWritable], classOf[BytesWritable], minSplits)
      .flatMap(x => Utils.deserialize[Array[T]](x._2.getBytes))
  }


  protected[spark] def checkpointFile[T: ClassManifest](
      path: String
    ): RDD[T] = {
    new CheckpointRDD[T](this, path)
  }

  /** Build the union of a list of RDDs. */
  def union[T: ClassManifest](rdds: Seq[RDD[T]]): RDD[T] = new UnionRDD(this, rdds)

  /** Build the union of a list of RDDs passed as variable-length arguments. */
  def union[T: ClassManifest](first: RDD[T], rest: RDD[T]*): RDD[T] =
    new UnionRDD(this, Seq(first) ++ rest)

  // Methods for creating shared variables

  /**
   * Create an [[spark.Accumulator]] variable of a given type, which tasks can "add" values
   * to using the `+=` method. Only the driver can access the accumulator's `value`.
   */
  def accumulator[T](initialValue: T)(implicit param: AccumulatorParam[T]) =
    new Accumulator(initialValue, param)

  /**
   * Create an [[spark.Accumulable]] shared variable, to which tasks can add values with `+=`.
   * Only the driver can access the accumuable's `value`.
   * @tparam T accumulator type
   * @tparam R type that can be added to the accumulator
   */
  def accumulable[T, R](initialValue: T)(implicit param: AccumulableParam[T, R]) =
    new Accumulable(initialValue, param)

  /**
   * Create an accumulator from a "mutable collection" type.
   *
   * Growable and TraversableOnce are the standard APIs that guarantee += and ++=, implemented by
   * standard mutable collections. So you can use this with mutable Map, Set, etc.
   */
  def accumulableCollection[R <% Growable[T] with TraversableOnce[T] with Serializable, T](initialValue: R) = {
    val param = new GrowableAccumulableParam[R,T]
    new Accumulable(initialValue, param)
  }

  /**
   * Broadcast a read-only variable to the cluster, returning a [[spark.broadcast.Broadcast]] object for
   * reading it in distributed functions. The variable will be sent to each cluster only once.
   */
  def broadcast[T](value: T) = env.broadcastManager.newBroadcast[T](value, isLocal)

  /**
   * Add a file to be downloaded with this Spark job on every node.
   * The `path` passed can be either a local file, a file in HDFS (or other Hadoop-supported
   * filesystems), or an HTTP, HTTPS or FTP URI.  To access the file in Spark jobs,
   * use `SparkFiles.get(path)` to find its download location.
   */
  def addFile(path: String) {
    val uri = new URI(path)
    val key = uri.getScheme match {
      case null | "file" => env.httpFileServer.addFile(new File(uri.getPath))
      case _ => path
    }
    addedFiles(key) = System.currentTimeMillis

    // Fetch the file locally in case a job is executed locally.
    // Jobs that run through LocalScheduler will already fetch the required dependencies,
    // but jobs run in DAGScheduler.runLocally() will not so we must fetch the files here.
    Utils.fetchFile(path, new File(SparkFiles.getRootDirectory))

    logInfo("Added file " + path + " at " + key + " with timestamp " + addedFiles(key))
  }

  def addSparkListener(listener: SparkListener) {
    dagScheduler.sparkListeners += listener
  }

  /**
   * Return a map from the slave to the max memory available for caching and the remaining
   * memory available for caching.
   */
  def getExecutorMemoryStatus: Map[String, (Long, Long)] = {
    env.blockManager.master.getMemoryStatus.map { case(blockManagerId, mem) =>
      (blockManagerId.host + ":" + blockManagerId.port, mem)
    }
  }

  /**
   * Return information about what RDDs are cached, if they are in mem or on disk, how much space
   * they take, etc.
   */
  def getRDDStorageInfo: Array[RDDInfo] = {
    StorageUtils.rddInfoFromStorageStatus(getExecutorStorageStatus, this)
  }

  def getStageInfo: Map[Stage,StageInfo] = {
    dagScheduler.stageToInfos
  }

  /**
   * Return information about blocks stored in all of the slaves
   */
  def getExecutorStorageStatus: Array[StorageStatus] = {
    env.blockManager.master.getStorageStatus
  }

  /**
   * Clear the job's list of files added by `addFile` so that they do not get downloaded to
   * any new nodes.
   */
  def clearFiles() {
    addedFiles.clear()
  }

  /**
   * Adds a JAR dependency for all tasks to be executed on this SparkContext in the future.
   * The `path` passed can be either a local file, a file in HDFS (or other Hadoop-supported
   * filesystems), or an HTTP, HTTPS or FTP URI.
   */
  def addJar(path: String) {
    if (null == path) {
      logWarning("null specified as parameter to addJar",
        new SparkException("null specified as parameter to addJar"))
    } else {
      val uri = new URI(path)
      val key = uri.getScheme match {
        case null | "file" => env.httpFileServer.addJar(new File(uri.getPath))
        case _ => path
      }
      addedJars(key) = System.currentTimeMillis
      logInfo("Added JAR " + path + " at " + key + " with timestamp " + addedJars(key))
    }
  }

  /**
   * Clear the job's list of JARs added by `addJar` so that they do not get downloaded to
   * any new nodes.
   */
  def clearJars() {
    addedJars.clear()
  }

  /** Shut down the SparkContext. */
  def stop() {
    ui.stop()
    // Do this only if not stopped already - best case effort.
    // prevent NPE if stopped more than once.
    val dagSchedulerCopy = dagScheduler
    dagScheduler = null
    if (dagSchedulerCopy != null) {
      metadataCleaner.cancel()
      dagSchedulerCopy.stop()
      taskScheduler = null
      // TODO: Cache.stop()?
      env.stop()
      // Clean up locally linked files
      clearFiles()
      clearJars()
      SparkEnv.set(null)
      ShuffleMapTask.clearCache()
      ResultTask.clearCache()
      logInfo("Successfully stopped SparkContext")
    } else {
      logInfo("SparkContext already stopped")
    }
  }


  /**
   * Get Spark's home location from either a value set through the constructor,
   * or the spark.home Java property, or the SPARK_HOME environment variable
   * (in that order of preference). If neither of these is set, return None.
   */
  private[spark] def getSparkHome(): Option[String] = {
    if (sparkHome != null) {
      Some(sparkHome)
    } else if (System.getProperty("spark.home") != null) {
      Some(System.getProperty("spark.home"))
    } else if (System.getenv("SPARK_HOME") != null) {
      Some(System.getenv("SPARK_HOME"))
    } else {
      None
    }
  }

  /**
   * Run a function on a given set of partitions in an RDD and pass the results to the given
   * handler function. This is the main entry point for all actions in Spark. The allowLocal
   * flag specifies whether the scheduler can run the computation on the driver rather than
   * shipping it out to the cluster, for short actions like first().
   */
  def runJob[T, U: ClassManifest](
      rdd: RDD[T],
      func: (TaskContext, Iterator[T]) => U,
      partitions: Seq[Int],
      allowLocal: Boolean,
      resultHandler: (Int, U) => Unit) {
    val callSite = Utils.formatSparkCallSite
    logInfo("Starting job: " + callSite)
    val start = System.nanoTime
    val result = dagScheduler.runJob(rdd, func, partitions, callSite, allowLocal, resultHandler, localProperties.value)
    logInfo("Job finished: " + callSite + ", took " + (System.nanoTime - start) / 1e9 + " s")
    rdd.doCheckpoint()
    result
  }

  /**
   * Run a function on a given set of partitions in an RDD and return the results as an array. The
   * allowLocal flag specifies whether the scheduler can run the computation on the driver rather
   * than shipping it out to the cluster, for short actions like first().
   */
  def runJob[T, U: ClassManifest](
      rdd: RDD[T],
      func: (TaskContext, Iterator[T]) => U,
      partitions: Seq[Int],
      allowLocal: Boolean
      ): Array[U] = {
    val results = new Array[U](partitions.size)
    runJob[T, U](rdd, func, partitions, allowLocal, (index, res) => results(index) = res)
    results
  }

  /**
   * Run a job on a given set of partitions of an RDD, but take a function of type
   * `Iterator[T] => U` instead of `(TaskContext, Iterator[T]) => U`.
   */
  def runJob[T, U: ClassManifest](
      rdd: RDD[T],
      func: Iterator[T] => U,
      partitions: Seq[Int],
      allowLocal: Boolean
      ): Array[U] = {
    runJob(rdd, (context: TaskContext, iter: Iterator[T]) => func(iter), partitions, allowLocal)
  }

  /**
   * Run a job on all partitions in an RDD and return the results in an array.
   */
  def runJob[T, U: ClassManifest](rdd: RDD[T], func: (TaskContext, Iterator[T]) => U): Array[U] = {
    runJob(rdd, func, 0 until rdd.partitions.size, false)
  }

  /**
   * Run a job on all partitions in an RDD and return the results in an array.
   */
  def runJob[T, U: ClassManifest](rdd: RDD[T], func: Iterator[T] => U): Array[U] = {
    runJob(rdd, func, 0 until rdd.partitions.size, false)
  }

  /**
   * Run a job on all partitions in an RDD and pass the results to a handler function.
   */
  def runJob[T, U: ClassManifest](
    rdd: RDD[T],
    processPartition: (TaskContext, Iterator[T]) => U,
    resultHandler: (Int, U) => Unit)
  {
    runJob[T, U](rdd, processPartition, 0 until rdd.partitions.size, false, resultHandler)
  }

  /**
   * Run a job on all partitions in an RDD and pass the results to a handler function.
   */
  def runJob[T, U: ClassManifest](
      rdd: RDD[T],
      processPartition: Iterator[T] => U,
      resultHandler: (Int, U) => Unit)
  {
    val processFunc = (context: TaskContext, iter: Iterator[T]) => processPartition(iter)
    runJob[T, U](rdd, processFunc, 0 until rdd.partitions.size, false, resultHandler)
  }

  /**
   * Run a job that can return approximate results.
   */
  def runApproximateJob[T, U, R](
      rdd: RDD[T],
      func: (TaskContext, Iterator[T]) => U,
      evaluator: ApproximateEvaluator[U, R],
      timeout: Long): PartialResult[R] = {
    val callSite = Utils.formatSparkCallSite
    logInfo("Starting job: " + callSite)
    val start = System.nanoTime
    val result = dagScheduler.runApproximateJob(rdd, func, evaluator, callSite, timeout, localProperties.value)
    logInfo("Job finished: " + callSite + ", took " + (System.nanoTime - start) / 1e9 + " s")
    result
  }

  /**
   * Clean a closure to make it ready to serialized and send to tasks
   * (removes unreferenced variables in $outer's, updates REPL variables)
   */
  private[spark] def clean[F <: AnyRef](f: F): F = {
    ClosureCleaner.clean(f)
    return f
  }

  /**
   * Set the directory under which RDDs are going to be checkpointed. The directory must
   * be a HDFS path if running on a cluster. If the directory does not exist, it will
   * be created. If the directory exists and useExisting is set to true, then the
   * exisiting directory will be used. Otherwise an exception will be thrown to
   * prevent accidental overriding of checkpoint files in the existing directory.
   */
  def setCheckpointDir(dir: String, useExisting: Boolean = false) {
    val path = new Path(dir)
    val fs = path.getFileSystem(SparkHadoopUtil.newConfiguration())
    if (!useExisting) {
      if (fs.exists(path)) {
        throw new Exception("Checkpoint directory '" + path + "' already exists.")
      } else {
        fs.mkdirs(path)
      }
    }
    checkpointDir = Some(dir)
  }

  /** Default level of parallelism to use when not given by user (e.g. parallelize and makeRDD). */
  def defaultParallelism: Int = taskScheduler.defaultParallelism

  /** Default min number of partitions for Hadoop RDDs when not given by user */
  def defaultMinSplits: Int = math.min(defaultParallelism, 2)

  private var nextShuffleId = new AtomicInteger(0)

  private[spark] def newShuffleId(): Int = nextShuffleId.getAndIncrement()

  private var nextRddId = new AtomicInteger(0)

  /** Register a new RDD, returning its RDD ID */
  private[spark] def newRddId(): Int = nextRddId.getAndIncrement()

  /** Called by MetadataCleaner to clean up the persistentRdds map periodically */
  private[spark] def cleanup(cleanupTime: Long) {
    persistentRdds.clearOldValues(cleanupTime)
  }
}

/**
 * The SparkContext object contains a number of implicit conversions and parameters for use with
 * various Spark features.
 */
object SparkContext {

  implicit object DoubleAccumulatorParam extends AccumulatorParam[Double] {
    def addInPlace(t1: Double, t2: Double): Double = t1 + t2
    def zero(initialValue: Double) = 0.0
  }

  implicit object IntAccumulatorParam extends AccumulatorParam[Int] {
    def addInPlace(t1: Int, t2: Int): Int = t1 + t2
    def zero(initialValue: Int) = 0
  }

  implicit object LongAccumulatorParam extends AccumulatorParam[Long] {
    def addInPlace(t1: Long, t2: Long) = t1 + t2
    def zero(initialValue: Long) = 0l
  }

  implicit object FloatAccumulatorParam extends AccumulatorParam[Float] {
    def addInPlace(t1: Float, t2: Float) = t1 + t2
    def zero(initialValue: Float) = 0f
  }

  // TODO: Add AccumulatorParams for other types, e.g. lists and strings

  implicit def rddToPairRDDFunctions[K: ClassManifest, V: ClassManifest](rdd: RDD[(K, V)]) =
    new PairRDDFunctions(rdd)

  implicit def rddToSequenceFileRDDFunctions[K <% Writable: ClassManifest, V <% Writable: ClassManifest](
      rdd: RDD[(K, V)]) =
    new SequenceFileRDDFunctions(rdd)

  implicit def rddToOrderedRDDFunctions[K <% Ordered[K]: ClassManifest, V: ClassManifest](
      rdd: RDD[(K, V)]) =
    new OrderedRDDFunctions(rdd)

  implicit def doubleRDDToDoubleRDDFunctions(rdd: RDD[Double]) = new DoubleRDDFunctions(rdd)

  implicit def numericRDDToDoubleRDDFunctions[T](rdd: RDD[T])(implicit num: Numeric[T]) =
    new DoubleRDDFunctions(rdd.map(x => num.toDouble(x)))

  // Implicit conversions to common Writable types, for saveAsSequenceFile

  implicit def intToIntWritable(i: Int) = new IntWritable(i)

  implicit def longToLongWritable(l: Long) = new LongWritable(l)

  implicit def floatToFloatWritable(f: Float) = new FloatWritable(f)

  implicit def doubleToDoubleWritable(d: Double) = new DoubleWritable(d)

  implicit def boolToBoolWritable (b: Boolean) = new BooleanWritable(b)

  implicit def bytesToBytesWritable (aob: Array[Byte]) = new BytesWritable(aob)

  implicit def stringToText(s: String) = new Text(s)

  private implicit def arrayToArrayWritable[T <% Writable: ClassManifest](arr: Traversable[T]): ArrayWritable = {
    def anyToWritable[U <% Writable](u: U): Writable = u

    new ArrayWritable(classManifest[T].erasure.asInstanceOf[Class[Writable]],
        arr.map(x => anyToWritable(x)).toArray)
  }

  // Helper objects for converting common types to Writable
  private def simpleWritableConverter[T, W <: Writable: ClassManifest](convert: W => T) = {
    val wClass = classManifest[W].erasure.asInstanceOf[Class[W]]
    new WritableConverter[T](_ => wClass, x => convert(x.asInstanceOf[W]))
  }

  implicit def intWritableConverter() = simpleWritableConverter[Int, IntWritable](_.get)

  implicit def longWritableConverter() = simpleWritableConverter[Long, LongWritable](_.get)

  implicit def doubleWritableConverter() = simpleWritableConverter[Double, DoubleWritable](_.get)

  implicit def floatWritableConverter() = simpleWritableConverter[Float, FloatWritable](_.get)

  implicit def booleanWritableConverter() = simpleWritableConverter[Boolean, BooleanWritable](_.get)

  implicit def bytesWritableConverter() = simpleWritableConverter[Array[Byte], BytesWritable](_.getBytes)

  implicit def stringWritableConverter() = simpleWritableConverter[String, Text](_.toString)

  implicit def writableWritableConverter[T <: Writable]() =
    new WritableConverter[T](_.erasure.asInstanceOf[Class[T]], _.asInstanceOf[T])

  /**
   * Find the JAR from which a given class was loaded, to make it easy for users to pass
   * their JARs to SparkContext
   */
  def jarOfClass(cls: Class[_]): Seq[String] = {
    val uri = cls.getResource("/" + cls.getName.replace('.', '/') + ".class")
    if (uri != null) {
      val uriStr = uri.toString
      if (uriStr.startsWith("jar:file:")) {
        // URI will be of the form "jar:file:/path/foo.jar!/package/cls.class", so pull out the /path/foo.jar
        List(uriStr.substring("jar:file:".length, uriStr.indexOf('!')))
      } else {
        Nil
      }
    } else {
      Nil
    }
  }

  /** Find the JAR that contains the class of a particular object */
  def jarOfObject(obj: AnyRef): Seq[String] = jarOfClass(obj.getClass)

  /** Get the amount of memory per executor requested through system properties or SPARK_MEM */
  private[spark] val executorMemoryRequested = {
    // TODO: Might need to add some extra memory for the non-heap parts of the JVM
    Option(System.getProperty("spark.executor.memory"))
      .orElse(Option(System.getenv("SPARK_MEM")))
      .map(Utils.memoryStringToMb)
      .getOrElse(512)
  }
}


/**
 * A class encapsulating how to convert some type T to Writable. It stores both the Writable class
 * corresponding to T (e.g. IntWritable for Int) and a function for doing the conversion.
 * The getter for the writable class takes a ClassManifest[T] in case this is a generic object
 * that doesn't know the type of T when it is created. This sounds strange but is necessary to
 * support converting subclasses of Writable to themselves (writableWritableConverter).
 */
private[spark] class WritableConverter[T](
    val writableClass: ClassManifest[T] => Class[_ <: Writable],
    val convert: Writable => T)
  extends Serializable