Spark-Dependency

1、Spark中采用依赖关系(Dependency)表示rdd之间的生成关系。Spark可利用Dependency计算出失效的RDD。在每个RDD中都存在一个依赖关系的列表

  private var dependencies_ : Seq[Dependency[_]] = null

用以记录各rdd中各partition的parent partition。

2、Spark中存在两类Dependency：

1）NarrowDependency表示的是一个父partition仅对应于一个子partition。这样的依赖关系是不需要shuffle的。在这类依赖中，可以根据getParents方法获取某个partition的父partitions：

/**
 * :: DeveloperApi ::
 * Base class for dependencies where each partition of the parent RDD is used by at most one
 * partition of the child RDD.  Narrow dependencies allow for pipelined execution.
 */
@DeveloperApi
abstract class NarrowDependency[T](rdd: RDD[T]) extends Dependency(rdd) {
  /**
   * 唯一的接口，获得该partition的所有parent partition
   * Get the parent partitions for a child partition.
   * @param partitionId a partition of the child RDD
   * @return the partitions of the parent RDD that the child partition depends upon
   */
  def getParents(partitionId: Int): Seq[Int]
}

这类又可分为：

a、OneToOneDependency：表示一一对应的依赖关系，由于在这种依赖中父partition与子partition Id是一致的，所以getParents直接原样返回。对应的转换操作有map和filter

class OneToOneDependency[T](rdd: RDD[T]) extends NarrowDependency[T](rdd) {
  /**
   * 其实partitionId就是partition在RDD中的序号, 所以如果是一一对应, 那么parent和child中的partition的序号应该是一样的
   */
  override def getParents(partitionId: Int) = List(partitionId)//原样返回
}

b、PruneDependency(org.apache.spark.rdd.PartitionPruningRDDPartition)：未详

/**
 * Represents a dependency between the PartitionPruningRDD and its parent. In this
 * case, the child RDD contains a subset of partitions of the parents'.
 */
private[spark] class PruneDependency[T](rdd: RDD[T], @transient partitionFilterFunc: Int => Boolean)
  extends NarrowDependency[T](rdd) {

  @transient
  val partitions: Array[Partition] = rdd.partitions
    .filter(s => partitionFilterFunc(s.index)).zipWithIndex
    .map { case(split, idx) => new PartitionPruningRDDPartition(idx, split) : Partition }

  override def getParents(partitionId: Int) = {
    List(partitions(partitionId).asInstanceOf[PartitionPruningRDDPartition].parentSplit.index)
  }
}

c、RangeDependency：这种是父rdd的连续多个partitions对应子rdd中的连续多个partitions，对应的转换有union

/**Union
 * :: DeveloperApi ::
 * Represents a one-to-one dependency between ranges of partitions in the parent and child RDDs.
 * @param rdd the parent RDD
 * @param inStart the start of the range in the parent RDD parent RDD中区间的起始点
 * @param outStart the start of the range in the child RDD child RDD中区间的起始点 
 * @param length the length of the range
 */
@DeveloperApi
class RangeDependency[T](rdd: RDD[T], inStart: Int, outStart: Int, length: Int)
  extends NarrowDependency[T](rdd) {

  override def getParents(partitionId: Int) = {
    if (partitionId >= outStart && partitionId < outStart + length) {//判断partitionId的合理性,必须在child RDD的合理partition范围
      List(partitionId - outStart + inStart)//算出parent RDD中对应的partition id
    } else {
      Nil
    }
  }
}

2）WideDependency：这种依赖是指一个父partition可以对应子rdd中多个partitions。由于需要对父partition进行划分，故需要用到shuffle，而shuffle一般是采用键值对的。

这里为每个shuffle分配了一个全局唯一的shuffleId。为了进行shuffle，需要指定如何进行shuffle，这对应于参数partitioner；由于shuffle是需要网络传输的，故需要进行序列化Serializer。在宽依赖中并无法获得partition对应的parent partitions？

/**
 * :: DeveloperApi ::
 * Represents a dependency on the output of a shuffle stage.
 * @param rdd the parent RDD
 * @param partitioner partitioner used to partition the shuffle output
 * @param serializer [[org.apache.spark.serializer.Serializer Serializer]] to use. If set to null,
 *                   the default serializer, as specified by `spark.serializer` config option, will
 *                   be used.
 */
@DeveloperApi
class ShuffleDependency[K, V](
    @transient rdd: RDD[_ <: Product2[K, V]],
    val partitioner: Partitioner,//需要给出partitioner, 指示如何完成shuffle
    val serializer: Serializer = null)//shuffle不象map可以在local进行, 往往需要网络传输或存储, 所以需要serializerClass
  extends Dependency(rdd.asInstanceOf[RDD[Product2[K, V]]]) {

  val shuffleId: Int = rdd.context.newShuffleId()//每个shuffle需要分配一个全局的id, context.newShuffleId()的实现就是把全局id累加

  rdd.sparkContext.cleaner.foreach(_.registerShuffleForCleanup(this))
}