RDD的源码

RDD是一个抽象类定义了所有RDD共有的一些属性和方法，下面介绍了主要的属性和方法。

abstract class RDD[T: ClassTag](
    @transient private var _sc: SparkContext,
    @transient private var deps: Seq[Dependency[_]]
  ) extends Serializable with Logging {

RDD有5个主要的属性

 *  - A list of partitions
 *  - A function for computing each split
 *  - A list of dependencies on other RDDs
 *  - Optionally, a Partitioner for key-value RDDs (e.g. to say that the RDD is hash-partitioned)
 *  - Optionally, a list of preferred locations to compute each split on (e.g. block locations for
 *    an HDFS file)

（一）子类

CoGroupedRDD, EdgeRDD, EdgeRDDImpl, HadoopRDD, JdbcRDD, NewHadoopRDD, PartitionPruningRDD, ShuffledRDD, UnionRDD, VertexRDD, VertexRDDImpl

（二）属性

1、SpackContext

@transient private var _sc: SparkContext

在主构建函数中定义，表示RDD所在运行环境，可用于获取配置，清理环境等。

2、Seq[Dependency[_]]

@transient private var deps: Seq[Dependency[_]]

定义了这个RDD对父RDD的依赖关系。

（三）方法

1、tranformation与action

RDD中定义了所有RDD所共用的tranformation与action，如map, filter, reduce, first等，举个filter的例子：

 def filter(f: T => Boolean): RDD[T] = withScope {
    val cleanF = sc.clean(f)
    new MapPartitionsRDD[T, T](
      this,
      (context, pid, iter) => iter.filter(cleanF),
      preservesPartitioning = true)
  }

2、缓存

包括pesist的多个实现及cache等，举个例子

 /**
   * Mark this RDD for persisting using the specified level.
   *
   * @param newLevel the target storage level
   * @param allowOverride whether to override any existing level with the new one
   */
  private def persist(newLevel: StorageLevel, allowOverride: Boolean): this.type = {
    // TODO: Handle changes of StorageLevel
    if (storageLevel != StorageLevel.NONE && newLevel != storageLevel && !allowOverride) {
      throw new UnsupportedOperationException(
        "Cannot change storage level of an RDD after it was already assigned a level")
    }
    // If this is the first time this RDD is marked for persisting, register it
    // with the SparkContext for cleanups and accounting. Do this only once.
    if (storageLevel == StorageLevel.NONE) {
      sc.cleaner.foreach(_.registerRDDForCleanup(this))
      sc.persistRDD(this)
    }
    storageLevel = newLevel
    this
  }