了解RDD之前，必读UCB的论文，个人认为这是最好的资料，没有之一。

http://www.cs.berkeley.edu/~matei/papers/2012/nsdi_spark.pdf  

A Resilient Distributed Dataset (RDD), the basic abstraction in Spark. Represents an immutable,
* partitioned collection of elements that can be operated on in parallel. This class contains the

* basic operations available on all RDDs, such as `map`, `filter`, and `persist`. In addition,* Internally, each 

* [[org.apache.spark.rdd.PairRDDFunctions]] contains operations available only on RDDs of key-value
* pairs, such as `groupByKey` and `join`;
* [[org.apache.spark.rdd.DoubleRDDFunctions]] contains operations available only on RDDs of
* Doubles; and
* [[org.apache.spark.rdd.SequenceFileRDDFunctions]] contains operations available on RDDs that
* can be saved as SequenceFiles.
* These operations are automatically available on any RDD of the right type (e.g. RDD[(Int, Int)]
* through implicit conversions when you `import org.apache.spark.SparkContext._`.

*RDD is characterized by five main properties:
*
*  - 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)

* All of the scheduling and execution in Spark is done based on these methods, allowing each RDD
* to implement its own way of computing itself. Indeed, users can implement custom RDDs (e.g. for
* reading data from a new storage system) by overriding these functions. Please refer to the
* [[http://www.cs.berkeley.edu/~matei/papers/2012/nsdi_spark.pdf Spark paper]] for more details
* on RDD internals.
*/
abstract class RDD[T: ClassTag](
    @transient private var sc: SparkContext,
    @transient private var deps: Seq[Dependency[_]]
  ) extends Serializable with Logging {
RDD是spark中最基础的数据表达形式，它的compute方法用来产生partition。由子类实现。
/**
 * :: DeveloperApi ::
 * Implemented by subclasses to compute a given partition.
 */
@DeveloperApi
def compute(split: Partition, context: TaskContext): Iterator[T]
RDD的persist是一个主要的功能，它负责将RDD以某个存储级别保留给后续的计算流程使用，是的迭代计算高效。
/**
 * Set this RDD‘s storage level to persist its values across operations after the first time
 * it is computed. This can only be used to assign a new storage level if the RDD does not
 * have a storage level set yet..
 */
def persist(newLevel: StorageLevel): this.type = {
  // TODO: Handle changes of StorageLevel
  if (storageLevel != StorageLevel.NONE && newLevel != storageLevel) {
    throw new UnsupportedOperationException(
      "Cannot change storage level of an RDD after it was already assigned a level")
  }
  sc.persistRDD(this)
  // Register the RDD with the ContextCleaner for automatic GC-based cleanup
  sc.cleaner.foreach(_.registerRDDForCleanup(this))
  storageLevel = newLevel
  this
}
RDD可以设置本地化优先策略，这是在使用Hadoop做存储时提高性能的主要手段。
/**
 * Get the preferred locations of a partition (as hostnames), taking into account whether the
 * RDD is checkpointed.
 */
final def preferredLocations(split: Partition): Seq[String] = {
  checkpointRDD.map(_.getPreferredLocations(split)).getOrElse {
    getPreferredLocations(split)
  }
}
RDD可以转化为其他的RDD，map/flatMap/filter是三个最常用的转化方式
// Transformations (return a new RDD)
/**
 * Return a new RDD by applying a function to all elements of this RDD.
 */
def map[U: ClassTag](f: T => U): RDD[U] = new MappedRDD(this, sc.clean(f))

/**
 *  Return a new RDD by first applying a function to all elements of this
 *  RDD, and then flattening the results.
 */
def flatMap[U: ClassTag](f: T => TraversableOnce[U]): RDD[U] =
  new FlatMappedRDD(this, sc.clean(f))

/**
 * Return a new RDD containing only the elements that satisfy a predicate.
 */
def filter(f: T => Boolean): RDD[T] = new FilteredRDD(this, sc.clean(f))
注意，大部分时候RDD是推迟计算的，也就是在做transformation时，其实只是记录“如何做”，而真正的转化，是等到“Actions”来出发的。这样做的优势是使得串行化成为可能，这是spark性能高于hadoop的主要原因之一。
// Actions (launch a job to return a value to the user program)

/**
 * Applies a function f to all elements of this RDD.
 */
def foreach(f: T => Unit) {
  val cleanF = sc.clean(f)
  sc.runJob(this, (iter: Iterator[T]) => iter.foreach(cleanF))
}

/**
 * Return an array that contains all of the elements in this RDD.
 */
def collect(): Array[T] = {
  val results = sc.runJob(this, (iter: Iterator[T]) => iter.toArray)
  Array.concat(results: _*)
}
/**
 * Reduces the elements of this RDD using the specified commutative and
 * associative binary operator.
 */
def reduce(f: (T, T) => T): T = {
  val cleanF = sc.clean(f)
  val reducePartition: Iterator[T] => Option[T] = iter => {
    if (iter.hasNext) {
      Some(iter.reduceLeft(cleanF))
    } else {
      None
    }
  }
  var jobResult: Option[T] = None
  val mergeResult = (index: Int, taskResult: Option[T]) => {
    if (taskResult.isDefined) {
      jobResult = jobResult match {
        case Some(value) => Some(f(value, taskResult.get))
        case None => taskResult
      }
    }
  }
  sc.runJob(this, reducePartition, mergeResult)
  // Get the final result out of our Option, or throw an exception if the RDD was empty
  jobResult.getOrElse(throw new UnsupportedOperationException("empty collection"))
}
/**
 * Return the number of elements in the RDD.
 */
def count(): Long = sc.runJob(this, Utils.getIteratorSize _).sum
/**
 * Returns the top K (largest) elements from this RDD as defined by the specified
 * implicit Ordering[T]. This does the opposite of [[takeOrdered]]. For example:
 * {{{
 *   sc.parallelize(Seq(10, 4, 2, 12, 3)).top(1)
 *   // returns Array(12)
 *
 *   sc.parallelize(Seq(2, 3, 4, 5, 6)).top(2)
 *   // returns Array(6, 5)
 * }}}
 *
 * @param num the number of top elements to return
 * @param ord the implicit ordering for T
 * @return an array of top elements
 */
def top(num: Int)(implicit ord: Ordering[T]): Array[T] = takeOrdered(num)(ord.reverse)
RDD的checkpoint功能意义也很重大，因为它会将RDD存到可靠存储设备，所以在这个RDD之前的历史记录就可以不用记录了（因为这个RDD已经是可靠的，不需要更老的历史了）。对于RDD以来很长的应用，选择合适的checkpiont显得格外重要。
/**
 * Mark this RDD for checkpointing. It will be saved to a file inside the checkpoint
 * directory set with SparkContext.setCheckpointDir() and all references to its parent
 * RDDs will be removed. This function must be called before any job has been
 * executed on this RDD. It is strongly recommended that this RDD is persisted in
 * memory, otherwise saving it on a file will require recomputation.
 */
def checkpoint() {
  if (context.checkpointDir.isEmpty) {
    throw new Exception("Checkpoint directory has not been set in the SparkContext")
  } else if (checkpointData.isEmpty) {
    checkpointData = Some(new RDDCheckpointData(this))
    checkpointData.get.markForCheckpoint()
  }
}
这个调试函数会打印绝大部分的RDD的状态和信息。
/** A description of this RDD and its recursive dependencies for debugging. */
def toDebugString: String = {
RDD的转换示意图：

spark 笔记 6: RDD