1. a list of partitions
2. a function for computing each split
3. a list dependencies on other RDDs
4. optionally, a partitioner for key-value RDDS
(比如按照key的hash值进行重分区)
5. optionally, a list of preferred locations to compute each split on
/**
* :: DeveloperApi ::
* Implemented by subclasses to compute a given partition.
*/
@DeveloperApi
def compute(split: Partition, context: TaskContext): Iterator[T]
/**
* Implemented by subclasses to return the set of partitions in this RDD. This method will only
* be called once, so it is safe to implement a time-consuming computation in it.
*
* The partitions in this array must satisfy the following property:
* `rdd.partitions.zipWithIndex.forall { case (partition, index) => partition.index == index }`
*/
protected def getPartitions: Array[Partition]
/**
* Implemented by subclasses to return how this RDD depends on parent RDDs. This method will only
* be called once, so it is safe to implement a time-consuming computation in it.
*/
protected def getDependencies: Seq[Dependency[_]] = deps
/**
* Optionally overridden by subclasses to specify placement preferences.
*/
protected def getPreferredLocations(split: Partition): Seq[String] = Nil
/** Optionally overridden by subclasses to specify how they are partitioned. */
@transient val partitioner: Option[Partitioner] = None
//
