# Principles of Overriding RDD

what should we override?

```scala
  /*
   RDD.scala
  */
  
  // =======================================================================
  // Methods that should be implemented by subclasses of RDD
  // =======================================================================

  /**
   * :: 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

```

first 3 are compulsory

compute 같은거는 경우에 따라 parent로부터 받거나 아니면 새로 만들거나 할 듯.

last two are optional

그 외 , 필요에 따라 clearDependencies(), 등 과 같은 함수를 override하기도 함. 

write examples (e.g. ShuffledRDD.scala)

###  Partition definition

trait partition 소개

```scala
trait Partition extends Serializable {
  /**
   * Get the partition's index within its parent RDD
   */
  def index: Int

  // A better default implementation of HashCode
  override def hashCode(): Int = index

  override def equals(other: Any): Boolean = super.equals(other)
}

```

normally, each RDD has its own Partition class (ex. ShuffledRDD + ShuffledRDDPartition )


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