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PairRDDFunctions

PairRDDFunctions is an extension of RDD API for extra methods for key-value RDDs (RDD[(K, V)]).

PairRDDFunctions is available in RDDs of key-value pairs via Scala implicit conversion.

saveAsNewAPIHadoopFile

saveAsNewAPIHadoopFile(
  path: String,
  keyClass: Class[_],
  valueClass: Class[_],
  outputFormatClass: Class[_ <: NewOutputFormat[_, _]],
  conf: Configuration = self.context.hadoopConfiguration): Unit
saveAsNewAPIHadoopFile[F <: NewOutputFormat[K, V]](
  path: String)(implicit fm: ClassTag[F]): Unit

saveAsNewAPIHadoopFile creates a new Job (Hadoop MapReduce) for the given Configuration (Hadoop).

saveAsNewAPIHadoopFile configures the Job (with the given keyClass, valueClass and outputFormatClass).

saveAsNewAPIHadoopFile sets mapreduce.output.fileoutputformat.outputdir configuration property to be the given path and saveAsNewAPIHadoopDataset.

saveAsNewAPIHadoopDataset

saveAsNewAPIHadoopDataset(
  conf: Configuration): Unit

saveAsNewAPIHadoopDataset creates a new HadoopMapReduceWriteConfigUtil (with the given Configuration) and writes the RDD out.

Configuration should have all the relevant output params set (an output format, output paths, e.g. a table name to write to) in the same way as it would be configured for a Hadoop MapReduce job.

groupByKey and reduceByKey

reduceByKey(
  func: (V, V) => V): RDD[(K, V)]
reduceByKey(
  func: (V, V) => V,
  numPartitions: Int): RDD[(K, V)]
reduceByKey(
  partitioner: Partitioner,
  func: (V, V) => V): RDD[(K, V)]

reduceByKey is sort of a particular case of aggregateByKey.

You may want to look at the number of partitions from another angle.

It may often not be important to have a given number of partitions upfront (at RDD creation time upon spark-data-sources.md[loading data from data sources]), so only "regrouping" the data by key after it is an RDD might be...the key (pun not intended).

You can use groupByKey or another PairRDDFunctions method to have a key in one processing flow.

groupByKey(): RDD[(K, Iterable[V])]
groupByKey(
  numPartitions: Int): RDD[(K, Iterable[V])]
groupByKey(
  partitioner: Partitioner): RDD[(K, Iterable[V])]

You could use partitionBy that is available for RDDs to be RDDs of tuples, i.e. PairRDD:

rdd.keyBy(_.kind)
  .partitionBy(new HashPartitioner(PARTITIONS))
  .foreachPartition(...)

Think of situations where kind has low cardinality or highly skewed distribution and using the technique for partitioning might be not an optimal solution.

You could do as follows:

rdd.keyBy(_.kind).reduceByKey(....)

or mapValues or plenty of other solutions. FIXME

combineByKeyWithClassTag

combineByKeyWithClassTag[C](
  createCombiner: V => C,
  mergeValue: (C, V) => C,
  mergeCombiners: (C, C) => C)(implicit ct: ClassTag[C]): RDD[(K, C)] // <1>
combineByKeyWithClassTag[C](
  createCombiner: V => C,
  mergeValue: (C, V) => C,
  mergeCombiners: (C, C) => C,
  numPartitions: Int)(implicit ct: ClassTag[C]): RDD[(K, C)] // <2>
combineByKeyWithClassTag[C](
  createCombiner: V => C,
  mergeValue: (C, V) => C,
  mergeCombiners: (C, C) => C,
  partitioner: Partitioner,
  mapSideCombine: Boolean = true,
  serializer: Serializer = null)(implicit ct: ClassTag[C]): RDD[(K, C)]

combineByKeyWithClassTag creates an Aggregator for the given aggregation functions.

combineByKeyWithClassTag branches off per the given Partitioner.

If the input partitioner and the RDD's are the same, combineByKeyWithClassTag simply mapPartitions on the RDD with the following arguments:

  • Iterator of the Aggregator

  • preservesPartitioning flag turned on

If the input partitioner is different than the RDD's, combineByKeyWithClassTag creates a ShuffledRDD (with the Serializer, the Aggregator, and the mapSideCombine flag).

Usage

combineByKeyWithClassTag lays the foundation for the following transformations:

  • aggregateByKey
  • combineByKey
  • countApproxDistinctByKey
  • foldByKey
  • groupByKey
  • reduceByKey

Requirements

combineByKeyWithClassTag requires that the mergeCombiners is defined (not-null) or throws an IllegalArgumentException:

mergeCombiners must be defined

combineByKeyWithClassTag throws a SparkException for the keys being of type array with the mapSideCombine flag enabled:

Cannot use map-side combining with array keys.

combineByKeyWithClassTag throws a SparkException for the keys being of type array with the partitioner being a HashPartitioner:

HashPartitioner cannot partition array keys.

Example

val nums = sc.parallelize(0 to 9, numSlices = 4)
val groups = nums.keyBy(_ % 2)
def createCombiner(n: Int) = {
  println(s"createCombiner($n)")
  n
}
def mergeValue(n1: Int, n2: Int) = {
  println(s"mergeValue($n1, $n2)")
  n1 + n2
}
def mergeCombiners(c1: Int, c2: Int) = {
  println(s"mergeCombiners($c1, $c2)")
  c1 + c2
}
val countByGroup = groups.combineByKeyWithClassTag(
  createCombiner,
  mergeValue,
  mergeCombiners)
println(countByGroup.toDebugString)
/*
(4) ShuffledRDD[3] at combineByKeyWithClassTag at <console>:31 []
 +-(4) MapPartitionsRDD[1] at keyBy at <console>:25 []
    |  ParallelCollectionRDD[0] at parallelize at <console>:24 []
*/

Last update: 2021-05-26
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