partitionBy 重新分区， repartition默认采用HashPartitioner分区，自己设计合理的分区方法(比如数量比较大的key 加个随机数 随机分到更多的分区， 这样处理数据倾斜更彻底一些)
/**
* An object that defines how the elements in a key-value pair RDD are partitioned by key.
* Maps each key to a partition ID, from 0 to `numPartitions - 1`.
*/
abstract class Partitioner extends Serializable {
def numPartitions: Int
def getPartition(key: Any): Int
}
import org.apache.spark.HashPartitioner
import org.apache.spark.sql.SparkSession
//查看rdd中的每个分区元素
object PartitionBy_Test {
def main(args: Array[String]): Unit = {
val spark = SparkSession.builder().master("local").appName(this.getClass.getSimpleName).getOrCreate()
val rdd = spark.sparkContext.parallelize(Array(("a", 1), ("a", 2), ("b", 1), ("b", 3), (("c", 1)), (("e", 1))), 2)
val result = rdd.mapPartitionsWithIndex {
(partIdx, iter) => {
val part_map = scala.collection.mutable.Map[String, List[(String, Int)]]()
while (iter.hasNext) {
val part_name = "part_" + partIdx
var elem = iter.next()
if (part_map.contains(part_name)) {
var elems = part_map(part_name)
elems ::= elem
part_map(part_name) = elems
} else {
part_map(part_name) = List[(String, Int)] {
elem
}
}
}
part_map.iterator
}
}.collect
result.foreach(x => println(x._1 + ":" + x._2.toString()))
}
}
这里的分区方法可以选择， 默认的分区就是HashPartition分区，
注意如果多次使用该RDD或者进行join操作， 分区后peresist持久化操作
/**
* A [[org.apache.spark.Partitioner]] that implements hash-based partitioning using
* Java's `Object.hashCode`.
*
* Java arrays have hashCodes that are based on the arrays' identities rather than their contents,
* so attempting to partition an RDD[Array[_]] or RDD[(Array[_], _)] using a HashPartitioner will
* produce an unexpected or incorrect result.
*/
class HashPartitioner(partitions: Int) extends Partitioner {
require(partitions >= 0, s"Number of partitions ($partitions) cannot be negative.")
def numPartitions: Int = partitions
def getPartition(key: Any): Int = key match {
case null => 0
case _ => Utils.nonNegativeMod(key.hashCode, numPartitions)
}
override def equals(other: Any): Boolean = other match {
case h: HashPartitioner =>
h.numPartitions == numPartitions
case _ =>
false
}
override def hashCode: Int = numPartitions
}
范围分区 RangePartitioner ：先键值排序， 确定样本大小，采样后不放回总体的随机采样方法， 分配键值的分区，通过样本采样避免数据倾斜。
class RangePartitioner[K : Ordering : ClassTag, V](
partitions: Int,
rdd: RDD[_ <: Product2[K, V]],
private var ascending: Boolean = true,
val samplePointsPerPartitionHint: Int = 20)
extends Partitioner {
// A constructor declared in order to maintain backward compatibility for Java, when we add the
// 4th constructor parameter samplePointsPerPartitionHint. See SPARK-22160.
// This is added to make sure from a bytecode point of view, there is still a 3-arg ctor.
def this(partitions: Int, rdd: RDD[_ <: Product2[K, V]], ascending: Boolean) = {
this(partitions, rdd, ascending, samplePointsPerPartitionHint = 20)
}
// We allow partitions = 0, which happens when sorting an empty RDD under the default settings.
require(partitions >= 0, s"Number of partitions cannot be negative but found $partitions.")
require(samplePointsPerPartitionHint > 0,
s"Sample points per partition must be greater than 0 but found $samplePointsPerPartitionHint")
private var ordering = implicitly[Ordering[K]]
// An array of upper bounds for the first (partitions - 1) partitions
private var rangeBounds: Array[K] = {
if (partitions <= 1) {
Array.empty
} else {
// This is the sample size we need to have roughly balanced output partitions, capped at 1M.
// Cast to double to avoid overflowing ints or longs
val sampleSize = math.min(samplePointsPerPartitionHint.toDouble * partitions, 1e6)
// Assume the input partitions are roughly balanced and over-sample a little bit.
val sampleSizePerPartition = math.ceil(3.0 * sampleSize / rdd.partitions.length).toInt
val (numItems, sketched) = RangePartitioner.sketch(rdd.map(_._1), sampleSizePerPartition)
if (numItems == 0L) {
Array.empty
} else {
// If a partition contains much more than the average number of items, we re-sample from it
// to ensure that enough items are collected from that partition.
val fraction = math.min(sampleSize / math.max(numItems, 1L), 1.0)
val candidates = ArrayBuffer.empty[(K, Float)]
val imbalancedPartitions = mutable.Set.empty[Int]
sketched.foreach { case (idx, n, sample) =>
if (fraction * n > sampleSizePerPartition) {
imbalancedPartitions += idx
} else {
// The weight is 1 over the sampling probability.
val weight = (n.toDouble / sample.length).toFloat
for (key <- sample) {
candidates += ((key, weight))
}
}
}
if (imbalancedPartitions.nonEmpty) {
// Re-sample imbalanced partitions with the desired sampling probability.
val imbalanced = new PartitionPruningRDD(rdd.map(_._1), imbalancedPartitions.contains)
val seed = byteswap32(-rdd.id - 1)
val reSampled = imbalanced.sample(withReplacement = false, fraction, seed).collect()
val weight = (1.0 / fraction).toFloat
candidates ++= reSampled.map(x => (x, weight))
}
RangePartitioner.determineBounds(candidates, math.min(partitions, candidates.size))
}
}
}
def numPartitions: Int = rangeBounds.length + 1
private var binarySearch: ((Array[K], K) => Int) = CollectionsUtils.makeBinarySearch[K]
def getPartition(key: Any): Int = {
val k = key.asInstanceOf[K]
var partition = 0
if (rangeBounds.length <= 128) {
// If we have less than 128 partitions naive search
while (partition < rangeBounds.length && ordering.gt(k, rangeBounds(partition))) {
partition += 1
}
} else {
// Determine which binary search method to use only once.
partition = binarySearch(rangeBounds, k)
// binarySearch either returns the match location or -[insertion point]-1
if (partition < 0) {
partition = -partition-1
}
if (partition > rangeBounds.length) {
partition = rangeBounds.length
}
}
if (ascending) {
partition
} else {
rangeBounds.length - partition
}
}
override def equals(other: Any): Boolean = other match {
case r: RangePartitioner[_, _] =>
r.rangeBounds.sameElements(rangeBounds) && r.ascending == ascending
case _ =>
false
}
override def hashCode(): Int = {
val prime = 31
var result = 1
var i = 0
while (i < rangeBounds.length) {
result = prime * result + rangeBounds(i).hashCode
i += 1
}
result = prime * result + ascending.hashCode
result
}
@throws(classOf[IOException])
private def writeObject(out: ObjectOutputStream): Unit = Utils.tryOrIOException {
val sfactory = SparkEnv.get.serializer
sfactory match {
case js: JavaSerializer => out.defaultWriteObject()
case _ =>
out.writeBoolean(ascending)
out.writeObject(ordering)
out.writeObject(binarySearch)
val ser = sfactory.newInstance()
Utils.serializeViaNestedStream(out, ser) { stream =>
stream.writeObject(scala.reflect.classTag[Array[K]])
stream.writeObject(rangeBounds)
}
}
}
@throws(classOf[IOException])
private def readObject(in: ObjectInputStream): Unit = Utils.tryOrIOException {
val sfactory = SparkEnv.get.serializer
sfactory match {
case js: JavaSerializer => in.defaultReadObject()
case _ =>
ascending = in.readBoolean()
ordering = in.readObject().asInstanceOf[Ordering[K]]
binarySearch = in.readObject().asInstanceOf[(Array[K], K) => Int]
val ser = sfactory.newInstance()
Utils.deserializeViaNestedStream(in, ser) { ds =>
implicit val classTag = ds.readObject[ClassTag[Array[K]]]()
rangeBounds = ds.readObject[Array[K]]()
}
}
}
}
自定义分区函数 自己根据业务数据减缓数据倾斜问题:
要实现自定义的分区器，你需要继承 org.apache.spark.Partitioner 类并实现下面三个方法
- numPartitions: Int：返回创建出来的分区数。
- getPartition(key: Any): Int：返回给定键的分区编号（ 0 到 numPartitions-1）。
//自定义分区类，需继承Partitioner类
class UsridPartitioner(numParts:Int) extends Partitioner{
//覆盖分区数
override def numPartitions: Int = numParts
//覆盖分区号获取函数
override def getPartition(key: Any): Int = {
if(key.toString == "A")
key.toString.toInt%10
else:
key.toString.toInt%5
}
}