[Spark]Spark RDD&算子总结

2019-09-19 本文已影响0人帅可儿妞

一、RDD概述

什么是RDD
- RDD（Resilient Distributed Dataset）叫做弹性分布式数据集，是Spark中最基本的数据抽象，它代表一个不可变、可分区、里面的元素可并行计算的集合。RDD具有数据流模型的特点：自动容错、位置感知性调度和可伸缩性。RDD允许用户在执行多个查询时显式地将工作集缓存在内存中，后续的查询能够重用工作集，这极大地提升了查询速度。
RDD属性

image.png
1. 一组分片（Partition），即数据集的基本组成单位。对于RDD来说，每个分片都会被一个计算任务处理，并决定并行计算的粒度。用户可以在创建RDD时指定RDD的分片个数，如果没有指定，那么就会采用默认值。默认值就是程序所分配到的CPU Core的数目。
2. 一个计算每个分片的函数。Spark中RDD的计算是以分片为单位的，每个RDD都会实现compute函数以达到这个目的。compute函数会对迭代器进行复合，不需要保存每次计算的结果。
3. RDD之间的依赖关系。RDD的每次转换都会生成一个新的RDD，所以RDD之间就会形成类似于流水线一样的前后依赖关系。在部分分区数据丢失时，Spark可以通过这个依赖关系重新计算丢失的分区数据，而不是对RDD的所有分区进行重新计算。
4. 一个Partitioner，即RDD的分片函数。当前Spark中实现了两种类型的分片函数，一个是基于哈希的HashPartitioner，另外一个是基于范围的RangePartitioner。只有对于于key-value的RDD，才会有Partitioner，非key-value的RDD的Parititioner的值是None。Partitioner函数不但决定了RDD本身的分片数量，也决定了parent RDD Shuffle输出时的分片数量。
5. 一个列表，存储存取每个Partition的优先位置（preferred location）。对于一个HDFS文件来说，这个列表保存的就是每个Partition所在的块的位置。按照“移动数据不如移动计算”的理念，Spark在进行任务调度的时候，会尽可能地将计算任务分配到其所要处理数据块的存储位置。
创建RDD
1. 由一个已经存在的Scala集合创建。
  val rdd1 = sc.parallelize(Array(1,2,3,4,5,6,7,8))
2. 由外部存储系统的数据集创建，包括本地的文件系统，还有所有Hadoop支持的数据集，比如HDFS、Cassandra、HBase等
```
val rdd2 = sc.textFile("hdfs://hadoop141:8020/words.txt")
```
3. 查看该rdd的分区数量，默认是程序所分配的cpu core的数量，也可以在创建的时候指定:rdd1.partitions.length, 创建的时候指定分区数量：val rdd1 = sc.parallelize(Array(1,2,3.4),3)

二、RDD编程API---包含两种算子

Transformation
RDD中的所有转换都是延迟加载的，也就是说，它们并不会直接计算结果。相反的，它们只是记住这些应用到基础数据集（例如一个文件）上的转换动作。只有当发生一个要求返回结果给Driver的动作时，这些转换才会真正运行。这种设计让Spark更加有效率地运行。

常用的Transformation操作：

map（func）：返回一个新的RDD，该RDD由每一个输入的元素经过func函数转换后组成。
filter（func）：返回一个新的RDD，该RDD由每一个输入的元素经过func函数计算后返回为true的输入元素组成。

sortBy（func，[ascending], [numTasks]）：返回一个新的RDD，输入元素经过func函数计算后，按照指定的方式进行排序。（默认方式为false，升序；true是降序）

val rdd1 = sc.parallelize(List(5,6,4,7,3,8,2,9,1,10))
val rdd2 = sc.parallelize(List(5,6,4,7,3,8,2,9,1,10)).map(_*2).sortBy(x=>x,true)
val rdd3 = rdd2.filter(_>10)
val rdd2 = sc.parallelize(List(5,6,4,7,3,8,2,9,1,10)).map(_*2).sortBy(x=>x+"",true)
val rdd2 = sc.parallelize(List(5,6,4,7,3,8,2,9,1,10)).map(_*2).sortBy(x=>x.toString,true)

flatMap（func）：类似于map，但是每一个输入元素可以被映射为0或多个输出元素（所以func应该返回一个序列，而不是单一元素）。类似于先map，然后再flatten。

val rdd4 = sc.parallelize(Array("a b c", "d e f", "h i j"))
rdd4.flatMap(_.split(' ')).collect
------------------------------------------------------------------
val rdd5 = sc.parallelize(List(List("a b c", "a b b"),List("e f g", "a f g"), List("h i j", "a a b")))
rdd5.flatMap(_.flatMap(_.split(" "))).collect

union：求并集，注意类型要一致
intersection：求交集

distinct：去重

val rdd6 = sc.parallelize(List(5,6,4,7))
val rdd7 = sc.parallelize(List(1,2,3,4))
val rdd8 = rdd6.union(rdd7)
rdd8.distinct.sortBy(x=>x).collect
--------------------------------------------
val rdd9 = rdd6.intersection(rdd7)

join、leftOuterJoin、rightOuterJoin

val rdd1 = sc.parallelize(List(("tom", 1), ("jerry", 2), ("kitty", 3)))
val rdd2 = sc.parallelize(List(("jerry", 9), ("tom", 8), ("shuke", 7)))
--------------------------------------------------------------------------
val rdd3 = rdd1.join(rdd2).collect
rdd3: Array[(String, (Int, Int))] = Array((tom,(1,8)), (jerry,(2,9)))
---------------------------------------------------------------------------
val rdd3 = rdd1.leftOuterJoin(rdd2).collect
rdd3: Array[(String, (Int, Option[Int]))] = Array((tom,(1,Some(8))), (jerry,(2,Some(9))), (kitty,(3,None)))
---------------------------------------------------------------------------
val rdd3 = rdd1.rightOuterJoin(rdd2).collect
rdd3: Array[(String, (Option[Int], Int))] = Array((tom,(Some(1),8)), (jerry,(Some(2),9)), (shuke,(None,7)))

groupByKey（[numTasks]）：在一个(K,V)的RDD上调用，返回一个(K, Iterator[V])的RDD----只针对数据是对偶元组的

val rdd1 = sc.parallelize(List(("tom", 1), ("jerry", 2), ("kitty", 3)))
val rdd2 = sc.parallelize(List(("jerry", 9), ("tom", 8), ("shuke", 7)))
val rdd3 = rdd1 union rdd2
val rdd4 = rdd3.groupByKey.collect
rdd4: Array[(String, Iterable[Int])] = Array((tom,CompactBuffer(8, 1)), (shuke,CompactBuffer(7)), (kitty,CompactBuffer(3)), (jerry,CompactBuffer(9, 2)))
-----------------------------------------------------------------------------------
val rdd5 = rdd4.map(x=>(x._1,x._2.sum))
rdd5: Array[(String, Int)] = Array((tom,9), (shuke,7), (kitty,3), (jerry,11))

groupBy：传入一个参数的函数，按照传入的参数为key，返回一个新的RDD[(K, Iterable[T])]，value是所有可以相同的传入数据组成的迭代器。以下为源码：

/**
  * Return an RDD of grouped items. Each group consists of a key and a sequence of elements
  * mapping to that key. The ordering of elements within each group is not guaranteed, and
  * may even differ each time the resulting RDD is evaluated.
  *
  * @note This operation may be very expensive. If you are grouping in order to perform an
  * aggregation (such as a sum or average) over each key, using `PairRDDFunctions.aggregateByKey`
  * or `PairRDDFunctions.reduceByKey` will provide much better performance.
  */
def groupBy[K](f: T => K)(implicit kt: ClassTag[K]): RDD[(K, Iterable[T])] = withScope {
  groupBy[K](f, defaultPartitioner(this))
}

具体代码案例：

scala> val rdd1=sc.parallelize(List(("a",1,2),("b",1,1),("a",4,5)))
rdd1: org.apache.spark.rdd.RDD[(String, Int, Int)] = ParallelCollectionRDD[47] at parallelize at <console>:24
scala> rdd1.groupBy(_._1).collect
res18: Array[(String, Iterable[(String, Int, Int)])] = Array((a,CompactBuffer((a,1,2), (a,4,5))), (b,CompactBuffer((b,1,1))))

reduceByKey（func，[numTasks]）：在一个(K,V)的RDD上调用，返回一个(K,V)的RDD，使用指定的reduce函数，将相同key的值聚合到一起，与groupByKey类似，reduce任务的个数可以通过第二个可选的参数来设置。

val rdd1 = sc.parallelize(List(("tom", 1), ("jerry", 2), ("kitty", 3)))
val rdd2 = sc.parallelize(List(("jerry", 9), ("tom", 8), ("shuke", 7)))
val rdd3 = rdd1 union rdd2
val rdd6 = rdd3.reduceByKey(_+_).collect
rdd6: Array[(String, Int)] = Array((tom,9), (shuke,7), (kitty,3), (jerry,11))

cogroup（otherDataset, [numTasks]）：在类型为(K,V)和(K,W)的RDD上调用，返回一个(K,(Iterable<V>,Iterable<W>))类型的RDD

val rdd1 = sc.parallelize(List(("tom", 1), ("tom", 2), ("jerry", 3), ("kitty", 2)))
val rdd2 = sc.parallelize(List(("jerry", 2), ("tom", 1), ("shuke", 2)))
val rdd3 = rdd1.cogroup(rdd2).collect
rdd3: Array[(String, (Iterable[Int], Iterable[Int]))] = Array((tom,(CompactBuffer(2, 1),CompactBuffer(1))), (jerry,(CompactBuffer(3),CompactBuffer(2))), (shuke,(CompactBuffer(),CompactBuffer(2))), (kitty,(CompactBuffer(2),CompactBuffer())))
----------------------------------------------------------------------------------------
val rdd4 = rdd3.map(x=>(x._1,x._2._1.sum+x._2._2.sum))
rdd4: Array[(String, Int)] = Array((tom,4), (jerry,5), (shuke,2), (kitty,2))

cartesian（otherDataset ）笛卡尔积

val rdd1 = sc.parallelize(List("tom", "jerry"))
val rdd2 = sc.parallelize(List("tom", "kitty", "shuke"))
val rdd3 = rdd1.cartesian(rdd2).collect
rdd3: Array[(String, String)] = Array((tom,tom), (tom,kitty), (tom,shuke), (jerry,tom), (jerry,kitty), (jerry,shuke))

Action

image.png

三、RDD编程----高级API

mapPartitions:针对每个分区进行操作，源码如下：要求传入一个Iterator，并且返回一个Iterator

/**
  * Return a new RDD by applying a function to each partition of this RDD.
  *
  * `preservesPartitioning` indicates whether the input function preserves the partitioner, which
  * should be `false` unless this is a pair RDD and the input function doesn't modify the keys.
  */
def mapPartitions[U: ClassTag](
    f: Iterator[T] => Iterator[U],
    preservesPartitioning: Boolean = false): RDD[U] = withScope {
  val cleanedF = sc.clean(f)
  new MapPartitionsRDD(
    this,
    (context: TaskContext, index: Int, iter: Iterator[T]) => cleanedF(iter),
    preservesPartitioning)
}

mapPartitionsWithIndex：针对每个partition操作，把每个partition中的分区号和对应的值拿出来。是Transformation
源码：

/**
* Return a new RDD by applying a function to each partition of this RDD, while tracking the index
* of the original partition.
*
* `preservesPartitioning` indicates whether the input function preserves the partitioner, which
* should be `false` unless this is a pair RDD and the input function doesn't modify the keys.
preservesPartitioning表示返回RDD是否留有分区器。仅当RDD为K-V型RDD，且key没有被修饰的情况下，可设为true。非K-V型RDD一般不存在分区器；K-V RDD key被修改后，元素将不再满足分区器的分区要求。这些情况下，须设为false，表示返回的RDD没有被分区器分过区。
*/
def mapPartitionsWithIndex[U: ClassTag](-------要求传入一个函数
    f: (Int, Iterator[T]) => Iterator[U],------函数要求传入两个参数
    preservesPartitioning: Boolean = false): RDD[U] = withScope {
  val cleanedF = sc.clean(f)
  new MapPartitionsRDD(
    this,
    (context: TaskContext, index: Int, iter: Iterator[T]) => cleanedF(index, iter),
    preservesPartitioning)
}

代码实例：

1. 首先自定义一个函数，符合mapPartitionsWithIndex参数要求的函数
scala> val func = (index : Int,iter : Iterator[Int]) => {
    | iter.toList.map(x=>"[PartID:" + index + ",val:" + x + "]").iterator
    | }
func: (Int, Iterator[Int]) => Iterator[String] = <function2>
2. 定义一个算子，分区数为2
scala> val rdd1 = sc.parallelize(List(1,2,3,4,5,6,7,8,9),2)
rdd1: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[0] at parallelize at <console>:24
3. 调用方法，传入自定义的函数
scala> rdd1.mapPartitionsWithIndex(func).collect
res0: Array[String] = Array([PartID:0,val:1], [PartID:0,val:2], [PartID:0,val:3], [PartID:0,val:4], [PartID:1,val:5], [PartID:1,val:6], [PartID:1,val:7], [PartID:1,val:8], [PartID:1,val:9])

aggregate：聚合操作，是Action

源码

/**
* Aggregate the elements of each partition, and then the results for all the partitions, using
* given combine functions and a neutral "zero value". This function can return a different result
* type, U, than the type of this RDD, T. Thus, we need one operation for merging a T into an U
* and one operation for merging two U's, as in scala.TraversableOnce. Both of these functions are
* allowed to modify and return their first argument instead of creating a new U to avoid memory
* allocation.
* 将RDD中元素聚集，须提供0初值（因为累积元素，所有要提供累积的初值）。先在分区内依照seqOp函数
* 聚集元素（把T类型元素聚集为U类型的分区“结果”），再在分区间按照combOp函数聚集分区计算结果，最后返回这个结果
*
* @param zeroValue the initial value for the accumulated result of each partition for the
*                  `seqOp` operator, and also the initial value for the combine results from
*                  different partitions for the `combOp` operator - this will typically be the
*                  neutral element (e.g. `Nil` for list concatenation or `0` for summation)
* @param seqOp an operator used to accumulate results within a partition
* @param combOp an associative operator used to combine results from different partitions
* 第一个参数是初始值, 第二个参数:是两个函数[每个函数都是2个参数(第一个参数:先对个个分区进行合并, 
* 第二个:对个个分区合并后的结果再进行合并), 输出一个参数]
*/
def aggregate[U: ClassTag](zeroValue: U)(seqOp: (U, T) => U, combOp: (U, U) => U): U = withScope {
  // Clone the zero value since we will also be serializing it as part of tasks
  var jobResult = Utils.clone(zeroValue, sc.env.serializer.newInstance())
  val cleanSeqOp = sc.clean(seqOp)
  val cleanCombOp = sc.clean(combOp)
  val aggregatePartition = (it: Iterator[T]) => it.aggregate(zeroValue)(cleanSeqOp, cleanCombOp)
  val mergeResult = (index: Int, taskResult: U) => jobResult = combOp(jobResult, taskResult)
  sc.runJob(this, aggregatePartition, mergeResult)
  jobResult
}

代码实例：

scala> val rdd1 = sc.parallelize(List(1,2,3,4,5,6,7,8,9), 2)
rdd1: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[0] at parallelize at <console>:24
//这里先对连个分区分别进行相加，然后两个的分区相加后的结果再相加得出最后的结果
scala> rdd1.aggregate(0)(_+_,_+_)
res0: Int = 45                                                                 
//先对每个分区比较求出最大值，然后每个分区求出的最大值再相加得出最后的结果
scala> rdd1.aggregate(0)(math.max(_,_),_+_)
res1: Int = 13
//这里需要注意，初始值是每次都要参与运算的，例如下面的代码：分区1是1,2,3,4；初始值为5，则他们比较最大值就是5，分区2是5,6,7,8,9；初始值为5，则他们比较结果最大值就是9；然后再相加，这里初始值也要参与运算，5+（5+9）=19
scala> rdd1.aggregate(5)(math.max(_,_),_+_)
res0: Int = 19
-----------------------------------------------------------------------------------------------
scala> val rdd2 = sc.parallelize(List("a","b","c","d","e","f"),2)
rdd2: org.apache.spark.rdd.RDD[String] = ParallelCollectionRDD[0] at parallelize at <console>:24
//这里需要注意，由于每个分区计算是并行计算，所以计算出的结果有先后顺序，所以结果会出现两种情况：如下
scala> rdd2.aggregate("")(_+_,_+_)
res0: String = defabc                                                                                                                    
scala> rdd2.aggregate("")(_+_,_+_)
res2: String = abcdef
//这里的例子更能说明上面提到的初始值参与计算的问题，我们可以看到初始值=号参与了三次计算
scala> rdd2.aggregate("=")(_+_,_+_)
res0: String = ==def=abc
--------------------------------------------------------------------------------------
scala> val rdd3 = sc.parallelize(List("12","23","345","4567"),2)
rdd3: org.apache.spark.rdd.RDD[String] = ParallelCollectionRDD[1] at parallelize at <console>:24
scala> rdd3.aggregate("")((x,y)=>math.max(x.length,y.length).toString,_+_)
res1: String = 42                                                               
scala> rdd3.aggregate("")((x,y)=>math.max(x.length,y.length).toString,_+_)
res3: String = 24
-------------------------------------------------------------------------------------------
scala> val rdd4 = sc.parallelize(List("12","23","345",""),2)
rdd4: org.apache.spark.rdd.RDD[String] = ParallelCollectionRDD[2] at parallelize at <console>:24
//这里需要注意：第一个分区加上初始值元素为"","12","23",两两比较，最小的长度为1；第二个分区加上初始值元素为"","345","",两两比较，最小的长度为0
scala> rdd4.aggregate("")((x,y)=>math.min(x.length,y.length).toString,_+_)
res4: String = 10                                                               
scala> rdd4.aggregate("")((x,y)=>math.min(x.length,y.length).toString,_+_)
res9: String = 01                                                               
------------------------------------------------------------------------------------
//注意与上面的例子的区别，这里定义的rdd里的元素的顺序跟上面不一样，导致结果不一样
scala> val rdd5 = sc.parallelize(List("12","23","","345"),2)
rdd5: org.apache.spark.rdd.RDD[String] = ParallelCollectionRDD[0] at parallelize at <console>:24
scala> rdd5.aggregate("")((x,y)=>math.min(x.length,y.length).toString,(x,y)=>x+y)
res1: String = 11

aggregateByKey：按照key值进行聚合

//定义RDD
scala> val pairRDD = sc.parallelize(List( ("cat",2), ("cat", 5), ("mouse", 4),("cat", 12), ("dog", 12), ("mouse", 2)), 2)
pairRDD: org.apache.spark.rdd.RDD[(String, Int)] = ParallelCollectionRDD[1] at parallelize at <console>:24
//自定义方法，用于传入mapPartitionsWithIndex
scala> val func=(index:Int,iter:Iterator[(String, Int)])=>{
    | iter.toList.map(x => "[partID:" +  index + ", val: " + x + "]").iterator
    | }
func: (Int, Iterator[(String, Int)]) => Iterator[String] = <function2>
//查看分区情况
scala> pairRDD.mapPartitionsWithIndex(func).collect
res2: Array[String] = Array([partID:0, val: (cat,2)], [partID:0, val: (cat,5)], [partID:0, val: (mouse,4)], [partID:1, val: (cat,12)], [partID:1, val: (dog,12)], [partID:1, val: (mouse,2)])
//注意：初始值为0和其他值的区别
scala> pairRDD.aggregateByKey(0)(_+_,_+_).collect
res4: Array[(String, Int)] = Array((dog,12), (cat,19), (mouse,6))               

scala> pairRDD.aggregateByKey(10)(_+_,_+_).collect
res5: Array[(String, Int)] = Array((dog,22), (cat,39), (mouse,26))
//下面三个的区别：，第一个比较好理解，由于初始值为0，所以每个分区输出不同动物中个数最多的那个，然后在累加
scala> pairRDD.aggregateByKey(0)(math.max(_,_),_+_).collect
res6: Array[(String, Int)] = Array((dog,12), (cat,17), (mouse,6))

//下面两个：由于有初始值，就需要考虑初始值参与计算，这里第一个分区的元素为("cat",2), ("cat", 5), ("mouse", 4)，初始值是10，不同动物之间两两比较value的大小，都需要将初始值加入比较，所以第一个分区输出为("cat", 10), ("mouse", 10)；第二个分区同第一个分区，输出结果为(dog,12), (cat,12), (mouse,10)；所以最后累加的结果为(dog,12), (cat,22), (mouse,20)，注意最后的对每个分区结果计算的时候，初始值不参与计算
scala> pairRDD.aggregateByKey(10)(math.max(_,_),_+_).collect
res7: Array[(String, Int)] = Array((dog,12), (cat,22), (mouse,20))
//这个和上面的类似
scala> pairRDD.aggregateByKey(100)(math.max(_,_),_+_).collect
res8: Array[(String, Int)] = Array((dog,100), (cat,200), (mouse,200))

coalesce：返回一个新的RDD
重新给RDD的元素分区。
当适当缩小分区数时，如1000->100，spark会把之前的10个分区当作一个分区，并行度变为100，不会引起数据shuffle。
当严重缩小分区数时，如1000->1，运算时的并行度会变成1。为了避免并行效率低下问题，可将shuffle设为true。shuffle之前的运算和之后的运算分为不同stage，它们的并行度分别为1000,1。
当把分区数增大时，必会存在shuffle，shuffle须设为true。
partitionBy：按照传入的参数进行分区，传入的参数为分区的实例对象，可以传入之定义分区的实例或者默认的HashPartitioner;源码如下：

/**
* Return a copy of the RDD partitioned using the specified partitioner.
*/
def partitionBy(partitioner: Partitioner): RDD[(K, V)] = self.withScope {
if (keyClass.isArray && partitioner.isInstanceOf[HashPartitioner]) {
    throw new SparkException("HashPartitioner cannot partition array keys.")
}
if (self.partitioner == Some(partitioner)) {
    self
} else {
    new ShuffledRDD[K, V, V](self, partitioner)
}
}

repartition：返回一个新的RDD
按指定分区数重新分区RDD，存在shuffle。
当指定的分区数比当前分区数目少时，考虑使用coalesce，这样能够避免shuffle。

scala> val rdd1 = sc.parallelize(Array(1,2,3,4,5,6,7,8),3)
rdd1: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[0] at parallelize at <console>:24

scala> val rdd2 = rdd1.repartition(6)
rdd2: org.apache.spark.rdd.RDD[Int] = MapPartitionsRDD[4] at repartition at <console>:26

scala> rdd2.partitions.length
res0: Int = 6

scala> val rdd3 = rdd2.coalesce(2,true)
rdd3: org.apache.spark.rdd.RDD[Int] = MapPartitionsRDD[8] at coalesce at <console>:28

scala> rdd3.partitions.length
res1: Int = 2

collectAsMap：将RDD转换成Map（注意RDD的数据应为对偶元组）

scala> val rdd1 = sc.parallelize(List(("a", 1), ("b", 2),("c", 2),("d", 4),("e", 1)))
rdd1: org.apache.spark.rdd.RDD[(String, Int)] = ParallelCollectionRDD[9] at parallelize at <console>:24
scala> rdd1.collectAsMap
res3: scala.collection.Map[String,Int] = Map(e -> 1, b -> 2, d -> 4, a -> 1, c -> 2)

combineByKey：和reduceByKey的效果相同，reduceByKey底层就是调用combineByKey

源码

/**
* Generic function to combine the elements for each key using a custom set of aggregation
* functions. This method is here for backward compatibility. It does not provide combiner
* classtag information to the shuffle.
*
* @see [[combineByKeyWithClassTag]]
*/
def combineByKey[C](
    createCombiner: V => C,
    mergeValue: (C, V) => C,
    mergeCombiners: (C, C) => C,
    partitioner: Partitioner,
    mapSideCombine: Boolean = true,
    serializer: Serializer = null): RDD[(K, C)] = self.withScope {
combineByKeyWithClassTag(createCombiner, mergeValue, mergeCombiners,
    partitioner, mapSideCombine, serializer)(null)
}

/**
* Simplified version of combineByKeyWithClassTag that hash-partitions the output RDD.
* This method is here for backward compatibility. It does not provide combiner
* classtag information to the shuffle.
*
* @see [[combineByKeyWithClassTag]]
*/
def combineByKey[C](
    createCombiner: V => C,
    mergeValue: (C, V) => C,
    mergeCombiners: (C, C) => C,
    numPartitions: Int): RDD[(K, C)] = self.withScope {
      combineByKeyWithClassTag(createCombiner, mergeValue, mergeCombiners, numPartitions)(null)
}

参数说明：
1. 第一个参数createCombiner: V => C：生成合并器，每组key，取出第一个value的值，然后返回你想合并的类型。
2. 第二个参数mergeValue: (C, V) => C：函数，局部计算
3. 第三个参数mergeCombiners: (C, C) => C：函数，对局部计算的结果再进行计算

代码实例

//首先声明两个rdd，然后利用zip将两个rdd合并成一个，rdd6
scala> val rdd4 = sc.parallelize(List("dog","cat","gnu","salmon","rabbit","turkey","wolf","bear","bee"), 3)
rdd4: org.apache.spark.rdd.RDD[String] = ParallelCollectionRDD[21] at parallelize at <console>:24

scala> val rdd5 = sc.parallelize(List(1,1,2,2,2,1,2,2,2), 3)
rdd5: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[22] at parallelize at <console>:24

scala> val rdd6 = rdd5.zip(rdd4)
rdd6: org.apache.spark.rdd.RDD[(Int, String)] = ZippedPartitionsRDD2[23] at zip at <console>:28

scala> rdd6.collect
res6: Array[(Int, String)] = Array((1,dog), (1,cat), (2,gnu), (2,salmon), (2,rabbit), (1,turkey), (2,wolf), (2,bear), (2,bee))

//我们需要将按照key进行分组合并，相同的key的value都放在List中
//这里我们第一个参数List(_)：表示将第一个value取出放进集合中
//第二个参数(x:List[String],y:String)=>x :+ y：表示局部计算，将value加入到List中
//第三个参数(m:List[String],n:List[String])=>m++n：表示对局部的计算结果再进行计算

scala> val rdd7 = rdd6.combineByKey(List(_),(x:List[String],y:String)=>x :+ y,(m:List[String],n:List[String])=>m++n)
rdd7: org.apache.spark.rdd.RDD[(Int, List[String])] = ShuffledRDD[24] at combineByKey at <console>:30

scala> rdd7.collect
res7: Array[(Int, List[String])] = Array((1,List(dog, cat, turkey)), (2,List(wolf, bear, bee, salmon, rabbit, gnu)))

//这里第一个参数，可以有另外的写法。如下面的两个
scala> val rdd7 = rdd6.combineByKey(_::List(),(x:List[String],y:String)=>x :+ y,(m:List[String],n:List[String])=>m++n).collect
rdd7: Array[(Int, List[String])] = Array((1,List(turkey, dog, cat)), (2,List(wolf, bear, bee, gnu, salmon, rabbit)))

scala> val rdd7 = rdd6.combineByKey(_::Nil,(x:List[String],y:String)=>x :+ y,(m:List[String],n:List[String])=>m++n).collect
rdd7: Array[(Int, List[String])] = Array((1,List(turkey, dog, cat)), (2,List(wolf, bear, bee, gnu, salmon, rabbit)))

countByKey、countByValue：按照key或者value计算出现的次数

scala> val rdd1 = sc.parallelize(List(("a", 1), ("b", 2), ("b", 2), ("c", 2), ("c", 1)))
rdd1: org.apache.spark.rdd.RDD[(String, Int)] = ParallelCollectionRDD[27] at parallelize at <console>:24
scala> rdd1.countByKey
res8: scala.collection.Map[String,Long] = Map(a -> 1, b -> 2, c -> 2)          
scala> rdd1.countByValue
res9: scala.collection.Map[(String, Int),Long] = Map((c,2) -> 1, (a,1) -> 1, (b,2) -> 2, (c,1) -> 1)

filterByRange

scala> val rdd1 = sc.parallelize(List(("e", 5), ("c", 3), ("d", 4), ("c", 2), ("a", 1),("b",6)))
rdd1: org.apache.spark.rdd.RDD[(String, Int)] = ParallelCollectionRDD[33] at parallelize at <console>:24
//注意：这里传入的参数，是左闭右闭的区间
scala> val rdd2 = rdd1.filterByRange("b","d")
rdd2: org.apache.spark.rdd.RDD[(String, Int)] = MapPartitionsRDD[34] at filterByRange at <console>:26
scala> rdd2.collect
res10: Array[(String, Int)] = Array((c,3), (d,4), (c,2), (b,6))

flatMapValues：对values进行处理，类似flatMap，会将key和每一个分出来的value组成映射

scala> val rdd3 = sc.parallelize(List(("a", "1 2"), ("b", "3 4")))
rdd3: org.apache.spark.rdd.RDD[(String, String)] = ParallelCollectionRDD[35] at parallelize at <console>:24
scala> val rdd4 = rdd3.flatMapValues(_.split(" "))
rdd4: org.apache.spark.rdd.RDD[(String, String)] = MapPartitionsRDD[36] at flatMapValues at <console>:26
scala> rdd4.collect
res11: Array[(String, String)] = Array((a,1), (a,2), (b,3), (b,4))

mapValues：不改变key，只针对传入的键值对的value进行计算，类似于map；注意与上面的flatMapValues的区别，它不会改变传入的key-value对，只是将value按照传入的函数进行处理；

scala> val rdd3 = sc.parallelize(List(("a",(1,2)),("b",(2,4))))
rdd3: org.apache.spark.rdd.RDD[(String, (Int, Int))] = ParallelCollectionRDD[57] at parallelize at <console>:24
scala> rdd3.mapValues(x=>x._1 + x._2).collect
res34: Array[(String, Int)] = Array((a,3), (b,6))
------------------------------------------------------------------------
如果使用flatMapValues，结果如下，它将value全部拆开跟key组成映射
scala> rdd3.flatMapValues(x=>x + "").collect
res36: Array[(String, Char)] = Array((a,(), (a,1), (a,,), (a,2), (a,)), (b,(), (b,2), (b,,), (b,4), (b,)))

foldByKey：根据key分组，对每一组的value进行计算

scala> val rdd1 = sc.parallelize(List("dog", "wolf", "cat", "bear"), 2)
rdd1: org.apache.spark.rdd.RDD[String] = ParallelCollectionRDD[37] at parallelize at <console>:24
scala> val rdd2 = rdd1.map(x=>(x.length,x))
rdd2: org.apache.spark.rdd.RDD[(Int, String)] = MapPartitionsRDD[38] at map at <console>:26
scala> rdd2.collect
res12: Array[(Int, String)] = Array((3,dog), (4,wolf), (3,cat), (4,bear))
-----------------------------------------------------------------------------
scala> val rdd3 = rdd2.foldByKey("")(_+_)
rdd3: org.apache.spark.rdd.RDD[(Int, String)] = ShuffledRDD[39] at foldByKey at <console>:28
scala> rdd3.collect
res13: Array[(Int, String)] = Array((4,bearwolf), (3,dogcat))
scala> val rdd3 = rdd2.foldByKey(" ")(_+_).collect
rdd3: Array[(Int, String)] = Array((4," bear wolf"), (3," dog cat"))
-----------------------------------------------------------------------------
//进行wordcout的计算
val rdd = sc.textFile("hdfs://node-1.itcast.cn:9000/wc").flatMap(_.split(" ")).map((_, 1))
rdd.foldByKey(0)(_+_)

keyBy：以传入的参数作为key，生成新的RDD

scala> val rdd1 = sc.parallelize(List("dog", "salmon", "salmon", "rat", "elephant"), 3)
rdd1: org.apache.spark.rdd.RDD[String] = ParallelCollectionRDD[41] at parallelize at <console>:24
scala> val rdd2 = rdd1.keyBy(_.length)
rdd2: org.apache.spark.rdd.RDD[(Int, String)] = MapPartitionsRDD[42] at keyBy at <console>:26
scala> rdd2.collect
res14: Array[(Int, String)] = Array((3,dog), (6,salmon), (6,salmon), (3,rat), (8,elephant))

keys、values：取出rdd的key或者value，生成新的RDD

scala> val rdd1 = sc.parallelize(List(("e", 5), ("c", 3), ("d", 4), ("c", 2), ("a", 1)))
rdd1: org.apache.spark.rdd.RDD[(String, Int)] = ParallelCollectionRDD[43] at parallelize at <console>:24
scala> rdd1.keys.collect
res16: Array[String] = Array(e, c, d, c, a)
scala> rdd1.values.collect
res17: Array[Int] = Array(5, 3, 4, 2, 1)

[Spark]Spark RDD&算子总结

一、RDD概述

二、RDD编程API---包含两种算子

三、RDD编程----高级API

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