RDD之六:Action算子
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本质上在Actions算子中通过SparkContext执行提交作业的runJob操作,触发了RDD DAG的执行。
根据Action算子的输出空间将Action算子进行分类:无输出、 HDFS、 Scala集合和数据类型。
无输出
foreach
对RDD中的每个元素都应用f函数操作,不返回RDD和Array,而是返回Uint。
图中,foreach算子通过用户自定义函数对每个数据项进行操作。 本例中自定义函数为println,控制台打印所有数据项。
源码:
- /**
- * Applies a function f to all elements of this RDD.
- */
- def foreach(f: T => Unit) {
- val cleanF = sc.clean(f)
- sc.runJob(this, (iter: Iterator[T]) => iter.foreach(cleanF))
- }
HDFS
(1)saveAsTextFile
函数将数据输出,存储到HDFS的指定目录。将RDD中的每个元素映射转变为(Null,x.toString),然后再将其写入HDFS。
图中,左侧的方框代表RDD分区,右侧方框代表HDFS的Block。 通过函数将RDD的每个分区存储为HDFS中的一个Block。
源码:
- /**
- * Save this RDD as a text file, using string representations of elements.
- */
- def saveAsTextFile(path: String) {
- // https://issues.apache.org/jira/browse/SPARK-2075
- //
- // NullWritable is a `Comparable` in Hadoop 1.+, so the compiler cannot find an implicit
- // Ordering for it and will use the default `null`. However, it‘s a `Comparable[NullWritable]`
- // in Hadoop 2.+, so the compiler will call the implicit `Ordering.ordered` method to create an
- // Ordering for `NullWritable`. That‘s why the compiler will generate different anonymous
- // classes for `saveAsTextFile` in Hadoop 1.+ and Hadoop 2.+.
- //
- // Therefore, here we provide an explicit Ordering `null` to make sure the compiler generate
- // same bytecodes for `saveAsTextFile`.
- val nullWritableClassTag = implicitly[ClassTag[NullWritable]]
- val textClassTag = implicitly[ClassTag[Text]]
- val r = this.mapPartitions { iter =>
- val text = new Text()
- iter.map { x =>
- text.set(x.toString)
- (NullWritable.get(), text)
- }
- }
- RDD.rddToPairRDDFunctions(r)(nullWritableClassTag, textClassTag, null)
- .saveAsHadoopFile[TextOutputFormat[NullWritable, Text]](path)
- }
- /**
- * Save this RDD as a compressed text file, using string representations of elements.
- */
- def saveAsTextFile(path: String, codec: Class[_ <: CompressionCodec]) {
- // https://issues.apache.org/jira/browse/SPARK-2075
- val nullWritableClassTag = implicitly[ClassTag[NullWritable]]
- val textClassTag = implicitly[ClassTag[Text]]
- val r = this.mapPartitions { iter =>
- val text = new Text()
- iter.map { x =>
- text.set(x.toString)
- (NullWritable.get(), text)
- }
- }
- RDD.rddToPairRDDFunctions(r)(nullWritableClassTag, textClassTag, null)
- .saveAsHadoopFile[TextOutputFormat[NullWritable, Text]](path, codec)
- }
(2)saveAsObjectFile
saveAsObjectFile将分区中的每10个元素组成一个Array,然后将这个Array序列化,映射为(Null,BytesWritable(Y))的元素,写入HDFS为SequenceFile的格式。
图中,左侧方框代表RDD分区,右侧方框代表HDFS的Block。 通过函数将RDD的每个分区存储为HDFS上的一个Block。
源码:
- /**
- * Save this RDD as a SequenceFile of serialized objects.
- */
- def saveAsObjectFile(path: String) {
- this.mapPartitions(iter => iter.grouped(10).map(_.toArray))
- .map(x => (NullWritable.get(), new BytesWritable(Utils.serialize(x))))
- .saveAsSequenceFile(path)
- }
Scala集合和数据类型
(1)collect
collect相当于toArray,toArray已经过时不推荐使用,collect将分布式的RDD返回为一个单机的scala Array数组。 在这个数组上运用scala的函数式操作。
图中,左侧方框代表RDD分区,右侧方框代表单机内存中的数组。通过函数操作,将结果返回到Driver程序所在的节点,以数组形式存储。
源码:
- /**
- * Return an array that contains all of the elements in this RDD.
- */
- def collect(): Array[T] = {
- val results = sc.runJob(this, (iter: Iterator[T]) => iter.toArray)
- Array.concat(results: _*)
- }
(2)collectAsMap
collectAsMap对(K,V)型的RDD数据返回一个单机HashMap。对于重复K的RDD元素,后面的元素覆盖前面的元素。
图中,左侧方框代表RDD分区,右侧方框代表单机数组。数据通过collectAsMap函数返回给Driver程序计算结果,结果以HashMap形式存储。
源码:
- /**
- * Return the key-value pairs in this RDD to the master as a Map.
- *
- * Warning: this doesn‘t return a multimap (so if you have multiple values to the same key, only
- * one value per key is preserved in the map returned)
- */
- def collectAsMap(): Map[K, V] = {
- val data = self.collect()
- val map = new mutable.HashMap[K, V]
- map.sizeHint(data.length)
- data.foreach { pair => map.put(pair._1, pair._2) }
- map
- }
(3)reduceByKeyLocally
实现的是先reduce再collectAsMap的功能,先对RDD的整体进行reduce操作,然后再收集所有结果返回为一个HashMap。
源码:
- /**
- * Merge the values for each key using an associative reduce function, but return the results
- * immediately to the master as a Map. This will also perform the merging locally on each mapper
- * before sending results to a reducer, similarly to a "combiner" in MapReduce.
- */
- def reduceByKeyLocally(func: (V, V) => V): Map[K, V] = {
- if (keyClass.isArray) {
- throw new SparkException("reduceByKeyLocally() does not support array keys")
- }
- val reducePartition = (iter: Iterator[(K, V)]) => {
- val map = new JHashMap[K, V]
- iter.foreach { pair =>
- val old = map.get(pair._1)
- map.put(pair._1, if (old == null) pair._2 else func(old, pair._2))
- }
- Iterator(map)
- } : Iterator[JHashMap[K, V]]
- val mergeMaps = (m1: JHashMap[K, V], m2: JHashMap[K, V]) => {
- m2.foreach { pair =>
- val old = m1.get(pair._1)
- m1.put(pair._1, if (old == null) pair._2 else func(old, pair._2))
- }
- m1
- } : JHashMap[K, V]
- self.mapPartitions(reducePartition).reduce(mergeMaps)
- }
(4)lookup
Lookup函数对(Key,Value)型的RDD操作,返回指定Key对应的元素形成的Seq。这个函数处理优化的部分在于,如果这个RDD包含分区器,则只会对应处理K所在的分区,然后返回由(K,V)形成的Seq。如果RDD不包含分区器,则需要对全RDD元素进行暴力扫描处理,搜索指定K对应的元素。
图中,左侧方框代表RDD分区,右侧方框代表Seq,最后结果返回到Driver所在节点的应用中。
源码:
- /**
- * Return the list of values in the RDD for key `key`. This operation is done efficiently if the
- * RDD has a known partitioner by only searching the partition that the key maps to.
- */
- def lookup(key: K): Seq[V] = {
- self.partitioner match {
- case Some(p) =>
- val index = p.getPartition(key)
- val process = (it: Iterator[(K, V)]) => {
- val buf = new ArrayBuffer[V]
- for (pair <- it if pair._1 == key) {
- buf += pair._2
- }
- buf
- } : Seq[V]
- val res = self.context.runJob(self, process, Array(index), false)
- res(0)
- case None =>
- self.filter(_._1 == key).map(_._2).collect()
- }
- }
(5)count
count返回整个RDD的元素个数。
图中,返回数据的个数为5。一个方块代表一个RDD分区。
源码:
- /**
- * Return the number of elements in the RDD.
- */
- def count(): Long = sc.runJob(this, Utils.getIteratorSize _).sum
(6)top
top可返回最大的k个元素。
相近函数说明:
- top返回最大的k个元素。
- take返回最小的k个元素。
- takeOrdered返回最小的k个元素, 并且在返回的数组中保持元素的顺序。
- first相当于top( 1) 返回整个RDD中的前k个元素, 可以定义排序的方式Ordering[T]。返回的是一个含前k个元素的数组。
源码:
- /**
- * Returns the top k (largest) elements from this RDD as defined by the specified
- * implicit Ordering[T]. This does the opposite of [[takeOrdered]]. For example:
- * {{{
- * sc.parallelize(Seq(10, 4, 2, 12, 3)).top(1)
- * // returns Array(12)
- *
- * sc.parallelize(Seq(2, 3, 4, 5, 6)).top(2)
- * // returns Array(6, 5)
- * }}}
- *
- * @param num k, the number of top elements to return
- * @param ord the implicit ordering for T
- * @return an array of top elements
- */
- def top(num: Int)(implicit ord: Ordering[T]): Array[T] = takeOrdered(num)(ord.reverse)
(7)reduce
reduce函数相当于对RDD中的元素进行reduceLeft函数的操作。
reduceLeft先对两个元素
- /**
- * Reduces the elements of this RDD using the specified commutative and
- * associative binary operator.
- */
- def reduce(f: (T, T) => T): T = {
- val cleanF = sc.clean(f)
- val reducePartition: Iterator[T] => Option[T] = iter => {
- if (iter.hasNext) {
- Some(iter.reduceLeft(cleanF))
- } else {
- None
- }
- }
- var jobResult: Option[T] = None
- val mergeResult = (index: Int, taskResult: Option[T]) => {
- if (taskResult.isDefined) {
- jobResult = jobResult match {
- case Some(value) => Some(f(value, taskResult.get))
- case None => taskResult
- }
- }
- }
- sc.runJob(this, reducePartition, mergeResult)
- // Get the final result out of our Option, or throw an exception if the RDD was empty
- jobResult.getOrElse(throw new UnsupportedOperationException("empty collection"))
- }
(8)fold
fold和reduce的原理相同,但是与reduce不同,相当于每个reduce时,迭代器取的第一个元素是zeroValue。
图中,通过用户自定义函数进行fold运算,图中的一个方框代表一个RDD分区。
源码:
- /**
- * Aggregate the elements of each partition, and then the results for all the partitions, using a
- * given associative function and a neutral "zero value". The function op(t1, t2) is allowed to
- * modify t1 and return it as its result value to avoid object allocation; however, it should not
- * modify t2.
- */
- def fold(zeroValue: T)(op: (T, T) => T): T = {
- // Clone the zero value since we will also be serializing it as part of tasks
- var jobResult = Utils.clone(zeroValue, sc.env.closureSerializer.newInstance())
- val cleanOp = sc.clean(op)
- val foldPartition = (iter: Iterator[T]) => iter.fold(zeroValue)(cleanOp)
- val mergeResult = (index: Int, taskResult: T) => jobResult = op(jobResult, taskResult)
- sc.runJob(this, foldPartition, mergeResult)
- jobResult
- }
(9)aggregate
aggregate先对每个分区的所有元素进行aggregate操作,再对分区的结果进行fold操作。
aggreagate与fold和reduce的不同之处在于,aggregate相当于采用归并的方式进行数据聚集,这种聚集是并行化的。 而在fold和reduce函数的运算过程中,每个分区中需要进行串行处理,每个分区串行计算完结果,结果再按之前的方式进行聚集,并返回最终聚集结果。
图中,通过用户自定义函数对RDD 进行aggregate的聚集操作,图中的每个方框代表一个RDD分区。
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.
- */
- def aggregate[U: ClassTag](zeroValue: U)(seqOp: (U, T) => U, combOp: (U, U) => U): U = {
- // Clone the zero value since we will also be serializing it as part of tasks
- var jobResult = Utils.clone(zeroValue, sc.env.closureSerializer.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
- }
原文链接:http://blog.csdn.net/jasonding1354
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