# Spark RDD Action 简单用例(一)

## collectAsMap(): Map[K, V]

返回key-value对，key是唯一的，如果rdd元素中同一个key对应多个value，则只会保留一个。/** * 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) * * @note this method should only be used if the resulting data is expected to be small, as * all the data is loaded into the driver's memory. */def collectAsMap(): Map[K, V]
scala> val rdd = sc.parallelize(List(("A",1),("A",2),("A",3),("B",1),("B",2),("C",3)),3)
rdd: org.apache.spark.rdd.RDD[(String, Int)] = ParallelCollectionRDD[0] at parallelize at <console>:24

scala> rdd.collectAsMap
res0: scala.collection.Map[String,Int] = Map(A -> 3, C -> 3, B -> 2)   

## countByKey(): Map[K, Long]

计算有多少个不同的key./** * Count the number of elements for each key, collecting the results to a local Map. * * Note that this method should only be used if the resulting map is expected to be small, as * the whole thing is loaded into the driver's memory. * To handle very large results, consider using rdd.mapValues(_ => 1L).reduceByKey(_ + _), which * returns an RDD[T, Long] instead of a map. */def countByKey(): Map[K, Long] = self.withScope {  self.mapValues(_ => 1L).reduceByKey(_ + _).collect().toMap}
scala> val rdd = sc.parallelize(List((1,1),(1,2),(1,3),(2,1),(2,2),(2,3)),3)
rdd: org.apache.spark.rdd.RDD[(Int, Int)] = ParallelCollectionRDD[5] at parallelize at <console>:24

scala> rdd.countByKey
res5: scala.collection.Map[Int,Long] = Map(1 -> 3, 2 -> 3)

## countByValue()

计算不同的value个数，该函数首先通过map将每个元素转成(value,null)的key-value（value为null）对，然后调用countByKey进行统计。/** * Return the count of each unique value in this RDD as a local map of (value, count) pairs. * * Note that this method should only be used if the resulting map is expected to be small, as * the whole thing is loaded into the driver's memory. * To handle very large results, consider using rdd.map(x =&gt; (x, 1L)).reduceByKey(_ + _), which * returns an RDD[T, Long] instead of a map. */def countByValue()(implicit ord: Ordering[T] = null): Map[T, Long] = withScope {  map(value => (value, null)).countByKey()}
scala> val rdd = sc.parallelize(List(1,2,3,4,5,4,4,3,2,1))
rdd: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[18] at parallelize at <console>:24

scala> rdd.countByValue
res12: scala.collection.Map[Int,Long] = Map(5 -> 1, 1 -> 2, 2 -> 2, 3 -> 2, 4 -> 3)

## lookup(key: K)

根据key值搜索所有的value./** * 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]
scala> val rdd = sc.parallelize(List(("A",1),("A",2),("A",3),("B",1),("B",2),("C",3)),3)
rdd: org.apache.spark.rdd.RDD[(String, Int)] = ParallelCollectionRDD[3] at parallelize at <console>:24

scala> rdd.lookup("A")
res2: Seq[Int] = WrappedArray(1, 2, 3)

## checkpoint()

将RDD数据根据设置的checkpoint目录保存至硬盘中。

/** * Mark this RDD for checkpointing. It will be saved to a file inside the checkpoint * directory set with SparkContext#setCheckpointDir and all references to its parent * RDDs will be removed. This function must be called before any job has been * executed on this RDD. It is strongly recommended that this RDD is persisted in * memory, otherwise saving it on a file will require recomputation. */def checkpoint(): Unit
/*通过linux命令创建/home/check目录后，设置checkpoint directory*/
scala> sc.setCheckpointDir("/home/check")

scala> val rdd = sc.parallelize(List(("A",1),("A",2),("A",3),("B",1),("B",2),("C",3)),3)
rdd: org.apache.spark.rdd.RDD[(String, Int)] = ParallelCollectionRDD[6] at parallelize at <console>:24

/*
*执行下面的代码会在/home/check目录下创建一个空的目录/home/check/5545e4ca-d53d-4d93-aaf4-fd3c74f1ea49
*/
scala> rdd.checkpoint

/*

*/
scala> rdd.count
res5: Long = 6           

[root@localhost ~]# ll -a /home/check/5545e4ca-d53d-4d93-aaf4-fd3c74f1ea49/
total 8
drwxr-xr-x. 2 root root 4096 Sep  4 10:29 .
drwxr-xr-x. 3 root root 4096 Sep  4 10:29 ..
[root@localhost ~]# ll -a /home/check/5545e4ca-d53d-4d93-aaf4-fd3c74f1ea49/
total 12
drwxr-xr-x. 3 root root 4096 Sep  4 10:30 .
drwxr-xr-x. 3 root root 4096 Sep  4 10:29 ..
drwxr-xr-x. 2 root root 4096 Sep  4 10:30 rdd-6
[root@localhost ~]# ll -a /home/check/5545e4ca-d53d-4d93-aaf4-fd3c74f1ea49/rdd-6/
total 32
drwxr-xr-x. 2 root root 4096 Sep  4 10:30 .
drwxr-xr-x. 3 root root 4096 Sep  4 10:30 ..
-rw-r--r--. 1 root root  171 Sep  4 10:30 part-00000
-rw-r--r--. 1 root root   12 Sep  4 10:30 .part-00000.crc
-rw-r--r--. 1 root root  170 Sep  4 10:30 part-00001
-rw-r--r--. 1 root root   12 Sep  4 10:30 .part-00001.crc
-rw-r--r--. 1 root root  170 Sep  4 10:30 part-00002
-rw-r--r--. 1 root root   12 Sep  4 10:30 .part-00002.crc

## collect()

返回RDD所有元素的数组。/** * Return an array that contains all of the elements in this RDD. * * @note this method should only be used if the resulting array is expected to be small, as * all the data is loaded into the driver's memory. */def collect(): Array[T]
scala> val rdd = sc.parallelize(1 to 10,3)
rdd: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[10] at parallelize at <console>:24

scala> rdd.collect
res8: Array[Int] = Array(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

## toLocalIterator: Iterator[T]

返回一个包含所有算的迭代器。/** * Return an iterator that contains all of the elements in this RDD. * * The iterator will consume as much memory as the largest partition in this RDD. * * Note: this results in multiple Spark jobs, and if the input RDD is the result * of a wide transformation (e.g. join with different partitioners), to avoid * recomputing the input RDD should be cached first. */def toLocalIterator: Iterator[T]
scala> val rdd = sc.parallelize(1 to 10,2)
rdd: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[0] at parallelize at <console>:24

scala> val it = rdd.toLocalIterator
it: Iterator[Int] = non-empty iterator

scala> while(it.hasNext){
| println(it.next)
| }
1
2
3
4
5
6
7
8
9
10

## count()

返回RDD中元素的数量。/** * Return the number of elements in the RDD. */def count(): Long
scala> val rdd = sc.parallelize(1 to 10,2)
rdd: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[0] at parallelize at <console>:24
scala> rdd.count
res1: Long = 10

## dependencies

返回该RDD的依赖RDD的地址。/** * Get the list of dependencies of this RDD, taking into account whether the * RDD is checkpointed or not. */final def dependencies: Seq[Dependency[_]]
scala> val rdd = sc.parallelize(1 to 10,2)
rdd: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[0] at parallelize at <console>:24
scala> val rdd1 = rdd.filter(_>3)
rdd1: org.apache.spark.rdd.RDD[Int] = MapPartitionsRDD[1] at filter at <console>:26

scala> val rdd2 = rdd1.filter(_<6)
rdd2: org.apache.spark.rdd.RDD[Int] = MapPartitionsRDD[2] at filter at <console>:28

scala> rdd2.dependencies
res2: Seq[org.apache.spark.Dependency[_]] = List(org.apache.spark.OneToOneDependency@21c882b5)

## partitions

以数组形式返回RDD各分区地址/** * Get the array of partitions of this RDD, taking into account whether the * RDD is checkpointed or not. */final def partitions: Array[Partition]
scala> val rdd = sc.parallelize(1 to 10,2)
rdd: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[3] at parallelize at <console>:24

scala> rdd.partitions
res4: Array[org.apache.spark.Partition] = Array(org.apache.spark.rdd.ParallelCollectionPartition@70c, org.apache.spark.rdd.ParallelCollectionPartition@70d)

## first()

返回RDD的第一个元素。/** * Return the first element in this RDD. */def first(): T
scala> val rdd = sc.parallelize(1 to 10,2)
rdd: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[3] at parallelize at <console>:24
scala> rdd.first
res5: Int = 1

## fold(zeroValue: T)(op: (T, T) => T)

使用zeroValue和每个分区的元素进行聚合运算，最后各分区结果和zeroValue再进行一次聚合运算。/** * @param zeroValue the initial value for the accumulated result of each partition for the op *                  operator, and also the initial value for the combine results from different *                  partitions for the op operator - this will typically be the neutral *                  element (e.g. Nil for list concatenation or 0 for summation) * @param op an operator used to both accumulate results within a partition and combine results *                  from different partitions */def fold(zeroValue: T)(op: (T, T) => T): T
scala> val rdd = sc.parallelize(1 to 5)
rdd: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[6] at parallelize at <console>:24

scala> rdd.fold(10)(_+_)
res13: Int = 35

posted @ 2016-09-04 15:31  alianblog  阅读(...)  评论(...编辑  收藏