mapPartitions(func) Similar to map, but runs separately on each partition (block) of the RDD, so func must be of type Iterator
mapPartitions
mapPartitions是map的一个变种。map的输入函数是应用于RDD中每个元素,而mapPartitions的输入函数是应用于每个分区,也就是把每个分区中的内容作为整体来处理的。
它的函数定义为:
def mapPartitions[U: ClassTag](f: Iterator[T] => Iterator[U], preservesPartitioning: Boolean = false): RDD[U]
f即为输入函数,它处理每个分区里面的内容。每个分区中的内容将以Iterator[T]传递给输入函数f,f的输出结果是Iterator[U]。最终的RDD由所有分区经过输入函数处理后的结果合并起来的。
举例:
scala> val a = sc.parallelize(1 to 9, 3)
scala> def myfunc[T](iter: Iterator[T]) : Iterator[(T, T)] = {
var res = List[(T, T)]()
var pre = iter.next while (iter.hasNext) {
val cur = iter.next;
res .::= (pre, cur) pre = cur;
}
res.iterator
}
scala> a.mapPartitions(myfunc).collect
res0: Array[(Int, Int)] = Array((2,3), (1,2), (5,6), (4,5), (8,9), (7,8))
上述例子中的函数myfunc是把分区中一个元素和它的下一个元素组成一个Tuple。因为分区中最后一个元素没有下一个元素了,所以(3,4)和(6,7)不在结果中。
mapPartitions还有些变种,比如mapPartitionsWithContext,它能把处理过程中的一些状态信息传递给用户指定的输入函数。还有mapPartitionsWithIndex,它能把分区的index传递给用户指定的输入函数。
在scala eclipse中运行
package yanan.spark.core.transformations.example
import org.apache.spark.SparkConf
import org.apache.spark.SparkContext
object TransformationsTest {
def mapPartitionsFunc[T](iter: Iterator[T]): Iterator[(T, T)] = {
var res = List[(T, T)]()
var pre = iter.next
while (iter.hasNext) {
val cur = iter.next
res.::=(pre, cur)
pre = cur;
}
res.iterator
}
def mapPartitionsTest(sc: SparkContext) = {
val a = sc.parallelize(1 to 9, 3)
a.mapPartitions(mapPartitionsFunc).collect.foreach(println)
}
def main(args: Array[String]) {
val conf = new SparkConf().setAppName(s"Book example: Scala").setMaster("local[2]")
val sc = new SparkContext(conf)
mapPartitionsTest(sc)
sc.stop()
}
}
pom.xml文件:
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>com.yanan.spark_maven</groupId>
<artifactId>spark1.3.1</artifactId>
<version>0.0.1-SNAPSHOT</version>
<packaging>jar</packaging>
<name>spark_maven</name>
<url>http://maven.apache.org</url>
<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<jackson.version>1.9.13</jackson.version>
<!-- <spark.version>1.3.1</spark.version> -->
<spark.version>1.4.0</spark.version>
</properties>
<dependencies>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>3.8.1</version>
<scope>test</scope>
</dependency>
<dependency>
<groupId>org.scala-lang</groupId>
<artifactId>scala-library</artifactId>
<version>2.10.4</version>
</dependency>
<dependency>
<groupId>org.apache.spark</groupId>
<artifactId>spark-core_2.10</artifactId>
<version>${spark.version}</version>
</dependency>
<dependency>
<groupId>org.apache.spark</groupId>
<artifactId>spark-mllib_2.10</artifactId>
<version>${spark.version}</version>
</dependency>
<!--<dependency> <groupId>org.apache.spark</groupId> <artifactId>spark-sql_2.10</artifactId>
<version>1.3.1</version> </dependency> <dependency> <groupId>org.apache.spark</groupId>
<artifactId>spark-hive_2.10</artifactId> <version>1.3.1</version> </dependency>
<dependency> <groupId>org.apache.spark</groupId> <artifactId>spark-bagel_2.10</artifactId>
<version>1.3.1</version> </dependency>
<dependency>
<groupId>org.apache.spark</groupId>
<artifactId>spark-graphx_2.10</artifactId>
<version>1.3.1</version>
</dependency> -->
<!-- specify the version for json_truple <dependency> <groupId>org.codehaus.jackson</groupId>
<artifactId>jackson-core-asl</artifactId> <version>${jackson.version}</version>
</dependency> <dependency> <groupId>org.codehaus.jackson</groupId> <artifactId>jackson-mapper-asl</artifactId>
<version>${jackson.version}</version> </dependency> -->
</dependencies>
<build>
<plugins>
<plugin>
<groupId>org.scala-tools</groupId>
<artifactId>maven-scala-plugin</artifactId>
<executions>
<execution>
<goals>
<goal>compile</goal>
<goal>testCompile</goal>
</goals>
</execution>
</executions>
</plugin>
</plugins>
</build>
<pluginRepositories>
<pluginRepository>
<id>scala-tools.org</id>
<name>Scala-tools Maven2 Repository</name>
<url>http://scala-tools.org/repo-releases</url>
</pluginRepository>
</pluginRepositories>
<repositories>
<repository>
<id>cloudera-repo-releases</id>
<url>https://repository.cloudera.com/artifactory/repo/</url>
</repository>
</repositories>
</project>