rdd之后的数字是什么意思

Question

rdd后括号中数字的含义是什么？

Answer 1

RDD之后的数字是其标识符：

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Using Scala version 2.11.8 (OpenJDK 64-Bit Server VM, Java 1.8.0_151)
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scala> val rdd = sc.range(0, 42)
rdd: org.apache.spark.rdd.RDD[Long] = MapPartitionsRDD[1] at range at <console>:24

scala> rdd.id
res0: Int = 1

它用于跟踪会话中的RDD，例如用于caching：

之类的目的

scala> rdd.cache
res1: rdd.type = MapPartitionsRDD[1] at range at <console>:24

scala> rdd.count
res2: Long = 42

scala> sc.getPersistentRDDs
res3: scala.collection.Map[Int,org.apache.spark.rdd.RDD[_]] = Map(1 -> MapPartitionsRDD[1] at range at <console>:24)

这个数字很简单incremental integer（nextRddId只是AtomicInteger）：

private[spark] def newRddId(): Int = nextRddId.getAndIncrement()

生成when RDD is constructed：

/** A unique ID for this RDD (within its SparkContext). */
val id: Int = sc.newRddId()

所以，如果我们遵循：

scala> val pairs1 = sc.parallelize(Seq((1, "foo")))
pairs1: org.apache.spark.rdd.RDD[(Int, String)] = ParallelCollectionRDD[2] at parallelize at <console>:24

scala> val pairs2 = sc.parallelize(Seq((1, "bar")))
pairs2: org.apache.spark.rdd.RDD[(Int, String)] = ParallelCollectionRDD[3] at parallelize at <console>:24


scala> pairs1.id
res5: Int = 2

scala> pairs2.id
res6: Int = 3

你会看到2和3，如果你执行

scala> pairs1.join(pairs2).foreach(_ => ())

您期望4，可通过检查用户界面确认：

我们还可以看到join在封面（RDDs和5）下创建了一些新的6。