What does the number meaning after the rdd

问题:

What does the meaning of the number in the bracket after rdd?

回答1:

The number after RDD is its identifier:

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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

It is used to track RDD across the session, for example for purposes like caching:

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)

This number is simple an incremental integer (nextRddId is just an AtomicInteger):

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

generated when RDD is constructed:

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

so if we followed:

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

you'll see 2 and 3, and if you execute

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

you'd expect 4, which can confirmed by checking the UI:

We can also see that join creates a few new RDDs under the covers (5 and 6).