How to join a random rdd to another rdd?

Question

I have an RDD of strings (but could be anything really) that I would like to innerjoin with a rdd of random normals. I know this can be solved with a .zipWithIndex on both RDDs but this doesn't seem like it will scale well, is there a way to initialize a random rdd with data from another RDD or another method that would be faster? Here is what I've done with .zipWithIndex :

import org.apache.spark.mllib.random.RandomRDDs
import org.apache.spark.rdd.RDD

val numExamples = 10 // number of rows in RDD 
val maNum   = 7
val commonStdDev   = 0.1 // common standard deviation 1/10, makes variance = 0.01
val normalVectorRDD = RandomRDDs.normalVectorRDD(sc, numRows = numExamples, numCols = maNum) 
val rescaledNormals = normalVectorRDD.map{myVec => myVec.toArray.map(x => x*commonStdDev)}
  .zipWithIndex
  .map{case (key,value) => (value,(key))} 

val otherRDD = sc.textFile(otherFilepath)
  .zipWithIndex
  .map{case (key,value) => (value,(key))} 

val joinedRDD = otherRDD.join(rescaledNormals).map{case(key,(other,dArray)) => (other,dArray)}

Answer 1

In general I wouldn't worry about zipWithIndex . While it requires additional actions it belongs to relatively cheap operations. join however is a different thing.

Since vector content doesn't depend on the value from the otherRDD , it makes more sense to generate it in place. All you have to do is to mimic RandomRDDs logic:

import org.apache.spark.mllib.random.StandardNormalGenerator 
import org.apache.spark.ml.linalg.DenseVector  // or org.apache.spark.mllib

val vectorSize = 42
val stdDev = 0.1
val seed = scala.util.Random.nextLong  // Or set manually

// Define seeds for each partition
val random = new scala.util.Random(seed)
val seeds = (0 until otherRDD.getNumPartitions).map(
  i => i -> random.nextLong
).toMap

otherRDD.mapPartitionsWithIndex((i, iter) => {
  val generator = new StandardNormalGenerator()
  generator.setSeed(seeds(i))
  iter.map(x => 
    (x, new DenseVector(Array.fill(vectorSize)(generator.nextValue() * stdDev)))
  )
})