Spark: How to union a List<RDD> to RDD

Question

I'm pretty new to spark and scala language and would like to union all RDDs in a List as below(List<RDD> to RDD) :

 val data = for (item <- paths) yield {
        val ad_data_path = item._1
        val ad_data = SparkCommon.sc.textFile(ad_data_path).map {
            line => {
                val ad_data = new AdData(line)
                (ad_data.ad_id, ad_data)
            }
        }.distinct()
    }
 val ret = SparkCommon.sc.parallelize(data).reduce(_ ++ _)

I run the code in IntelliJ while always get an error as:

ava.lang.NullPointerException
at org.apache.spark.rdd.RDD.<init>(RDD.scala:125)
at org.apache.spark.rdd.UnionRDD.<init>(UnionRDD.scala:59)
at org.apache.spark.rdd.RDD.union(RDD.scala:438)
at org.apache.spark.rdd.RDD.$plus$plus(RDD.scala:444)
at data.GenerateData$$anonfun$load_data$1.apply(GenerateData.scala:99)
at data.GenerateData$$anonfun$load_data$1.apply(GenerateData.scala:99)
at scala.collection.TraversableOnce$$anonfun$reduceLeft$1.apply(TraversableOnce.scala:177)
at scala.collection.TraversableOnce$$anonfun$reduceLeft$1.apply(TraversableOnce.scala:172)
at scala.collection.Iterator$class.foreach(Iterator.scala:727)
at org.apache.spark.InterruptibleIterator.foreach(InterruptibleIterator.scala:28)
at scala.collection.TraversableOnce$class.reduceLeft(TraversableOnce.scala:172)
at org.apache.spark.InterruptibleIterator.reduceLeft(InterruptibleIterator.scala:28)
at org.apache.spark.rdd.RDD$$anonfun$18.apply(RDD.scala:847)
at org.apache.spark.rdd.RDD$$anonfun$18.apply(RDD.scala:845)
at org.apache.spark.SparkContext$$anonfun$26.apply(SparkContext.scala:1157)
at org.apache.spark.SparkContext$$anonfun$26.apply(SparkContext.scala:1157)
at org.apache.spark.scheduler.ResultTask.runTask(ResultTask.scala:62)
at org.apache.spark.scheduler.Task.run(Task.scala:54)
at org.apache.spark.executor.Executor$TaskRunner.run(Executor.scala:177)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:745)

Anyone has any idea about the error? Thanks in advance:)

Answer 1

This might be the cause,

val listA = 1 to 10
for(i <- listA; if i%2 == 0)yield {i}

will return Vector(2,4,6,8,10), whereas

for(i <- listA; if i%2 == 0)yield {val c = i}

will return Vector ((),(),(),(),())

That is exactly what is happening in your case. You are initializing ad_data but not returning it back to yield.

As far as your question is concerned ,i.e List[RDD] to RDD

here is the solution:

val listA = sc.parallelize(1 to 10)
val listB = sc.parallelize(10 to 1 by -1)

creating list of 2 RDDS

val listC = List(listA,listB)

convert List[RDD] to RDD

val listD = listC.reduce(_ union _)

Hope, this answers your question.