Merge multiple RDD generated in loop

Question

I am calling a function in scala which gives an RDD[(Long,Long,Double)] as its output.

def helperfunction(): RDD[(Long, Long, Double)]

I call this function in loop in another part of the code and I want to merge all the generated RDDs. The loop calling the function looks something like this

for (i <- 1 to n){
    val tOp = helperfunction()
    // merge the generated tOp
}

What I want to do is something similar to what StringBuilder would do for you in Java when you wanted to merge the strings. I have looked at techniques of merging RDDs, which mostly point to using union function like this

RDD1.union(RDD2)

But this requires both RDDs to be generated before taking their union. I though of initializing a var RDD1 to accumulate the results outside the for loop but I am not sure how can I initialize a blank RDD of type [(Long,Long,Double)]. Also I am starting out with spark, so I am not even sure if this is the most elegant method to solve this problem.

Answer 1

Instead of using vars, you can use functional programming paradigms to achieve what you want :

val rdd = (1 to n).map(x => helperFunction()).reduce(_ union _)

Also, if you still need to create an empty RDD, you can do it using :

val empty = sc.emptyRDD[(long, long, String)]

Answer 2

You're correct that this might not be the optimal way to do this, but we would need more info on what you're trying to accomplish with generating a new RDD with each call to your helper function.

You could define 1 RDD prior to the loop and assign it a var then run it through your loop. Here's an example:

val rdd = sc.parallelize(1 to 100)
val rdd_tuple = rdd.map(x => (x.toLong, (x*10).toLong, x.toDouble))
var new_rdd = rdd_tuple
println("Initial RDD count: " + new_rdd.count())
for (i <- 2 to 4) {
  new_rdd = new_rdd.union(rdd_tuple)
}
println("New count after loop: " + new_rdd.count())