What is Spark execution order with function calls in scala? - scala

Question

I have a spark program as follows:
object A {
var id_set: Set[String] = _
def init(argv: Array[String]) = {
val args = new AArgs(argv)
id_set = args.ids.split(",").toSet
}
def main(argv: Array[String]) {
init(argv)
val conf = new SparkConf().setAppName("some.name")
val rdd1 = getRDD(paras)
val rdd2 = getRDD(paras)
//......
}
def getRDD(paras) = {
//function details
getRDDDtails(paras)
}
def getRDDDtails(paras) = {
//val id_given = id_set
id_set.foreach(println) //worked normal, not empty
someRDD.filter{ x =>
val someSet = x.getOrElse(...)
//id_set.foreach(println) ------wrong, id_set just empty set
(some_set & id_set).size > 0
}
}
class AArgs(args: Array[String]) extends Serializable {
//parse args
}
I have a global variable id_set. At first, it is just an empty set. In main function, I call init which sets id_set to a non-empty set from args. After that, I call getRDD function which calls getRDDDtails. In getRDDDtails, I filter a rdd based on contents in id_set. However, the result semms to be empty. I tried to print is_set in executor, and it is just an empty line. So, the problem seems to be is_set is not well initilized(in init function). However, when I try to print is_set in driver(in head lines of function getRDDDtails), it worked normal, not empty.
So, I have tried to add val id_given = id_set in function getRDDDtails, and use id_given later. This seems to fix the problem. But I'm totally confused why should this happen? What is the execution order of Spark programs? Why does my solution work?