Understanding Apache Spark RDD task serialization - scala

Question

I am trying to understand how task serialization works in Spark and am a bit confused by some mixed results I'm getting in a test I've written.
I have some test code (simplified for sake of post) that does the following over more than one node:
object TestJob {
def run(): Unit = {
val rdd = ...
val helperObject = new Helper() // Helper does NOT impl Serializable and is a vanilla class
rdd.map(element => {
helperObject.transform(element)
}).collect()
}
}
When I execute run(), the job bombs out with a "task not serializable" exception as expected since helperObject is not serializable. HOWEVER, when I alter it a little, like this:
trait HelperComponent {
val helperObject = new Helper()
}
object TestJob extends HelperComponent {
def run(): Unit = {
val rdd = ...
rdd.map(element => {
helperObject.transform(element)
}).collect()
}
}
The job executes successfully for some reason. Could someone help me to understand why this might be? What exactly gets serialized by Spark and sent to the workers in each case above?
I am using Spark version 2.1.1.
Thank you!

Answer 1

Could someone help me to understand why this might be?
In your first snippet, helperObject is a local variable declared inside run. As such, it will be closed over (lifted) by the function such that where ever this code executes, all information would be available, and because of that Sparks ClosureCleaner yells at you for trying to serialize it.
In your second snippet, the value is no longer a local variable in the method scope, it is part of the class instance (technically, this is an object declaration but it will be transformed into a JVM class after all).
This is meaningful in Spark for the reason that all worker nodes in the cluster contain the JARs needed to execute your code. Thus, instead of serializing TestObject in its entirety for rdd.map, when Spark spins up an Executor process in one of your workers, it will load TestObject locally via a ClassLoader, and create an instance of it, just like every other JVM class in a non distributed application.
To conclude, the reason you don't see this blowing up is because the class is no longer serialized due to the changes in the way you've declared the type instance.