Spark Source code: How to understand withScope method

Question

I can not understand the functionality of withScope method (Actually, I do not really know the meaning of the RDDOperationScope Class)

Especially, what's the meaning of (body: => T) in the parameter list of withScope method:

private[spark] def withScope[T](
  sc: SparkContext,
  name: String,
  allowNesting: Boolean,
  ignoreParent: Boolean)(body: => T): T = {
// Save the old scope to restore it later
val scopeKey = SparkContext.RDD_SCOPE_KEY
val noOverrideKey = SparkContext.RDD_SCOPE_NO_OVERRIDE_KEY
val oldScopeJson = sc.getLocalProperty(scopeKey)
val oldScope = Option(oldScopeJson).map(RDDOperationScope.fromJson)
val oldNoOverride = sc.getLocalProperty(noOverrideKey)
try {
  if (ignoreParent) {
    // Ignore all parent settings and scopes and start afresh with our own root scope
    sc.setLocalProperty(scopeKey, new RDDOperationScope(name).toJson)
  } else if (sc.getLocalProperty(noOverrideKey) == null) {
    // Otherwise, set the scope only if the higher level caller allows us to do so
    sc.setLocalProperty(scopeKey, new RDDOperationScope(name, oldScope).toJson)
  }
  // Optionally disallow the child body to override our scope
  if (!allowNesting) {
    sc.setLocalProperty(noOverrideKey, "true")
  }
  body
} finally {
  // Remember to restore any state that was modified before exiting
  sc.setLocalProperty(scopeKey, oldScopeJson)
  sc.setLocalProperty(noOverrideKey, oldNoOverride)
}
}

You can find the source code with this link: https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/rdd/RDDOperationScope.scala

Can anyone help me? Thanks, I was confused at that for a long time.

Answer 1

The following code may help you

object TestWithScope {
    def withScope(func: => String) = {
        println("withscope")
        func
    }

    def bar(foo: String) = withScope {
        println("Bar: " + foo)
        "BBBB"
    }

    def main(args: Array[String]): Unit = {
        println(bar("AAAA"));
    }
}

Possible output

withscope
Bar: AAAA
BBBB