Github user tdas commented on a diff in the pull request:
https://github.com/apache/spark/pull/6607#discussion_r32781077
--- Diff:
streaming/src/main/scala/org/apache/spark/streaming/scheduler/ReceiverTracker.scala
---
@@ -308,12 +336,25 @@ class ReceiverTracker(ssc: StreamingContext,
skipReceiverLaunch: Boolean = false
supervisor.start()
supervisor.awaitTermination()
}
+
// Run the dummy Spark job to ensure that all slaves have registered.
// This avoids all the receivers to be scheduled on the same node.
if (!ssc.sparkContext.isLocal) {
ssc.sparkContext.makeRDD(1 to 50, 50).map(x => (x,
1)).reduceByKey(_ + _, 20).collect()
}
+ // Get the list of executors and schedule receivers
+ val executors = getExecutors(ssc)
+ val locations = scheduleReceivers(receivers, executors)
+ val tempRDD =
+ if (locations(0) != null) {
--- End diff --
It may be so, but its hard to read and understand the condition (which is
why I asked). Also checking for null at location(0) to detect whether an
ArrayBuffer was assigned to the position to detect whether very very brittle
check and ties the logic deep with the implementation of the
`scheduleReceiver`. If someone changes the implementation of the function to
apply a different way to allocate receivers (say, always assign a ArrayBuffer
even if it is empty), this may totally break. So this condition makes
non-intuitive assumptions about the implementation logic of the
scheduleReceiver. This is BAD code design.
We try to design the code as intuitive and modular as possible.
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