@Override
      public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
        overflowQueue.add(r);
      }
Once the ThreadPoolExecutor is at full capacity, it starts to reject
submissions which are queued for later submission.
ambari-server/src/main/java/org/apache/ambari/server/controller/utilities/BufferedThreadPoolExecutorCompletionService.java