public void scheduleCancellingRequest(long requestId, String reason) {
    synchronized (requestsToBeCancelled) {
      requestsToBeCancelled.add(requestId);
      requestCancelReasons.put(requestId, reason);
    }
  }
@param requestId request will be cancelled on next scheduler wake up
(if it is in state that allows cancellation, e.g. QUEUED, PENDING, IN_PROGRESS)
@param reason why request is being cancelled
ambari-server/src/main/java/org/apache/ambari/server/actionmanager/ActionScheduler.java