code: @Override
    public synchronized IdleMessage getIdleStatus() {
  
      if (this.isIdle()) {
        return IDLE_MESSAGE;
      }
  
      final String message = String.format(
          "There are %d outstanding container requests and %d allocated containers",
          this.outstandingContainerRequests, this.containerAllocationCount);
  
      return new IdleMessage(COMPONENT_NAME, message, false);
    }
docs: Driver is idle if, regardless of status, it has no evaluators allocated
  and no pending container requests. This method is used in the DriverIdleManager.
  If all DriverIdlenessSource components are idle, DriverIdleManager will initiate Driver shutdown.
  @return idle, if there are no outstanding requests or allocations. Not idle otherwise.