code: private void rejectPendingSlotRequest(PendingSlotRequest pendingSlotRequest, Exception cause) {
  		CompletableFuture<Acknowledge> request = pendingSlotRequest.getRequestFuture();
  
  		if (null != request) {
  			request.completeExceptionally(new SlotAllocationException(cause));
  		} else {
  			LOG.debug("Cannot reject pending slot request {}, since no request has been sent.", pendingSlotRequest.getAllocationId());
  		}
  	}
docs: Rejects the pending slot request by failing the request future with a
  {@link SlotAllocationException}.
  
  @param pendingSlotRequest to reject
  @param cause of the rejection