code: public void triggerCheckpointOnBarrier(CheckpointMetaData checkpointMetaData, CheckpointOptions checkpointOptions, CheckpointMetrics checkpointMetrics) throws Exception {
  		throw new UnsupportedOperationException(String.format("triggerCheckpointOnBarrier not supported by %s", this.getClass().getName()));
  	}
docs: This method is called when a checkpoint is triggered as a result of receiving checkpoint
  barriers on all input streams.
  
  @param checkpointMetaData Meta data for about this checkpoint
  @param checkpointOptions Options for performing this checkpoint
  @param checkpointMetrics Metrics about this checkpoint
  
  @throws Exception Exceptions thrown as the result of triggering a checkpoint are forwarded.