code: @Override
  	public boolean triggerCheckpoint(CheckpointMetaData checkpointMetaData, CheckpointOptions checkpointOptions, boolean advanceToEndOfEventTime) throws Exception {
  		if (!externallyInducedCheckpoints) {
  			return super.triggerCheckpoint(checkpointMetaData, checkpointOptions, advanceToEndOfEventTime);
  		}
  		else {
  			// we do not trigger checkpoints here, we simply state whether we can trigger them
  			synchronized (getCheckpointLock()) {
  				return isRunning();
  			}
  		}
  	}
docs: ------------------------------------------------------------------------