code: public boolean triggerCheckpoint(CheckpointMetaData checkpointMetaData, CheckpointOptions checkpointOptions, boolean advanceToEndOfEventTime) throws Exception {
  		throw new UnsupportedOperationException(String.format("triggerCheckpoint not supported by %s", this.getClass().getName()));
  	}
docs: This method is called to trigger a checkpoint, asynchronously by the checkpoint
  coordinator.
  
  <p>This method is called for tasks that start the checkpoints by injecting the initial barriers,
  i.e., the source tasks. In contrast, checkpoints on downstream operators, which are the result of
  receiving checkpoint barriers, invoke the {@link #triggerCheckpointOnBarrier(CheckpointMetaData, CheckpointOptions, CheckpointMetrics)}
  method.
  
  @param checkpointMetaData Meta data for about this checkpoint
  @param checkpointOptions Options for performing this checkpoint
  @param advanceToEndOfEventTime Flag indicating if the source should inject a {@code MAX_WATERMARK} in the pipeline
  to fire any registered event-time timers
  
  @return {@code false} if the checkpoint can not be carried out, {@code true} otherwise