docs: @Override
  	public void snapshotState(FunctionSnapshotContext context) throws Exception {
  		Preconditions.checkState(this.checkpointedState != null,
  			"The " + getClass().getSimpleName() + " state has not been properly initialized.");
  
  		this.checkpointedState.clear();
  		this.checkpointedState.add(this.globalModificationTime);
  
  		if (LOG.isDebugEnabled()) {
  			LOG.debug("{} checkpointed {}.", getClass().getSimpleName(), globalModificationTime);
  		}
  	}