@PublicEvolving
public abstract class SplitFetcherManager<E, SplitT extends SourceSplit> {        
	@Deprecated
    public SplitFetcherManager(
            FutureCompletingBlockingQueue<RecordsWithSplitIds<E>> elementsQueue,
            Supplier<SplitReader<E, SplitT>> splitReaderFactory,
            Configuration configuration) {
        this(elementsQueue, splitReaderFactory, configuration, (ignore) -> {
        });
    }       

    @Deprecated
    @VisibleForTesting
    public SplitFetcherManager(
            FutureCompletingBlockingQueue<RecordsWithSplitIds<E>> elementsQueue,
            Supplier<SplitReader<E, SplitT>> splitReaderFactory,
            Configuration configuration,
            Consumer<Collection<String>> splitFinishedHook) {

	}



 // todo: provide a new constructor without FutureCompletingBlockingQueue.     
 public SplitFetcherManager(
            Supplier<SplitReader<E, SplitT>> splitReaderFactory,
            Configuration configuration) {
        this(splitReaderFactory, configuration, (ignore) -> {
        });
        
  }

 public SplitFetcherManager(
            Supplier<SplitReader<E, SplitT>> splitReaderFactory,
            Configuration configuration,
            Consumer<Collection<String>> splitFinishedHook) {
        this.elementsQueue = new FutureCompletingBlockingQueue<>(
                configuration.getInteger(SourceReaderOptions.ELEMENT_QUEUE_CAPACITY));
		// ......
	}

	/**
	 * returns the RecordsWithSplitIds produced by SplitReader.
	**/
	public RecordsWithSplitIds<E> poll(){
		return elementsQueue.poll();
	}

 	/**
     * Returns the availability future. If the queue is non-empty, then this future will already be
     * complete. Otherwise the obtained future is guaranteed to get completed the next time the
     * queue becomes non-empty, or a notification happens via {@link #notifyAvailable()}.
     *
     * <p>It is important that a completed future is no guarantee that the next call to {@link
     * #poll()} will return a non-null element. If there are concurrent consumer, another consumer
     * may have taken the available element. Or there was no element in the first place, because the
     * future was completed through a call to {@link #notifyAvailable()}.
     *
     * <p>For that reason, it is important to call this method (to obtain a new future) every time
     * again after {@link #poll()} returned null and you want to wait for data.
     */
	public CompletableFuture<Void> getAvailabilityFuture(){
 		return elementsQueue.getAvailabilityFuture();
	}

     /**
     * Makes sure the availability future is complete, if it is not complete already. All futures
     * returned by previous calls to {@link #getAvailabilityFuture()} are guaranteed to be
     * completed.
     *
     * <p>All future calls to the method will return a completed future, until the point that the
     * availability is reset via calls to {@link #poll()} that leave the queue empty.
     */
	public void notifyAvailable(){
		elementsQueue.notifyAvailable();
	}

   /** Checks whether is no data available. */
 	public boolean noAvailableElement(){
		return elementsQueue.isEmpty();
 	}
}

@PublicEvolving
public abstract class SingleThreadMultiplexSourceReaderBase<E, T, SplitT extends SourceSplit, SplitStateT> extends SourceReaderBase<E, T, SplitT, SplitStateT> { 

	@Depricated
	public SingleThreadMultiplexSourceReaderBase(
            FutureCompletingBlockingQueue<RecordsWithSplitIds<E>> elementsQueue,
            SingleThreadFetcherManager<E, SplitT> splitFetcherManager,
            RecordEmitter<E, T, SplitStateT> recordEmitter,
            Configuration config,
            SourceReaderContext context) {
        super(elementsQueue, splitFetcherManager, recordEmitter, config, context);
 	}

	@Depricated
	public SingleThreadMultiplexSourceReaderBase(
            FutureCompletingBlockingQueue<RecordsWithSplitIds<E>> elementsQueue,
            Supplier<SplitReader<E, SplitT>> splitReaderSupplier,
            RecordEmitter<E, T, SplitStateT> recordEmitter,
            Configuration config,
            SourceReaderContext context) {
        super(
                elementsQueue,
                new SingleThreadFetcherManager<>(elementsQueue, splitReaderSupplier, config),
                recordEmitter,
                config,
                context);
	}


	// todo: Add new constructors without FutureCompletingBlockingQueue
 	public SingleThreadMultiplexSourceReaderBase(
            Supplier<SplitReader<E, SplitT>> splitReaderSupplier,
            RecordEmitter<E, T, SplitStateT> recordEmitter,
            Configuration config,
            SourceReaderContext context) {
        	super(
                new SingleThreadFetcherManager<>(splitReaderSupplier, config),,
                recordEmitter,
                config,
                context);
    }

    public SingleThreadMultiplexSourceReaderBase(
            SingleThreadFetcherManager<E, SplitT> splitFetcherManager,
            RecordEmitter<E, T, SplitStateT> recordEmitter,
            @Nullable RecordEvaluator<T> eofRecordEvaluator,
            Configuration config,
            SourceReaderContext context) {
        super(
                splitFetcherManager,
                recordEmitter,
                eofRecordEvaluator,
                config,
                context);
    }
}