...

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

...

For FutureCompletingBlockingQueue, as a hindsight, it might be better to not expose it to the Source developers. They are unlikely to use it anywhere other than just constructing it. The reason that FutureCompletingBlockingQueue is currently exposed in the SourceReaderBase constructor is because both the SplitFetcherManager and SourceReaderBase need it. One way to hide the FutureCompletingBlockingQueue from the public API is to make SplitFetcherManager the only owner class of the queue, and expose some of its methods via SplitFetcherManager. This way, the SourceReaderBase can invoke the methods via SplitFetcherManager. I believe this also makes .  This will make the code slightly cleaner.

...

• To expose the SplitFetcherManager / SingleThreadFetcheManager as Public, allowing connector developers to easily create their own threading models in the SourceReaderBase.
• To hide the element queue from the connector developers and simplify the SourceReaderBase to consist of only SplitFetcherManager and RecordEmitter as major components. make SplitFetcherManager the only owner class of the queue

Public Interfaces

SplitFetcherManager

• Change SplitFetcherManager from Internal to PublicEvolving.
• Deprecate the old constructor exposing the FutureCompletingBlockingQueue, and add new constructors as replacements which creates the FutureCompletingBlockingQueue instance internally.
• Add a few new methods to expose the functionality of the internal FutureCompletingBlockingQueue via the SplitFetcherManagerinternal method SplitFetcherManager#getQueue() for SourceReaderBase usage.

@PublicEvolving
public abstract class SplitFetcherManager<E, SplitT extends SourceSplit> {    
    
	@Deprecated
    
	public SplitFetcherManagerSingleThreadMultiplexSourceReaderBase(
            FutureCompletingBlockingQueue<RecordsWithSplitIds<E>> elementsQueueSupplier<SplitReader<E, SplitT>> splitReaderSupplier,
            Supplier<SplitReader<ERecordEmitter<E, T, SplitT>>SplitStateT> splitReaderFactoryrecordEmitter,
            Configuration configuration) {
config,
          this(elementsQueue, splitReaderFactory, configuration,SourceReaderContext (ignorecontext) -> {
        });super(
    }       

    @Deprecated
    @VisibleForTesting
  new SingleThreadFetcherManager<>(splitReaderSupplier, config),
  public SplitFetcherManager(
            FutureCompletingBlockingQueue<RecordsWithSplitIds<E>> elementsQueuerecordEmitter,
            Supplier<SplitReader<E,   SplitT>> splitReaderFactoryconfig,
            Configuration configuration,
   context);
    }

    @Deprecated
   Consumer<Collection<String>> splitFinishedHook) {

	}


public SplitFetcherManager(
 // todo: provide a new constructor without FutureCompletingBlockingQueue.     
 publicFutureCompletingBlockingQueue<RecordsWithSplitIds<E>> SplitFetcherManager(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 this.elementsQueue = new FutureCompletingBlockingQueue<>FutureCompletingBlockingQueue.     
 public SplitFetcherManager(
            Supplier<SplitReader<E, SplitT>>   configuration.getInteger(SourceReaderOptions.ELEMENT_QUEUE_CAPACITY));
		// ......
	}

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

 	/**
splitReaderFactory,
            Configuration configuration) {
        this(splitReaderFactory, configuration, (ignore) -> {
        });
      * Returns the
 availability future.}

 If the// queuetodo: is non-empty, then this future will already beprovide a new constructor without FutureCompletingBlockingQueue.
  public SplitFetcherManager(
     * complete. Otherwise the obtained future is guaranteedSupplier<SplitReader<E, toSplitT>> getsplitReaderFactory,
 completed the next time the
     * queue becomesConfiguration non-empty, or a notification happens via {@link #notifyAvailable()}.
configuration,
            Consumer<Collection<String>> splitFinishedHook) *{
     * <p>It is importantthis.elementsQueue that= anew completedFutureCompletingBlockingQueue<>(
 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()}.configuration.getInteger(SourceReaderOptions.ELEMENT_QUEUE_CAPACITY));
		// ......
	}

	
	@Internal
	public FutureCompletingBlockingQueue getQueue();
 }

SingleThreadFetcherManager

• Change SingleThreadFetcherManager from Internal to PublicEvolving.
• Deprecate the old constructor exposing the FutureCompletingBlockingQueue, and add new constructors without FutureCompletingBlockingQueue as replacements which creates the FutureCompletingBlockingQueue instance by its parent class(SplitFetcherManager) internally.

@PublicEvolving
public class SingleThreadFetcherManager<E, SplitT extends SourceSplit>
        extends SplitFetcherManager<E, SplitT> {  

   	@Deprecated
    public SingleThreadFetcherManager(
     *
     * <p>For thatFutureCompletingBlockingQueue<RecordsWithSplitIds<E>> reasonelementsQueue,
   it is important to call this method (to obtain aSupplier<SplitReader<E, newSplitT>> futuresplitReaderSupplier) every time{
     * again after {@link #poll()} returned null and you want to wait for data.this(elementsQueue, splitReaderSupplier, new Configuration());
    }

  	@Deprecated
     */
	public CompletableFuture<Void> getAvailabilityFutureSingleThreadFetcherManager(){
 		return elementsQueue.getAvailabilityFuture();
	}

     /**
     * Makes sureFutureCompletingBlockingQueue<RecordsWithSplitIds<E>> the availability future is complete, if it is not complete already. All futureselementsQueue,
            Supplier<SplitReader<E, SplitT>> splitReaderSupplier,
     *  returned by previous calls to {@linkConfiguration #getAvailabilityFuture()} are guaranteed to be
configuration) {
         * completed.super(elementsQueue, splitReaderSupplier, configuration);
     *}

    @Deprecated
 * <p>All future calls@VisibleForTesting
 to the method willpublic SingleThreadFetcherManager(
 return a completed future, until the point that the
   FutureCompletingBlockingQueue<RecordsWithSplitIds<E>> elementsQueue,
 * availability is reset via calls to {@link #poll()} that leave theSupplier<SplitReader<E, queueSplitT>> empty.splitReaderSupplier,
     */
	public void notifyAvailable(){
		elementsQueue.notifyAvailable();
	}

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

SingleThreadFetcherManager

• Change SingleThreadFetcherManager from Internal to PublicEvolving.
• Deprecate the old constructor exposing the FutureCompletingBlockingQueue, and add new constructors without FutureCompletingBlockingQueue as replacements which creates the FutureCompletingBlockingQueue instance by its parent class(SplitFetcherManager) internally.

@PublicEvolving
public class SingleThreadFetcherManager<E, SplitT extends SourceSplit>
        extends SplitFetcherManager<E, SplitT> {  

   	@Deprecated
    public SingleThreadFetcherManager(
            FutureCompletingBlockingQueue<RecordsWithSplitIds<E>> elementsQueue,
            Supplier<SplitReader<E,   Configuration configuration,
            Consumer<Collection<String>> splitFinishedHook) {
        super(elementsQueue, splitReaderSupplier, configuration, splitFinishedHook);
    }

  // todo: provide a new constructor without FutureCompletingBlockingQueue.     
  public SingleThreadFetcherManager(
            Supplier<SplitReader<E, SplitT>> splitReaderSupplier) {
        thissuper(elementsQueue, splitReaderSupplier, new Configuration());
    }  

  	@Deprecated
   // todo: provide a new constructor without FutureCompletingBlockingQueue. 
     public SingleThreadFetcherManager(
            FutureCompletingBlockingQueue<RecordsWithSplitIds<E>> elementsQueue,
            Supplier<SplitReader<ESupplier<SplitReader<E, SplitT>> splitReaderSupplier,
            Configuration configuration) {
        super(elementsQueue, splitReaderSupplier, configuration);
    }

 // todo: provide a @Deprecated
new constructor without FutureCompletingBlockingQueue. @VisibleForTesting
      public SingleThreadFetcherManager(
            FutureCompletingBlockingQueue<RecordsWithSplitIds<E>> elementsQueue,
            Supplier<SplitReader<E, SplitT>> splitReaderSupplier,
            Configuration configuration,
            Consumer<Collection<String>> splitFinishedHook) {
        super(elementsQueue, splitReaderSupplier, configuration, splitFinishedHook);
    }

  // todo: provide a new constructor without FutureCompletingBlockingQueue.     
  public SingleThreadFetcherManager(
            Supplier<SplitReader<E, SplitT>> splitReaderSupplier) {
        this(splitReaderSupplier, new Configuration());
    }


    public SingleThreadFetcherManager}

SplitFetcherTask

Change SplitFetcherTask from Internal to PublicEvolving.

@PublicEvolving
public interface SplitFetcherTask {}

SplitFetcher

Change SplitFetcher from Internal to PublicEvolving, but still not to expose the construct of SplitFetcher, so it can only created by org.apache.flink.connector.base.source.reader.fetcher.SplitFetcherManager#createSplitFetcher

@PublicEvolving
 public class SplitFetcher<E, SplitT extends SourceSplit> implements Runnable {    
	SplitFetcher(
            Supplier<SplitReader<E, SplitT>> splitReaderSupplierint id,
            Configuration configuration) {FutureCompletingBlockingQueue<RecordsWithSplitIds<E>> elementsQueue,
        super(splitReaderSupplier, configuration);
    }

   SplitReader<E, publicSplitT> SingleThreadFetcherManager(splitReader,
            Supplier<SplitReader<E, SplitT>> splitReaderSupplierConsumer<Throwable> errorHandler,
            ConfigurationRunnable configurationshutdownHook,
            Consumer<Collection<String>> splitFinishedHook) {,
        super(splitReaderSupplier, configuration, splitFinishedHook);
  boolean  }allowUnalignedSourceSplits);
}

SplitFetcherTask

Change SplitFetcherTask from Internal to PublicEvolving.

@PublicEvolving
public interface SplitFetcherTask {}

SplitFetcher

Change SplitFetcher from Internal to PublicEvolving, but still not to expose the construct of SplitFetcher.

the constructor of SplitFetcher to the end users?

SourceReaderBase

Deprecate the old constructor exposing the FutureCompletingBlockingQueue, and add new constructors as replacements which creates the FutureCompletingBlockingQueue instance in SplitFetcherManager internally.

@PublicEvolving
public abstract class SourceReaderBase<E, T

@Internal
public class SplitFetcher<E, SplitT extends SourceSplit> implements RunnableSourceSplit, SplitStateT>
        implements SourceReader<T, SplitT> {    
	SplitFetcher( 
 
    @Deprecated
       int id,public SourceReaderBase(
            FutureCompletingBlockingQueue<RecordsWithSplitIds<E>> elementsQueue,
            SplitReader<ESplitFetcherManager<E, SplitT> splitReadersplitFetcherManager,
            Consumer<Throwable> errorHandlerRecordEmitter<E, T, SplitStateT> recordEmitter,
            RunnableConfiguration shutdownHookconfig,
            SourceReaderContext Consumer<Collection<String>>context) splitFinishedHook,{
        this(elementsQueue, splitFetcherManager, recordEmitter, null, booleanconfig, allowUnalignedSourceSplitscontext);
}

the constructor of SplitFetcher to the end users?

SourceReaderBase

• Deprecate the old constructor exposing the FutureCompletingBlockingQueue, and add new constructors as replacements which creates the FutureCompletingBlockingQueue instance in SplitFetcherManager internally.
• Remove the direct operations to FutureCompletingBlockingQueue and utilize the functionality of SplitFetcherManager as an alternative.

@PublicEvolving
public abstract class SourceReaderBase<E, T, SplitT extends SourceSplit, SplitStateT>
        implements SourceReader<T, SplitT> {  
 
 
        }
 
 
    @Deprecated
    public SourceReaderBase(
            FutureCompletingBlockingQueue<RecordsWithSplitIds<E>> elementsQueue,
            SplitFetcherManager<E, SplitT> splitFetcherManager,
            RecordEmitter<E, T, SplitStateT> recordEmitter,
            Configuration@Nullable RecordEvaluator<T> configeofRecordEvaluator,
            SourceReaderContext context) {Configuration config,
        this(elementsQueue, splitFetcherManager, recordEmitter, null, config,SourceReaderContext context); {
    }
 
 
  //this.elementsQueue = @DeprecatedelementsQueue;
    public SourceReaderBase(
   this.splitFetcherManager = splitFetcherManager;
       FutureCompletingBlockingQueue<RecordsWithSplitIds<E>> elementsQueue,
   this.recordEmitter = recordEmitter;
        this.splitStates = SplitFetcherManager<E, SplitT> splitFetcherManager,new HashMap<>();
        this.options =   RecordEmitter<E, T, SplitStateT> recordEmitter,new SourceReaderOptions(config);
        this.config    @Nullable RecordEvaluator<T> eofRecordEvaluator,
            Configuration config,= config;
        this.context =   SourceReaderContext context) {context;
        //this.elementsQueuenoMoreSplitsAssignment = elementsQueuefalse;
        this.splitFetcherManagereofRecordEvaluator = splitFetcherManagereofRecordEvaluator;
 
       this.recordEmitter numRecordsInCounter = recordEmittercontext.metricGroup().getIOMetricGroup().getNumRecordsInCounter();
    } 
 
   this.splitStates =// new HashMap<>();
     todo: provide a new constructor without FutureCompletingBlockingQueue
   this.options =public new SourceReaderOptions(config);SourceReaderBase(
        this.config = config;
  SplitFetcherManager<E, SplitT> splitFetcherManager,
    this.context = context;
      RecordEmitter<E, T, this.noMoreSplitsAssignment = false;SplitStateT> recordEmitter,
        this.eofRecordEvaluator = eofRecordEvaluator;
 
   Configuration config,
           numRecordsInCounter =SourceReaderContext context.metricGroup().getIOMetricGroup().getNumRecordsInCounter() {
        this(splitFetcherManager, recordEmitter, null, config, context);
    }
 
 
     // todo: provide a new constructor without FutureCompletingBlockingQueue
      public SourceReaderBase(
            SplitFetcherManager<E, SplitT> splitFetcherManager,
            RecordEmitter<E, T, SplitStateT> recordEmitter,
            Configuration@Nullable RecordEvaluator<T> configeofRecordEvaluator,
            SourceReaderContext context) {Configuration config,
        this(splitFetcherManager, recordEmitter, null, config,    SourceReaderContext context); {
    }
 
 
  this.splitFetcherManager  public SourceReaderBase(= splitFetcherManager;
        this.recordEmitter = recordEmitter;
  SplitFetcherManager<E, SplitT> splitFetcherManager,
    this.splitStates = new HashMap<>();
     RecordEmitter<E, T, SplitStateT> recordEmitter,
    this.options = new SourceReaderOptions(config);
         @Nullable RecordEvaluator<T> eofRecordEvaluator,
            Configuration config,this.config = config;
        this.context   = SourceReaderContext context) {;
        this.splitFetcherManagernoMoreSplitsAssignment = splitFetcherManagerfalse;
        this.recordEmittereofRecordEvaluator = recordEmittereofRecordEvaluator;
 
        this.splitStatesnumRecordsInCounter = new HashMap<>context.metricGroup().getIOMetricGroup().getNumRecordsInCounter();
        this.options = new SourceReaderOptions(config);
        this.config = config;
        this.context = context;
}
}

SingleThreadMultiplexSourceReaderBase

Deprecate the old constructor exposing the FutureCompletingBlockingQueue, and add new constructors as replacements which creates the FutureCompletingBlockingQueue instance in SplitFetcherManager internally.

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

	@Depricated
	public SingleThreadMultiplexSourceReaderBase(
           this.noMoreSplitsAssignment =FutureCompletingBlockingQueue<RecordsWithSplitIds<E>> false;
elementsQueue,
            this.eofRecordEvaluator = eofRecordEvaluator;
 
SingleThreadFetcherManager<E, SplitT> splitFetcherManager,
            RecordEmitter<E, numRecordsInCounterT, = context.metricGroup().getIOMetricGroup().getNumRecordsInCounter();SplitStateT> recordEmitter,
    }
}

SingleThreadMultiplexSourceReaderBase

Deprecate the old constructor exposing the FutureCompletingBlockingQueue, and add new constructors as replacements which creates the FutureCompletingBlockingQueue instance in SplitFetcherManager internally.

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

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

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

	@Depricated
	public SingleThreadMultiplexSourceReaderBase(

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


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


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

    // SourceReaderContexttodo: context)provide {
a new constructor without FutureCompletingBlockingQueue
     public 	superSingleThreadMultiplexSourceReaderBase(
            SingleThreadFetcherManager<E,    new SingleThreadFetcherManager<>(splitReaderSupplier, config),,SplitT> splitFetcherManager,
            RecordEmitter<E, T,  SplitStateT> recordEmitter,
            @Nullable RecordEvaluator<T>   configeofRecordEvaluator,
            Configuration config,
   context);
    }

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

Proposed Changes

• By exposing the SplitFetcherManager / SingleThreadFetcheManager, connector developers can easily create their own threading models in the SourceReaderBase, by implementing addSplits(), removeSplits() and 

  maybeShutdownFinishedFetchers() functions.

• Note that the SplitFetcher constructor is package private, so users can only create SplitFetchers via SplitFetcherManager.createSplitFetcher(). This ensures each SplitFetcher is always owned by the SplitFetcherManager.
•  This FLIP essentially embedded the element queue (a FutureCompletingBlockingQueue) instance into the SplitFetcherManager. This hides the element queue from the connector developers and simplifies the SourceReaderBase to consist of only SplitFetcherManager and RecordEmitter as major components.

Compatibility, Deprecation, and Migration Plan

The Connectors that utilize constructors (including param elementsQueue) of SingleThreadMultiplexSourceReaderBase, SourceReaderBase, and SingleThreadFetcherManager should be migrated to the new constructor that does not have elementsQueue parameter.

Mark the impacted constructors as deprecated in 1.19, and remove them in release of 2.0.

Test Plan

nothing else to do.

Rejected Alternatives

If SplitFetcherManager becomes PublicEvolving, that also means SplitFetcher needs to be PublicEvolving, because it is returned by the protected method SplitFetcherManager.createSplitFetcher()

However,  SplitFetcher requires FutureCompletingBlockingQueue as a constructor parameter. SplitFetcher is a class rather than Interface. Therefore, I want to  change SplitFetcher to a public Interface and moving its implementation details to an implement subclass .

Reject Reason

}
}

Proposed Changes

• By exposing the SplitFetcherManager / SingleThreadFetcheManager, connector developers can easily create their own threading models in the SourceReaderBase, by implementing addSplits(), removeSplits() and 

  maybeShutdownFinishedFetchers() functions.

• Note that the SplitFetcher constructor is package private, so users can only create SplitFetchers via SplitFetcherManager.createSplitFetcher(). This ensures each SplitFetcher is always owned by the SplitFetcherManager.
•  This FLIP essentially embedded the element queue (a FutureCompletingBlockingQueue) instance into the SplitFetcherManager. This hides the element queue from the connector developers and simplifies the SourceReaderBase to consist of only SplitFetcherManager and RecordEmitter as major components.

Compatibility, Deprecation, and Migration Plan

The Connectors that utilize constructors (including param elementsQueue) of SingleThreadMultiplexSourceReaderBase, SourceReaderBase, and SingleThreadFetcherManager should be migrated to the new constructor that does not have elementsQueue parameter.

Mark the impacted constructors as deprecated in 1.19, and remove them in release of 2.0.

Test Plan

nothing else to do.

Rejected Alternatives

change SplitFetcher to a public Interface

If SplitFetcherManager becomes PublicEvolving,  that also means SplitFetcher needs to be PublicEvolving, because it is returned by the protected method SplitFetcherManager.createSplitFetcher()

However,  SplitFetcher requires FutureCompletingBlockingQueue as a constructor parameter. SplitFetcher is a class rather than Interface. Therefore, I want to  change SplitFetcher to a public Interface and moving its implementation details to an implement subclass .

Reject Reason

The constructor of the SplitFetcher is already package private. So it can only be accessed from the classes in the package org.apache.flink.connector.base.source.reader.fetcher.  And apparently,  user classes should not be in this package.  Therefore,  even if we mark the
SplitFetcher class as PublicEvolving, the constructor is not available to the users. Only the public and protected methods are considered public API
in this case. Private / package private methods and fields are still internal.

SplitFetcherManager expose poll / getAvailabilityFuture / notifyAvailable / noAvailableElement

• Change SplitFetcherManager from Internal to PublicEvolving.
• Deprecate the old constructor exposing the FutureCompletingBlockingQueue, and add new constructors as replacements which creates the FutureCompletingBlockingQueue instance internally.
• Add a new internal method SplitFetcherManager#getQueue() for SourceReaderBase usage.

Reject Reason

SplitFetcherManager exposes 4 more methods, poll / getAvailabilityFuture / notifyAvailable / noAvailableElement, which are tightly coupled with the implementation of the elementQueue. The naming of these methods look weird, like what does it mean to "poll from a SplitFetcherManager" / "notify a SplitFetcherManager available"? To clarify these methods we have to explain to developers that "well we hide a queue inside SplitFetcherMamager and the poll method is actually polling from the queue". I'm afraid these methods will implicitly expose the concept and the implementation of the queue to developers. 

A cleaner solution would be having a new interface that extracts SplitFetcher creating and managing logic from the current SplitFetcherManager, but having too many concepts might make the entire Source API even harder to understand. To make a compromise,  only expose constructors of SplitFetcherManager as public APIs, and adding a new internal method SplitFetcherManager#getQueue() for SourceReaderBase (well it's a bit hacky , more worthy than exposing methods like poll and notifyAvailable on SplitFetcherManager )The constructor of the SplitFetcher is already package private. So it can only be accessed from the classes in the package org.apache.flink.connector.base.source.reader.fetcher. And apparently, user classes should not be in this package. Therefore, even if we mark the
SplitFetcher class as PublicEvolving, the constructor is not available to the users. Only the public and protected methods are considered public API
in this case. Private / package private methods and fields are still internal.