public interface SplitReader<E, SplitT extends SourceSplit> {

	RecordsWithSplitIds<E> fetch() throws InterruptedException;

	void handleSplitsChanges(Queue<SplitsChange<SplitT>> splitsChanges);

	void wakeUp();
}

/**
 * An interface for all the Split types to implement.
 */
public interface SourceSplit {

	/**
	 * Get the split id of this source split.
	 * @return id of this source split.
	 */
	String splitId();
}

public interface SourceOutput<E> extends WatermarkOutput {

	void emitRecord(E record);

	void emitRecord(E record, long timestamp);
}

/**
 * An output for watermarks. The output accepts watermarks and idleness (inactivity) status.
 */
@Public
public interface WatermarkOutput {

	/**
	 * Emits the given watermark

/**
 * The interface for Source. It acts like a factory class that helps construct
 * the {@link SplitEnumerator} and {@link SourceReader} and corresponding
 * serializers.
 *
 * @param <T>        The type of records produced by the source.
 * @param <SplitT>   The type of splits handled by the source.
 * @param <EnumChkT> The type of the enumerator checkpoints.
 */
public interface Source<T, SplitT extends SourceSplit, EnumChkT> extends Serializable {

	/**
	 * Checks whether the source supports the given boundedness.
	 *
	 * <p>Some sources might only support either continuous unbounded streams, or
	 * bounded streams.
	 *
	 * <p>Emitting @parama boundednesswatermark Thealso boundednessimplicitly to check.
	 * @return <code>true</code> if the given boundedness is supported, <code>false</code> otherwisemarks the stream as <i>active</i>, ending
	 * previously marked idleness.
	 */
	booleanvoid supportsBoundednessemitWatermark(BoundednessWatermark boundednesswatermark);

	/**
	 * CreatesMarks athis newoutput readeras toidle, readmeaning data from the spits it gets assigned.
	 * The reader starts fresh and does not have any state to resume.
	 *that downstream operations do not
	 * @paramwait configfor A flat config forwatermarks from this source operatoroutput.
	 *
	 @param* readerContext<p>An Theoutput {@linkbecomes SourceReaderContextactive context}again foras thesoon source reader.
	 * @return A new SourceReaderas the next watermark is emitted.
	 */
	SourceReader<T, SplitT> createReader(
			Configuration config,
			SourceReaderContext readerContext);

	void markIdle();
}

/**
	 * Watermarks Createsare athe newprogress SplitEnumeratorindicators forin thisthe source,data startingstreams. aA newwatermark input.signifies
	 *
	 *that @paramno configevents Thewith configurationa fortimestamp this operator.
	 * @param enumContext The {@link SplitEnumeratorContext context} for the split enumerator.
	 * @return A new SplitEnumerator.
	 */
	SplitEnumerator<SplitT, EnumChkT> createEnumerator(
			Configuration config,
			SplitEnumeratorContext<SplitT> enumContext);

	/**
	 * Restores an enumerator from a checkpoint.
	 *
	 * @param config The configuration of this operator.
	 * @param enumContext The {@link SplitEnumeratorContext context} for the restored split enumerator.
	 * @param checkpoint The checkpoint to restore the SplitEnumerator from.
	 * @return A SplitEnumerator restored from the given checkpoint.
	 */
	SplitEnumerator<SplitT, EnumChkT> restoreEnumerator(
			Configuration config,
			SplitEnumeratorContext<SplitT> enumContext,
			EnumChkT checkpoint) throws IOException;

	// ------------------------------------------------------------------------
	//  serializers for the metadata
	// ------------------------------------------------------------------------

	/**
	 * Creates a serializer for the source splits. Splits are serialized when sending them
	 * from enumerator to reader, and when checkpointing the reader's current state.
	 *
	 * @return The serializer for the split type.
	 */
	SimpleVersionedSerializer<SplitT> getSplitSerializer();

	/**
	 * Creates the serializer for the {@link SplitEnumerator} checkpoint.
	 * The serializer is used for the result of the {@link SplitEnumerator#snapshotState()}
	 * method.
	 *
	 * @return The serializer for the SplitEnumerator checkpoint.
	 */
	SimpleVersionedSerializer<EnumChkT> getEnumeratorCheckpointSerializer();
}


/**
 * The boundedness of the source: "bounded" for the currently available data (batch style),
 * "continuous unbounded" for a continuous streaming style source.
 */
public enum Boundedness {smaller or equal to the watermark's time will occur after the
 * water. A watermark with timestamp <i>T</i> indicates that the stream's event time has progressed
 * to time <i>T</i>.
 *
 * <p>Watermarks are created at the sources and propagate through the streams and operators.
 *
 * <p>In some cases a watermark is only a heuristic, meaning some events with a lower timestamp
 * may still follow. In that case, it is up to the logic of the operators to decide what to do
 * with the "late events". Operators can for example ignore these late events, route them to a
 * different stream, or send update to their previously emitted results.
 *
 * <p>When a source reaches the end of the input, it emits a final watermark with timestamp
 * {@code Long.MAX_VALUE}, indicating the "end of time".
 *
 * <p>Note: A stream's time starts with a watermark of {@code Long.MIN_VALUE}. That means that all records
 * in the stream with a timestamp of {@code Long.MIN_VALUE} are immediately late.
 */
@Public
public final class Watermark implements Serializable {

	private static final long serialVersionUID = 1L;

	/** Thread local formatter for stringifying the timestamps. */
	private static final ThreadLocal<SimpleDateFormat> TS_FORMATTER = ThreadLocal.withInitial(
		() -> new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS"));

	// ------------------------------------------------------------------------

	/** The watermark that signifies end-of-event-time. */
	public static final Watermark MAX_WATERMARK = new Watermark(Long.MAX_VALUE);

	// ------------------------------------------------------------------------

	/** The timestamp of the watermark in milliseconds. */
	private final long timestamp;

	/**
	 * Creates Aa boundednew sourcewatermark processeswith the datagiven thattimestamp is currently available and will end after thatin milliseconds.
	 */
	public * <p>When a source produces a bounded stream, the runtime may activate additional optimizations
	 * that are suitable only for bounded input. Incorrectly producing unbounded data when the source
	 * is set to produce a bounded stream will often result in programs that do not output any results
	 * and may eventually fail due to runtime errors (out of memory or storage).
	 */
	BOUNDED,

	/**
	 * A continuous unbounded source continuously processes all data as it comes.
	 *
	 * <p>The source may run forever (until the program is terminated) or might actually end at some point,
	 * based on some source-specific conditions. Because that is not transparent to the runtime,
	 * the runtime will use an execution mode for continuous unbounded streams whenever this mode
	 * is chosen.
	 */
	CONTINUOUS_UNBOUNDED
}

/**
 * <p>AlthoughA theinterface implementationof cana emitsplit multipleenumerator recordsresponsible intofor the given SourceOutput,
	followings:
 * it1. isdiscover recommendedthe notsplits doingfor so. Instead, emit one record into the SourceOutput
	 * and return a {@link Status#AVAILABLE_NOW} to let the caller thread
	 * know there are more records available.
	 the {@link SourceReader} to read.
 * 2. assign the splits to the source reader.
 */
public interface SplitEnumerator<SplitT extends SourceSplit, CheckpointT> extends AutoCloseable {

	/**
	 * @returnStart Thethe {@link Status} of the SourceReader after the method invocationsplit enumerator.
	 *
	 * <p>The default behavior does nothing.
	 */
	Statusvoid pollNextstart(SourceOutput<T> sourceOutput) throws Exception;

	/**
	 * Handles the source event from the source reader.
	 *
	 * Checkpoint on the state of @param subtaskId the subtask id of the source reader who sent the source event.
	 *
	 *@param @returnsourceEvent the source stateevent offrom the source reader.
	 */
	List<SplitT>void snapshotState(handleSourceEvent(int subtaskId, SourceEvent sourceEvent);

	/**
	 * @returnAdd a future that will be completed once there is a record available to poll split back to the split enumerator. It will only happen when a {@link SourceReader} fails
	 * and there are splits assigned to it after the last successful checkpoint.
	 */
	CompletableFuture<Void> isAvailable();

	/**
	 * Adds a list of splits for this reader to read.
	 ** @param splits The split to add back to the enumerator for reassignment.
	 * @param splitssubtaskId The splits assigned by the split enumerator id of the subtask to which the returned splits belong.
	 */
	void addSplitsaddSplitsBack(List<SplitT> splits, int subtaskId);

	/**
	 * HandleAdd a new source eventreader sent bywith the {@link SplitEnumerator}
	 *
	 * @param sourceEvent the event sent by the {@link SplitEnumerator}.
	 */
	default void handleSourceEvents(SourceEvent sourceEvent) {
		// Do nothing.
	}

	/*given subtask ID.
	 *
	 * The status of this @param subtaskId the subtask ID of the new source reader.
	 */
	enumvoid Status {
	addReader(int subtaskId);

	/**
	 * TheCheckpoints nextthe recordstate isof availablethis rightsplit nowenumerator.
	 */
		AVAILABLE_NOW,
	CheckpointT snapshotState();

	/**
	 * The next record will be available later. */
		AVAILABLE_LATER,
		/** The source reader has completed all the reading work.Called to close the enumerator, in case it holds on to any resources, like threads or
	 * network connections.
	 */
		FINISHED@Override
	}void close() throws IOException;
}

/**
 * A class containing the splits assignment to the source readers.
 *
 * <p>The assignment is always incremental. In another word, splits in the assignment are simply
 * added to the existing assignment.
 */
public class SplitsAssignment<SplitT extends SourceSplit> {
	private final Map<Integer, List<SplitT>> assignment;

	public SplitsAssignment(Map<Integer, List<SplitT>> assignment) {
		this.assignment = assignment;
	}

/**
 * A context class for the {@link SplitEnumerator}. This class serves the following purposes:
 * 1. Host information necessary for the SplitEnumerator to make split assignment decisions.
 * 2. Accept and track the split assignment from the enumerator.
 * 3. Provide a managed threading model so the split enumerators do not need to create their
 *    own internal threads.
 *
 * @param <SplitT> the type of the splits.
 */
public interface SplitEnumeratorContext<SplitT extends SourceSplit> {

	MetricGroup metricGroup();

	/**
	 * Send a source event to a source reader. The source reader is identified by its subtask id.
	 *
	 * @param subtaskId the subtask id of the source reader to send this event to.
	 * @param event the source event to send.
	 * @return a completable future which will be completed when the event is successfully sent.
	 */
	void sendEventToSourceReader(int subtaskId, SourceEvent event);

	/**
	 * Get the number of subtasks @return A mapping from subtask ID to their split assignment.
	 */
	 * @return the number of subtasks.
	 */
	int numSubtaskspublic Map<Integer, List<SplitT>> assignment() {
		return assignment;
	}

	@Override
	public String toString() {
		return assignment.toString();

		}
}

/**
	 * GetAn theinterface currentlyfor registeredthe readers.events Thepassed mappingbetween isthe fromSourceReaders subtask id to the reader infoand Enumerators.
	 */
	 * @return the currently registered readers.
	 */
	Map<Integer, ReaderInfo> registeredReaders();

	/**
	 * Assign the splits.
	 *
	 * @param newSplitAssignments the new split assignments to add.
	 */
	void assignSplits(SplitsAssignment<SplitT> newSplitAssignments);

	/**
	 * Invoke the callable and handover the return value to the handler which will be executed
	 * by the source coordinator.
	 *
	 * <p>It is important to make sure that the callable should not modify
	 * any shared state. Otherwise the there might be unexpected behavior.
	 *
	 * @param callable a callable to call.
	 * @param handler a handler that handles the return value of or the exception thrown from the callable.
	 */
	<T> void callAsync(Callable<T> callable, BiConsumer<T, Throwable> handler);

	/**
	 * Invoke the callable and handover the return value to the handler which will be executed
	 * by the source coordinator.
	 *
	 * <p>It is important to make sure that the callable should not modify
	 * any shared state. Otherwise the there might be unexpected behavior.
	 *
	 * @param callable the callable to call.
	 * @param handler a handler that handles the return value of or the exception thrown from the callable.
	 * @param initialDelay the initial delay of calling the callable.
	 * @param period the period between two invocations of the callable.
	 */
	<T> void callAsync(Callable<T> callable,
					   BiConsumer<T, Throwable> handler,
					   long initialDelay,
					   long period);
}

public class StreamExecutionEnvironment {
...
    public <T> DataStream<T> continuousSource(Source<T, ?, ?> source) {...}

    public <T> DataStream<T> continuousSource(Source<T, ?, ?> source, TypeInformation<T> type) {...}

    public <T> DataStream<T> boundedSource(Source<T, ?, ?> source) {...}

    public <T> DataStream<T> boundedSource(Source<T, ?, ?> source, TypeInformation<T> type) {...}
...
}

/**
 * An abstract implementation of {@link SourceReader} which provides some sychronization between
 * the mail box main thread and the SourceReader internal threads. This class allows user to have
 * a SourceReader implementation by just providing the following:
 * <ul>
 *     <li>A {@link SplitReader}.</li>
 *     <li>A {@link RecordEmitter}</li>
 *     <li>The logic to clean up a split state after it is finished.</li>
 *     <li>The logic to get the state from a {@link SourceSplit}.</li>
 *     <li>The logic to restore a {@link SourceSplit} from its state.</li>
 * </ul>
 *
 * @param <E> The rich element type that contains information for split state update or timestamp extraction.
 * @param <T> The final element type to emit.
 * @param <SplitT> the immutable split type.
 * @param <SplitStateT> the mutable type of split state.
 */
public abstract class SourceReaderBase<E, T, SplitT extends SourceSplit, SplitStateT>
		implements SourceReader<T, SplitT> {
	
	// -------------------- Abstract method to allow different implementations ------------------
	/**
	 * Handles the finished splits to clean the state if needed.
	 */
	protected abstract void onSplitFinished(Collection<String> finishedSplitIds);

	/**
	 * When new splits are added to the reader. The initialize the state of the new splits.
	 *
	 * @param split a newly added split.
	 */
	protected abstract SplitStateT initializedState(SplitT split);

	/**
	 * Convert a mutable SplitStateT to immutable SplitT.
	 *
	 * @param splitState splitState.
	 * @return an immutable Split state.
	 */
	protected abstract SplitT toSplitType(String splitId, SplitStateT splitState);
}