/**
 * 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 {

	/**
	 * Get the boundedness of this source.
	 * 
	 * @return the boundedness of this source.
	 */
	Boundedness getBoundedness

/**
 * 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 {

	/**
	 * Get the boundedness of this source.
	 * 
	 * @return the boundedness of this source.
	 */
	Boundedness getBoundedness();

	/**
	 * Creates a new reader to read data from the spits it gets assigned.
	 * The reader starts fresh and does not have any state to resume.
	 *
	 * @param readerContext The {@link SourceReaderContext context} for the source reader.
	 * @return A new SourceReader.
	 */
	SourceReader<T, SplitT> createReader(SourceReaderContext readerContext);

	/**
	 * Creates a new SplitEnumerator for this source, starting a new input.
	 *
	 * @param enumContext The {@link SplitEnumeratorContext context} for the split enumerator.
	 * @return A new SplitEnumerator.
	 */
	SplitEnumerator<SplitT, EnumChkT> createEnumerator(SplitEnumeratorContext<SplitT> enumContext);

	/**
	 * Restores an enumerator from a checkpoint.
	 *
	 * @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(
			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 fora new reader to read data from the {@link SplitEnumerator} checkpointspits it gets assigned.
	 * The serializerreader isstarts usedfresh forand thedoes resultnot ofhave theany {@link SplitEnumerator#snapshotState()}
	 * methodstate to resume.
	 *
	 * @return@param readerContext The serializer {@link SourceReaderContext context} for the SplitEnumeratorsource checkpointreader.
	 */
	SimpleVersionedSerializer<EnumChkT> getEnumeratorCheckpointSerializer();
}


 @return A new SourceReader.
	 */
	SourceReader<T, SplitT> createReader(SourceReaderContext readerContext);

	/**
	 * Creates a Thenew boundednessSplitEnumerator offor thethis source: "bounded" for the currently available data (batch style),
 * "continuous unbounded" for a continuous streaming style source.
 */
public enum Boundedness {, starting a new input.
	 *
	 * @param enumContext The {@link SplitEnumeratorContext context} for the split enumerator.
	 * @return A new SplitEnumerator.
	 */
	SplitEnumerator<SplitT, EnumChkT> createEnumerator(SplitEnumeratorContext<SplitT> enumContext);

	/**
	 * ARestores boundedan sourceenumerator processesfrom the data that is currently available and will end after thata checkpoint.
	 *
	 * <p>When@param aenumContext sourceThe produces{@link aSplitEnumeratorContext boundedcontext} stream,for the runtimerestored may activate additional optimizationssplit enumerator.
	 * 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
}

/**
	 * @returnAn ainterface futurefor thatall willthe beSplit completedtypes once there is a record available to pollto implement.
	 */
	CompletableFuture<Void> isAvailable();public interface SourceSplit {

	/**
	 * Get Addsthe asplit listid of splits for this readersource to readsplit.
	 *
	 *@return @paramid splitsof Thethis splits assigned by the split enumeratorsource split.
	 */
	voidString addSplitssplitId(List<SplitT> splits);
}

	/**
	 * The interface Handlefor a source eventreader sentwhich byis the {@link SplitEnumerator}
	 *
	responsible for reading the records from
 * @paramthe sourceEventsource thesplits event sentassigned by the {@link SplitEnumerator}.
	 */
	void handleSourceEvents(SourceEvent sourceEvent);

	/**
	 ** @param <T> The statustype of the record emitted by this source reader.
	 */
	enum Status {
		/**@param <SplitT> The nexttype recordof isthe availablethe rightsource nowsplits.
 */
		AVAILABLE_NOW,
		/** The next record will be available later. */
		AVAILABLE_LATER,
		/** The source reader has completed all the reading work.public interface SourceReader<T, SplitT extends SourceSplit> extends Serializable, AutoCloseable {

	/**
	 * Start the reader;
	 */
		FINISHED
	}
}

public interface SourceOutput<E> extends WatermarkOutput {

	void emitRecord(E record);

	void emitRecord(E record, long timestamp);
}

/**
 * A context for the source reader. It allows the source reader to get the context information and
 * allows the SourceReader to send source event to its split enumerator.
 */
public interface SourceReaderContext {

	/**
	 * AddReturns athe newmetric sourcegroup readerfor withthis the givenparallel subtask ID.
	 *
	 * @param@return subtaskIdmetric thegroup subtaskfor IDthis of the new source readerparallel subtask.
	 */
	voidMetricGroup addReadergetMetricGroup(int subtaskId);

	/**
	 * CheckpointsSend a thesource stateevent ofto thisthe splitcorresponding enumeratorSplitEnumerator.
	 */
	CheckpointT snapshotState();

	/**
	 *@param Calledevent toThe closesource theevent enumerator, in case it holds on to any resources, like threads or
	 * network connections.to send.
	 */
	@Override
	void closesendEventToEnumerator() throws IOExceptionSourceEvent event);
}

/**
 * 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@param callable a callable to call.
	 * @param handler a handler that handles the return value of or the exception thrown from the callableenumerator.
	 */
	<T> void callAsync(Callable<T> callable, BiConsumer<T, Throwable> handler 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();

	/**
	 * 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);
}

/**
 * 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;
	}

	/**
	 * @return A mapping from subtask ID to their split assignment.
	 */
	public Map<Integer, List<SplitT>> assignment() {
		return assignment;
	}

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

/**
 * An interface for the events passed between the SourceReaders and Enumerators.
 */
public interface SourceEvent extends Serializable {}

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);
}