/**
 * 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.
	 *
	 * @param boundedness The boundedness to check.
	 * @return <code>true</code> if the given boundedness is supported, <code>false</code> otherwise.
	 */
	boolean supportsBoundedness(Boundedness boundedness);

	/**
	 * 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 config A flat config for this source operator.
	 * @param readerContext The {@link SourceReaderContext context} for the source reader.
	 * @return A new SourceReader.
	 */
	SourceReader<T, SplitT> createReader(
			Configuration config,
			SourceReaderContext readerContext);

	/**
	 * Creates a new SplitEnumerator for this source, starting a new input.
	 *
	 * @param config The configuration for 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 {

	/**
	 * A bounded source processes the data that is currently available and will end after that.
	 *
	 * <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
}

/**
 * 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();
}

/**
 * The interface for a source reader which is responsible for reading the records from
 * the source splits assigned by {@link SplitEnumerator}.
 *
 * @param <T> The type of the record emitted by this source reader.
 * @param <SplitT> The type of the the source splits.
 */
public interface SourceReader<T, SplitT extends SourceSplit> extends Serializable, AutoCloseable {

	/**
	 * Start the reader;
	 */
	void start();

	/**
	 * Poll the next available record into the {@link SourceOutput}.
	 *
	 * <p>The implementation must make sure this method is non-blocking.
	 *
	 * <p>Although the implementation can emit multiple records into the given SourceOutput,
	 * it is recommended not doing 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.
	 *
	 * @return The {@link Status} of the SourceReader after the method invocation.
	 */
	Status pollNext(SourceOutput<T> sourceOutput) throws Exception;

	/**
	 * Checkpoint on the state of the source.
	 *
	 * @return the state of the source.
	 */
	List<SplitT> snapshotState();

	/**
	 * @return a future that will be completed once there is a record available to poll.
	 */
	CompletableFuture<Void> isAvailable();

	/**
	 * Adds a list of splits for this reader to read.
	 *
	 * @param splits The splits assigned by the split enumerator.
	 */
	void addSplits(List<SplitT> splits);

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

	/**
	 * The status of this reader.
	 */
	enum Status {
		/** The next record is available right now. */
		AVAILABLE_NOW,
		/** The next record will be available later. */
		AVAILABLE_LATER,
		/** The source reader has completed all the reading work. */
		FINISHED
	}
}

/**
 * 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 {

	/**
	 * Returns the metric group for this parallel subtask.
	 *
	 * @return metric group for this parallel subtask.
	 */
	MetricGroup getMetricGroup();

	/**
	 * Send a source event to the corresponding SplitEnumerator.
	 *
	 * @param event The source event to send.
	 */
	void sendEventToEnumerator(SourceEvent event);
}

/**
 * A interface of a split enumerator responsible for the followings:
 * 1. discover the splits for the {@link SourceReader} to read.
 * 2. assign the splits to the source reader.
 */
public interface SplitEnumerator<SplitT extends SourceSplit, CheckpointT> extends AutoCloseable {

	/**
	 * Start the split enumerator.
	 *
	 * <p>The default behavior does nothing.
	 */
	default void start() {
		// By default do nothing.
	}

	/**
	 * Handles the source event from the source reader.
	 *
	 * @param subtaskId the subtask id of the source reader who sent the source event.
	 * @param sourceEvent the source event from the source reader.
	 */
	void handleSourceEvent(int subtaskId, SourceEvent sourceEvent);

	/**
	 * Add a 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.
	 *
	 * @param splits The split to add back to the enumerator for reassignment.
	 * @param subtaskId The id of the subtask to which the returned splits belong.
	 */
	void addSplitsBack(List<SplitT> splits, int subtaskId);

/**
 * 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(); ctx) throws IOException;

	/**
	 * Creates a new SplitEnumerator for this source, starting a new input.
	 */
	SplitEnumerator<SplitT, EnumChkT> createEnumerator(Boundedness mode) throws IOException;

	/**
	 * Restores an enumerator from a checkpoint.
	 */
	SplitEnumerator<SplitT, EnumChkT> restoreEnumerator(Boundedness mode, EnumChkT checkpoint) throws IOException;

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

	/**
	 * CreatesSend a source serializerevent forto thea inputsource splitsreader. Splits are serialized when sending them The source reader is identified by its subtask id.
	 *
	 from* enumerator@param tosubtaskId reader,the andsubtask whenid checkpointingof the source reader's current state to send this event to.
	 */
	SimpleVersionedSerializer<SplitT> getSplitSerializer();

	/**
	 * Creates @param event the serializersource forevent the {@link SplitEnumerator} checkpointto send.
	 * @return Thea serializercompletable isfuture usedwhich forwill thebe resultcompleted ofwhen the {@link SplitEnumerator#snapshotState()}
	 * methodevent is successfully sent.
	 */
	SimpleVersionedSerializer<EnumChkT>void getEnumeratorCheckpointSerializer(sendEventToSourceReader(int subtaskId, SourceEvent event);
}


	/**
 * The boundedness of the source: "bounded" for	 * Get the currentlynumber available data (batch style),
of subtasks.
	 *
	 * "continuous@return unbounded"the fornumber a continuous streaming style sourceof subtasks.
	 */
public enum Boundedness {	int numSubtasks();

	/**
	 * AGet boundedthe sourcecurrently processesregistered thereaders. dataThe thatmapping is currentlyfrom availablesubtask andid willto endthe afterreader thatinfo.
	 *
	 * <p>When a source produces a bounded stream, the runtime may activate additional optimizations *
	 * @return the currently registered readers.
	 */
	Map<Integer, ReaderInfo> registeredReaders();

	/**
	 * thatAssign are suitable only for bounded input. Incorrectly producing unbounded data when the sourcethe splits.
	 *
	 * @param newSplitAssignments the new split assignments to add.
	 * is set to produce a bounded stream will often result in programs that do not output any results/
	void assignSplits(SplitsAssignment<SplitT> newSplitAssignments);

	/**
	 * Invoke the callable and checks its return value. If the return value is true then
	 * andnotify maythe eventuallysource failcoordinator duethat toa runtimenew errorssplit (outassignment of memory or storage)is available.
	 */
	BOUNDED,

	/**
	 * A continuous unbounded source continuously processes all data as it comes.
	 * * <p>It is important to make sure that the callable and handler does not modify
	 * <p>Theany sourceshared may run forever (until the program is terminated) orstate. Otherwise the there might actuallybe end at some point,unexpected behavior.
	 *
	 * based@param oncallable some source-specific conditions. Because that is not transparent to the runtime,a callable to call.
	 * the@param runtimehandler willa usehandler anthat executionhandles modethe forreturn continuousvalue unboundedof streamsor wheneverthe thisexception mode
	thrown *from isthe chosencallable.
	 */
	CONTINUOUS_UNBOUNDED
}

/**
 * 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;
	}

	/**
	 * Called periodically @return A mapping from subtask ID to discovertheir furthersplit splitsassignment.
	 */
	public *Map<Integer, @returnList<SplitT>> Returns true if further splits were discovered, false if not.
	 */
	boolean discoverMoreSplits() throws IOException;

	/**
	 * Continuous enumeration is only applicable to unbounded sources.
	 */
	default boolean isEndOfInput() {
		return false;
	}assignment() {
		return assignment;
	}

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

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