/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.flink.streaming.api.checkpoint;

import org.apache.flink.annotation.PublicEvolving;
import org.apache.flink.streaming.api.functions.source.SourceFunction;
import org.apache.flink.util.FlinkException;

/**
 * Sources that implement this interface do not trigger checkpoints when receiving a
 * trigger message from the checkpoint coordinator, but when their input data/events
 * indicate that a checkpoint should be triggered.
 *
 * <p>Since sources cannot simply create a new checkpoint on their own, this mechanism
 * always goes together with a {@link WithMasterCheckpointHook hook on the master side}.
 * In a typical setup, the hook on the master tells the source system (for example
 * the message queue) to prepare a checkpoint. The exact point when the checkpoint is
 * taken is then controlled by the event stream received from the source, and triggered
 * by the source function (implementing this interface) in Flink when seeing the relevant
 * events.
 *
 * @param <T> Type of the elements produced by the source function
 * @param <CD> The type of the data stored in the checkpoint by the master that triggers
 */
@PublicEvolving
public interface ExternallyInducedSource<T, CD> extends SourceFunction<T>, WithMasterCheckpointHook<CD> {

	/**
	 * Sets the checkpoint trigger through which the source can trigger the checkpoint.
	 *
	 * @param checkpointTrigger The checkpoint trigger to set
	 */
	void setCheckpointTrigger(CheckpointTrigger checkpointTrigger);

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

	/**
	 * Through the {@code CheckpointTrigger}, the source function notifies the Flink
	 * source operator when to trigger the checkpoint.
	 */
	interface CheckpointTrigger {

		/**
		 * Triggers a checkpoint. This method should be called by the source
		 * when it sees the event that indicates that a checkpoint should be triggered.
		 *
		 * <p>When this method is called, the parallel operator instance in which the
		 * calling source function runs will perform its checkpoint and insert the
		 * checkpoint barrier into the data stream.
		 *
		 * @param checkpointId The ID that identifies the checkpoint.
		 *
		 * @throws FlinkException Thrown when the checkpoint could not be triggered, for example
		 *                        because of an invalid state or errors when storing the
		 *                        checkpoint state.
		 */
		void triggerCheckpoint(long checkpointId) throws FlinkException;
	}
}