org.apache.flink.streaming.runtime.tasks.SourceStreamTask Maven / Gradle / Ivy

Go to download
/*
 * 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.runtime.tasks;

import org.apache.flink.streaming.api.operators.Output;
import org.apache.flink.streaming.api.operators.StreamSource;
import org.apache.flink.streaming.api.watermark.Watermark;
import org.apache.flink.streaming.runtime.streamrecord.StreamRecord;

/**
 * Task for executing streaming sources.
 *
 * One important aspect of this is that the checkpointing and the emission of elements must never
 * occur at the same time. The execution must be serial. This is achieved by having the contract
 * with the StreamFunction that it must only modify its state or emit elements in
 * a synchronized block that locks on the lock Object. Also, the modification of the state
 * and the emission of elements must happen in the same block of code that is protected by the
 * synchronized block.
 *
 * @param  Type of the output elements of this source.
 */
public class SourceStreamTask extends StreamTask> {

	@Override
	protected void init() {
		// does not hold any resources, so no initialization needed
	}

	@Override
	protected void cleanup() {
		// does not hold any resources, so no cleanup needed
	}
	

	@Override
	protected void run() throws Exception {
		final Object checkpointLock = getCheckpointLock();
		final SourceOutput> output = new SourceOutput<>(getHeadOutput(), checkpointLock);
		headOperator.run(checkpointLock, output);
	}
	
	@Override
	protected void cancelTask() throws Exception {
		headOperator.cancel();
	}

	// ------------------------------------------------------------------------
	
	/**
	 * Special output for sources that ensures that sources synchronize on  the lock object before
	 * emitting elements.
	 *
	 * 
	 * This is required to ensure that no concurrent method calls on operators later in the chain
	 * can occur. When operators register a timer the timer callback is synchronized
	 * on the same lock object.
	 *
	 * @param  The type of elements emitted by the source.
	 */
	private class SourceOutput implements Output {
		
		private final Output output;
		private final Object lockObject;

		public SourceOutput(Output output, Object lockObject) {
			this.output = output;
			this.lockObject = lockObject;
		}

		@Override
		public void emitWatermark(Watermark mark) {
			synchronized (lockObject) {
				output.emitWatermark(mark);
			}
		}

		@Override
		public void collect(T record) {
			synchronized (lockObject) {
				checkTimerException();
				output.collect(record);
			}
		}

		@Override
		public void close() {
			output.close();
		}
	}
}