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From tzulitai <...@git.apache.org>
Subject [GitHub] flink pull request #2789: [FLINK-5048] [Kafka Consumer] Change thread model ...
Date Fri, 11 Nov 2016 17:57:11 GMT
Github user tzulitai commented on a diff in the pull request:

    https://github.com/apache/flink/pull/2789#discussion_r87625136
  
    --- Diff: flink-streaming-connectors/flink-connector-kafka-0.9/src/main/java/org/apache/flink/streaming/connectors/kafka/internal/Handover.java
---
    @@ -0,0 +1,136 @@
    +/*
    + * 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.connectors.kafka.internal;
    +
    +import org.apache.flink.annotation.VisibleForTesting;
    +import org.apache.flink.util.ExceptionUtils;
    +import org.apache.kafka.clients.consumer.ConsumerRecords;
    +
    +import javax.annotation.Nonnull;
    +import java.io.Closeable;
    +
    +import static org.apache.flink.util.Preconditions.checkNotNull;
    +
    +public final class Handover implements Closeable {
    +
    +	private final Object lock = new Object();
    +
    +	private ConsumerRecords<byte[], byte[]> next;
    +	private Throwable error;
    +	private boolean wakeup;
    +
    +	@Nonnull
    +	public ConsumerRecords<byte[], byte[]> pollNext() throws Exception {
    +		synchronized (lock) {
    +			while (next == null && error == null) {
    +				lock.wait();
    +			}
    +
    +			ConsumerRecords<byte[], byte[]> n = next;
    +			if (n != null) {
    +				next = null;
    +				lock.notifyAll();
    +				return n;
    +			}
    +			else {
    +				ExceptionUtils.rethrowException(error, error.getMessage());
    +
    +				// this statement cannot be reached since the above method always throws an exception
    +				// this is only here to silence the compiler and any warnings
    +				return ConsumerRecords.empty(); 
    +			}
    +		}
    +	}
    +
    +	public void produce(final ConsumerRecords<byte[], byte[]> element)
    +			throws InterruptedException, WakeupException, ClosedException {
    +
    +		checkNotNull(element);
    +
    +		synchronized (lock) {
    +			while (next != null && !wakeup) {
    +				lock.wait();
    +			}
    +
    +			wakeup = false;
    +
    +			// if there is still an element, we must have been woken up
    +			if (next != null) {
    +				throw new WakeupException();
    +			}
    +			// if there is no error, then this is open and can accept this element
    +			else if (error == null) {
    +				next = element;
    +				lock.notifyAll();
    +			}
    +			// an error marks this as closed for the producer
    +			else {
    +				throw new ClosedException();
    +			}
    +		}
    +	}
    +
    +	public void reportError(Throwable t) {
    +		checkNotNull(t);
    +
    +		synchronized (lock) {
    +			// do not override the initial exception
    +			if (error == null) {
    +				error = t;
    +			}
    +			next = null;
    +			lock.notifyAll();
    +		}
    +	}
    +
    +	@Override
    +	public void close() {
    +		synchronized (lock) {
    +			next = null;
    +			wakeup = false;
    +
    +			if (error == null) {
    +				error = new ClosedException();
    +			}
    +			lock.notifyAll();
    +		}
    +	}
    +
    +	public void wakeupProducer() {
    +		synchronized (lock) {
    +			wakeup = true;
    +			lock.notifyAll();
    +		}
    +	}
    +
    +	@VisibleForTesting
    +	Object getLock() {
    +		return lock;
    +	}
    +
    +	// ------------------------------------------------------------------------
    +
    +	public static final class ClosedException extends IllegalStateException {
    --- End diff --
    
    Not really sure if extending `IllegalStateException` is good here, because `ClosedException`
will be rethrown to the fetcher even on a normal call to `Handover#close()`.
    
    I understand it's to allow the cancellation process be faster, but somehow I think a normal
`close()` _after_ `poll()` was called doesn't add up to me as an illegal state.


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