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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (FLINK-5048) Kafka Consumer (0.9/0.10) threading model leads problematic cancellation behavior
Date Fri, 11 Nov 2016 17:57:59 GMT

    [ https://issues.apache.org/jira/browse/FLINK-5048?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15657657#comment-15657657
] 

ASF GitHub Bot commented on FLINK-5048:
---------------------------------------

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.


> Kafka Consumer (0.9/0.10) threading model leads problematic cancellation behavior
> ---------------------------------------------------------------------------------
>
>                 Key: FLINK-5048
>                 URL: https://issues.apache.org/jira/browse/FLINK-5048
>             Project: Flink
>          Issue Type: Bug
>          Components: Kafka Connector
>    Affects Versions: 1.1.3
>            Reporter: Stephan Ewen
>            Assignee: Stephan Ewen
>             Fix For: 1.2.0
>
>
> The {{FLinkKafkaConsumer}} (0.9 / 0.10) spawns a separate thread that operates the KafkaConsumer.
That thread is shielded from interrupts, because the Kafka Consumer has not been handling
thread interrupts well.
> Since that thread is also the thread that emits records, it may block in the network
stack (backpressure) or in chained operators. The later case leads to situations where cancellations
get very slow unless that thread would be interrupted (which it cannot be).
> I propose to change the thread model as follows:
>   - A spawned consumer thread pull from the KafkaConsumer and pushes its pulled batch
of records into a blocking queue (size one)
>   - The main thread of the task will pull the record batches from the blocking queue
and emit the records.
> This allows actually for some additional I/O overlay while limiting the additional memory
consumption - only two batches are ever held, one being fetched and one being emitted.



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