如何在kafka 0.9.0中使用多线程消费者？

Question

kafka 的文档给出了一个方法，描述如下：

One Consumer Per Thread:A simple option is to give each thread its own consumer > instance.

我的代码：

public class KafkaConsumerRunner implements Runnable {

    private final AtomicBoolean closed = new AtomicBoolean(false);
    private final CloudKafkaConsumer consumer;
    private final String topicName;

    public KafkaConsumerRunner(CloudKafkaConsumer consumer, String topicName) {
        this.consumer = consumer;
        this.topicName = topicName;
    }

    @Override
    public void run() {
        try {
            this.consumer.subscribe(topicName);
            ConsumerRecords<String, String> records;
            while (!closed.get()) {
                synchronized (consumer) {
                    records = consumer.poll(100);
                }
                for (ConsumerRecord<String, String> tmp : records) {
                    System.out.println(tmp.value());
                }
            }
        } catch (WakeupException e) {
            // Ignore exception if closing
            System.out.println(e);
            //if (!closed.get()) throw e;
        }
    }

    // Shutdown hook which can be called from a separate thread
    public void shutdown() {
        closed.set(true);
        consumer.wakeup();
    }

    public static void main(String[] args) {
        CloudKafkaConsumer kafkaConsumer = KafkaConsumerBuilder.builder()
                .withBootstrapServers("172.31.1.159:9092")
                .withGroupId("test")
                .build();
        ExecutorService executorService = Executors.newFixedThreadPool(5);
        executorService.execute(new KafkaConsumerRunner(kafkaConsumer, "log"));
        executorService.execute(new KafkaConsumerRunner(kafkaConsumer, "log.info"));
        executorService.shutdown();
    }
}

但它不起作用并抛出异常：

java.util.ConcurrentModificationException: KafkaConsumer is not safe for multi-threaded access

此外，我阅读了Flink（分布式流和批数据处理的开源平台）的源码。 Flink使用多线程消费者和我的差不多

long pollTimeout = Long.parseLong(flinkKafkaConsumer.properties.getProperty(KEY_POLL_TIMEOUT, Long.toString(DEFAULT_POLL_TIMEOUT)));
pollLoop: while (running) {
    ConsumerRecords<byte[], byte[]> records;
    //noinspection SynchronizeOnNonFinalField
    synchronized (flinkKafkaConsumer.consumer) {
        try {
            records = flinkKafkaConsumer.consumer.poll(pollTimeout);
        } catch (WakeupException we) {
            if (running) {
                throw we;
            }
            // leave loop
            continue;
        }
    }

flink code of mutli-thread

怎么了？

Answer 1

Kafka 消费者不是线程安全的。正如您在问题中指出的那样，该文件指出

A simple option is to give each thread its own consumer instance

但是在您的代码中，您拥有由不同 KafkaConsumerRunner 实例包装的相同消费者实例。因此，多个线程正在访问同一个消费者实例。 kafka 文档明确说明

The Kafka consumer is NOT thread-safe. All network I/O happens in the thread of the application making the call. It is the responsibility of the user to ensure that multi-threaded access is properly synchronized. Un-synchronized access will result in ConcurrentModificationException.

这正是您收到的异常。

Answer 2

它在您调用订阅时抛出异常。 this.consumer.subscribe(topicName);

将该块移动到同步块中，如下所示：

@Override
public void run() {
    try {
        synchronized (consumer) {
            this.consumer.subscribe(topicName);
        }
        ConsumerRecords<String, String> records;
        while (!closed.get()) {
            synchronized (consumer) {
                records = consumer.poll(100);
            }
            for (ConsumerRecord<String, String> tmp : records) {
                System.out.println(tmp.value());
            }
        }
    } catch (WakeupException e) {
        // Ignore exception if closing
        System.out.println(e);
        //if (!closed.get()) throw e;
    }
}

Answer 3

也许不是你的情况，但如果你正在合并处理多个主题的数据，那么你可以使用同一个消费者从多个主题读取数据。如果没有，那么最好创建单独的作业来处理每个主题。

如何在kafka 0.9.0中使用多线程消费者？

How to use multi-thread consumer in kafka 0.9.0?

java

multithreading

apache-kafka

distributed-computing

apache-flink