在上一篇《Kafka Consumer多线程实例》中我们讨论了KafkaConsumer多线程的两种写法：多KafkaConsumer多线程以及单KafkaConsumer多线程。在第二种用法中我使用的是自动提交的方式，省去了多线程提交位移的麻烦。很多人跑来问如果是手动提交应该怎么写？由于KafkaConsumer不是线程安全的，因此我们不能简单地在多个线程中直接调用consumer.commitSync来提交位移。本文将给出一个实际的例子来模拟多线程消费以及手动提交位移。
本例中包含3个类：
- ConsumerThreadHandler类：consumer多线程的管理类，用于创建线程池以及为每个线程分配任务。另外consumer位移的提交也在这个类中进行
- ConsumerWorker类：本质上是一个Runnable，执行真正的消费逻辑并上报位移信息给ConsumerThreadHandler
- Main类：测试主方法类
测试代码
ConsumerWorker类
package huxi.test.consumer.multithreaded;
import org.apache.kafka.clients.consumer.ConsumerRecord;
import org.apache.kafka.clients.consumer.ConsumerRecords;
import org.apache.kafka.clients.consumer.OffsetAndMetadata;
import org.apache.kafka.common.TopicPartition;
import java.util.List;
import java.util.Map;
public class ConsumerWorker<K, V> implements Runnable {
private final ConsumerRecords<K, V> records;
private final Map<TopicPartition, OffsetAndMetadata> offsets;
public ConsumerWorker(ConsumerRecords<K, V> record, Map<TopicPartition, OffsetAndMetadata> offsets) {
this.records = record;
this.offsets = offsets;
}
@Override
public void run() {
for (TopicPartition partition : records.partitions()) {
List<ConsumerRecord<K, V>> partitionRecords = records.records(partition);
for (ConsumerRecord<K, V> record : partitionRecords) {
// 插入消息处理逻辑，本例只是打印消息
System.out.println(String.format("topic=%s, partition=%d, offset=%d",
record.topic(), record.partition(), record.offset()));
}
// 上报位移信息
long lastOffset = partitionRecords.get(partitionRecords.size() - 1).offset();
synchronized (offsets) {
if (!offsets.containsKey(partition)) {
offsets.put(partition, new OffsetAndMetadata(lastOffset + 1));
} else {
long curr = offsets.get(partition).offset();
if (curr <= lastOffset + 1) {
offsets.put(partition, new OffsetAndMetadata(lastOffset + 1));
}
}
}
}
}
}
ConsumerThreadHandler类
package huxi.test.consumer.multithreaded;
import org.apache.kafka.clients.consumer.ConsumerRebalanceListener;
import org.apache.kafka.clients.consumer.ConsumerRecords;
import org.apache.kafka.clients.consumer.KafkaConsumer;
import org.apache.kafka.clients.consumer.OffsetAndMetadata;
import org.apache.kafka.common.TopicPartition;
import org.apache.kafka.common.errors.WakeupException;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import java.util.Properties;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
public class ConsumerThreadHandler<K, V> {
private final KafkaConsumer<K, V> consumer;
private ExecutorService executors;
private final Map<TopicPartition, OffsetAndMetadata> offsets = new HashMap<>();
public ConsumerThreadHandler(String brokerList, String groupId, String topic) {
Properties props = new Properties();
props.put("bootstrap.servers", brokerList);
props.put("group.id", groupId);
props.put("enable.auto.commit", "false");
props.put("auto.offset.reset", "earliest");
props.put("key.deserializer", "org.apache.kafka.common.serialization.ByteArrayDeserializer");
props.put("value.deserializer", "org.apache.kafka.common.serialization.ByteArrayDeserializer");
consumer = new KafkaConsumer<>(props);
consumer.subscribe(Arrays.asList(topic), new ConsumerRebalanceListener() {
@Override
public void onPartitionsRevoked(Collection<TopicPartition> partitions) {
consumer.commitSync(offsets);
}
@Override
public void onPartitionsAssigned(Collection<TopicPartition> partitions) {
offsets.clear();
}
});
}
/**
* 消费主方法
* @param threadNumber 线程池中线程数
*/
public void consume(int threadNumber) {
executors = new ThreadPoolExecutor(
threadNumber,
threadNumber,
0L,
TimeUnit.MILLISECONDS,
new ArrayBlockingQueue<Runnable>(1000),
new ThreadPoolExecutor.CallerRunsPolicy());
try {
while (true) {
ConsumerRecords<K, V> records = consumer.poll(1000L);
if (!records.isEmpty()) {
executors.submit(new ConsumerWorker<>(records, offsets));
}
commitOffsets();
}
} catch (WakeupException e) {
// swallow this exception
} finally {
commitOffsets();
consumer.close();
}
}
private void commitOffsets() {
// 尽量降低synchronized块对offsets锁定的时间
Map<TopicPartition, OffsetAndMetadata> unmodfiedMap;
synchronized (offsets) {
if (offsets.isEmpty()) {
return;
}
unmodfiedMap = Collections.unmodifiableMap(new HashMap<>(offsets));
offsets.clear();
}
consumer.commitSync(unmodfiedMap);
}
public void close() {
consumer.wakeup();
executors.shutdown();
}
}
Main类
package huxi.test.consumer.multithreaded;
public class Main {
public static void main(String[] args) {
String brokerList = "localhost:9092";
String topic = "test-topic";
String groupID = "test-group";
final ConsumerThreadHandler<byte[], byte[]> handler = new ConsumerThreadHandler<>(brokerList, groupID, topic);
final int cpuCount = Runtime.getRuntime().availableProcessors();
Runnable runnable = new Runnable() {
@Override
public void run() {
handler.consume(cpuCount);
}
};
new Thread(runnable).start();
try {
// 20秒后自动停止该测试程序
Thread.sleep(20000L);
} catch (InterruptedException e) {
// swallow this exception
}
System.out.println("Starting to close the consumer...");
handler.close();
}
}
测试步骤
1. 首先创建一个测试topic： test-topic，10个分区，并使用kafka-producer-perf-test.sh脚本生产50万条消息
2. 运行Main，假定group.id设置为test-group
3. 新开一个终端，不断地运行以下脚本监控consumer group的消费进度
bin/kafka-consumer-groups.sh --bootstrap-server localhost:9092 --describe --group test-group
测试结果
LAG列全部为0表示consumer group的位移提交正常。值得一提的是，各位可以通过控制consumer.poll的超时时间来控制ConsumerThreadHandler类提交位移的频率。
感谢QQ群友的提醒，这种方式有丢失数据的时间窗口——假设T1线程在t0时间消费分区0的位移=100的消息M1，而T2线程在t1时间消费分区0的位移=101的消息M2。现在假设t3时T2线程先完成处理，于是上报位移101给Handler，但此时T1线程尚未处理完成。t4时handler提交位移101，之后T1线程发生错误，抛出异常导致位移100的消息消费失败，但由于位移已经提交到101，故消息丢失~。
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