kafka（三）—Kafka的Java代码示例和配置说明

本示以同步至个人博客liaosi's blog-Kafka的Java代码示例和配置说明，代码已上传至我的GitHub

配置类:Constants.java

消费者和生产者使用的几个常量。

package com.lzumetal.mq.kafka.demo;

/**
 * <p>Description: </p>
 *
 * @author: liaosi
 * @date: 2018-01-30
 */
public class Constants {

    final static String GROUP_ID = "test_group";
    final static String MY_TOPIC = "myTest";
    final static String KAFKA_SERVER_ADRESS = "192.168.128.1";
    final static int KAFKA_SERVER_PORT = 9092;


}

消息生产者：MyKafkaProducer.java

KafkaProducer(org.apache.kafka.clients.producer.KafkaProducer)是一个用于向kafka集群发送数据的客户端。producer是线程安全的，多个线程可以共享同一个 producer实例，而且这通常比在多个线程中每个线程创建一个实例速度要快些。参考KafkaProducer官方文档

package com.lzumetal.mq.kafka.demo;


import org.apache.kafka.clients.producer.KafkaProducer;
import org.apache.kafka.clients.producer.Producer;
import org.apache.kafka.clients.producer.ProducerRecord;
import org.junit.Before;
import org.junit.Test;

import java.util.Properties;
import java.util.concurrent.TimeUnit;

/**
 * <p>Description: </p>
 * Producer由一个持有未发送消息记录的资源池和一个用来向Kafka集群发送消息记录的后台IO线程组成。
 * 使用后未关闭producer将导致这些资源泄露。
 *
 * @author: liaosi
 * @date: 2018-01-30
 */
public class MyKafkaProducer {

    private Producer<String, String> producer;


    @Before
    public void init() {
        Properties props = new Properties();
        props.put("bootstrap.servers", Constants.KAFKA_SERVER_ADRESS + ":" + Constants.KAFKA_SERVER_PORT);

        /*ack 配置项用来控制producer要求leader确认多少消息后返回调用成功。
        当值为0时producer不需要等待任何确认消息。
        当值为1时只需要等待leader确认。
        当值为-1或all时需要全部ISR集合返回确认才可以返回成功。
        */
        //props.put("acks", "all");

        //当 retries > 0 时，如果发送失败，会自动尝试重新发送数据。发送次数为retries设置的值。
        props.put("retries", 0);
        props.put("batch.size", 16384);
        props.put("linger.ms", 1);
        props.put("buffer.memory", 33554432);

        //key.serializer 和 value.serializer 指定使用什么序列化方式将用户提供的key和value进行序列化。
        props.put("key.serializer", "org.apache.kafka.common.serialization.StringSerializer");
        props.put("value.serializer", "org.apache.kafka.common.serialization.StringSerializer");

        producer = new KafkaProducer<>(props);
    }

    @Test
    public void produceMsg() {

        for (int i = 0; i < 10; i++) {
            String msg = "Message_test_" + i;
            System.out.println("produce : " + msg);
            //send方法是异步的。当它被调用时，它会将消息记录添加到待发送缓冲区并立即返回。
            //使用这种方式可以使生产者聚集一批消息记录后一起发送，从而提高效率。
            producer.send(new ProducerRecord<>(Constants.MY_TOPIC, Integer.toString(i), msg));
            sleep(1);
        }
        producer.close();

    }

    private void sleep(int seconds) {
        try {
            TimeUnit.SECONDS.sleep(seconds);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }


}

producer的配置说明

bootstrap.servers

A list of host/port pairs to use for establishing the initial connection to the Kafka cluster. The client will make use of all servers irrespective of which servers are specified here for bootstrapping—this list only impacts the initial hosts used to discover the full set of servers. This list should be in the form host1:port1,host2:port2,.... Since these servers are just used for the initial connection to discover the full cluster membership (which may change dynamically), this list need not contain the full set of servers (you may want more than one, though, in case a server is down).

Kafka集群连接的host/port组,格式：host1:port1,host2:port2,…
这些server仅仅是用于初始化的连接，以发现集群所有成员（成员可能会动态的变化），这个列表不需要包含所有的servers（数量尽量不止一个，以防其中一个down机了）。

acks

The number of acknowledgments the producer requires the leader to have received before considering a request complete. This controls the durability of records that are sent. The following settings are allowed:


acks=0 If set to zero then the producer will not wait for any acknowledgment from the server at all. The record will be immediately added to the socket buffer and considered sent. No guarantee can be made that the server has received the record in this case, and the retries configuration will not take effect (as the client won't generally know of any failures). The offset given back for each record will always be set to -1.


acks=1 This will mean the leader will write the record to its local log but will respond without awaiting full acknowledgement from all followers. In this case should the leader fail immediately after acknowledging the record but before the followers have replicated it then the record will be lost.


acks=all This means the leader will wait for the full set of in-sync replicas to acknowledge the record. This guarantees that the record will not be lost as long as at least one in-sync replica remains alive. This is the strongest available guarantee. This is equivalent to the acks=-1 setting.

producer需要server接收到数据之后发出的确认接收的信号，此项配置就是指procuder需要多少个这样的确认信号。此配置实际上代表了数据备份的可用性。以下设置为常用选项：
（1）acks=0： 设置为0表示producer不需要等待任何确认收到的信息。副本将立即加到socket buffer并认为已经发送。没有任何保障可以保证此种情况下server已经成功接收数据，同时重试配置不会发生作用（因为客户端不知道是否失败）回馈的offset会总是设置为-1；
（2）acks=1： 这意味着至少要等待leader已经成功将数据写入本地log，但是并没有等待所有follower是否成功写入。这种情况下，如果follower没有成功备份数据，而此时leader又挂掉，则消息会丢失。
（3）acks=all： 这意味着leader需要等待所有备份都成功写入日志，这种策略会保证只要有一个备份存活就不会丢失数据。这是最强的保证。

key.serializer/key.serializer

Message record 的key, value的序列化类。

buffer.memory

The total bytes of memory the producer can use to buffer records waiting to be sent to the server. If records are sent faster than they can be delivered to the server the producer will block for max.block.ms after which it will throw an exception.


This setting should correspond roughly to the total memory the producer will use, but is not a hard bound since not all memory the producer uses is used for buffering. Some additional memory will be used for compression (if compression is enabled) as well as for maintaining in-flight requests.

producer可以用来缓存数据的内存大小。该值实际为RecordAccumulator类中的BufferPool，即Producer所管理的最大内存。但不是所有producer管理的内存都用作缓存，一些额外的内存会用于压缩（如果引入压缩机制），同样还有一些用于维护请求。
如果数据产生速度大于向broker发送的速度，producer会阻塞配置项max.block.ms所设定的值，超出这个时间则抛出异常。

retries

Setting a value greater than zero will cause the client to resend any record whose send fails with a potentially transient error. Note that this retry is no different than if the client resent the record upon receiving the error. Allowing retries without setting max.in.flight.requests.per.connection to 1 will potentially change the ordering of records because if two batches are sent to a single partition, and the first fails and is retried but the second succeeds, then the records in the second batch may appear first.

当设置 retries > 0 时，如果发送失败，会自动尝试重新发送数据，发送次数为retries设置的值。如果设定了retries但没有把max.in.flight.requests.per.connection 设置成 1则可能会改变数据的顺序，因为如果这两个batch都是发送到同一个partition，并且第一个batch发送失败而第二个发送成功，则第二个batch中的消息记录会比第一个的达到得早。
retries 的默认值是0。

batch.size

The producer will attempt to batch records together into fewer requests whenever multiple records are being sent to the same partition. This helps performance on both the client and the server. This configuration controls the default batch size in bytes.
No attempt will be made to batch records larger than this size.


Requests sent to brokers will contain multiple batches, one for each partition with data available to be sent.
A small batch size will make batching less common and may reduce throughput (a batch size of zero will disable batching entirely). A very large batch size may use memory a bit more wastefully as we will always allocate a buffer of the specified batch size in anticipation of additional records.

为了改善客户端和Kafka集群的性能，减少请求次数，producer会把要发送到同一个partition的批量消息作为batch发送，batch.size 是用来设置batch的字节大小。如果 batch.size 太小则可能会降低吞吐量，设置太大则可能会导致浪费，因为我们预先就需要腾出一个batch.size 大小的缓冲区来存贮将要发送达到该缓冲区的消息。
若将该值设为0，则不会进行批处理。

linger.ms

The producer groups together any records that arrive in between request transmissions into a single batched request. Normally this occurs only under load when records arrive faster than they can be sent out. However in some circumstances the client may want to reduce the number of requests even under moderate load. This setting accomplishes this by adding a small amount of artificial delay—that is, rather than immediately sending out a record the producer will wait for up to the given delay to allow other records to be sent so that the sends can be batched together. This can be thought of as analogous to Nagle's algorithm in TCP. This setting gives the upper bound on the delay for batching: once we get batch.size worth of records for a partition it will be sent immediately regardless of this setting, however if we have fewer than this many bytes accumulated for this partition we will 'linger' for the specified time waiting for more records to show up. This setting defaults to 0 (i.e. no delay). Setting linger.ms=5, for example, would have the effect of reducing the number of requests sent but would add up to 5ms of latency to records sent in the absence of load.

producer会将request传输之间到达的所有records聚合到一个批请求。通常这个值发生在欠负载情况下，record到达速度快于发送。但是在某些场景下，client即使在正常负载下也期望减少请求数量。这个设置就是如此，通过人工添加少量时延，而不是立马发送一个record，producer会等待所给的时延，以让其他records发送出去，这样就会被聚合在一起。这个类似于TCP的Nagle算法。该设置给了batch的时延上限：当我们获得一个partition的batch.size大小的records，就会立即发送出去，而不管该设置；但是如果对于这个partition没有累积到足够的record，会linger指定的时间等待更多的records出现。该设置的默认值为0(无时延)。例如，设置linger.ms=5，会减少request发送的数量，但是在无负载下会增加5ms的发送时延。

消息消费者：MyKafkaConsumer.java

package com.lzumetal.mq.kafka.demo;

import org.apache.kafka.clients.consumer.ConsumerRecord;
import org.apache.kafka.clients.consumer.ConsumerRecords;
import org.apache.kafka.clients.consumer.KafkaConsumer;
import org.junit.Test;

import java.util.*;
import java.util.concurrent.TimeUnit;

/**
 * <p>Description: </p>
 *
 * @author: liaosi
 * @date: 2018-01-30
 */
public class MyKafkaConsumer {


       /**
     * 自动提交offset
     */
    @Test
    public void comsumeMsgAutoCommit() {

        Properties props = new Properties();
        props.put("bootstrap.servers", Constants.KAFKA_SERVER_ADRESS + ":" + Constants.KAFKA_SERVER_PORT);
        props.put("group.id", Constants.GROUP_ID);
        props.put("enable.auto.commit", "true");
        props.put("auto.commit.interval.ms", "1000");
        props.put("key.deserializer", "org.apache.kafka.common.serialization.StringDeserializer");
        props.put("value.deserializer", "org.apache.kafka.common.serialization.StringDeserializer");
        KafkaConsumer<String, String> consumer = new KafkaConsumer<>(props);
        consumer.subscribe(Arrays.asList(Constants.MY_TOPIC));
        
        while (true) {
            ConsumerRecords<String, String> records = consumer.poll(100);
            for (ConsumerRecord<String, String> record : records) {
                System.out.printf("offset = %d, key = %s, value = %s%n", record.offset(), record.key(), record.value());
            }
            sleep(1);
        }
    }

    /**
     * 手动提交offset
     */
    @Test
    public void consumerMsgManualCommit() {
        Properties props = new Properties();
        props.put("bootstrap.servers", "localhost:9092");
        props.put("group.id", "test");
        props.put("enable.auto.commit", "false");
        props.put("auto.commit.interval.ms", "1000");
        props.put("session.timeout.ms", "30000");
        props.put("key.deserializer", "org.apache.kafka.common.serialization.StringDeserializer");
        props.put("value.deserializer", "org.apache.kafka.common.serialization.StringDeserializer");
        KafkaConsumer<String, String> consumer = new KafkaConsumer<>(props);
        consumer.subscribe(Arrays.asList(Constants.MY_TOPIC));
        final int minBatchSize = 200;
        List<ConsumerRecord<String, String>> buffer = new ArrayList<>();
        while (true) {
            ConsumerRecords<String, String> records = consumer.poll(100);
            for (ConsumerRecord<String, String> record : records) {
                buffer.add(record);
            }
            if (buffer.size() >= minBatchSize) {
                insertIntoDb(buffer);
                consumer.commitSync();
                buffer.clear();
            }
        }
    }

    private void insertIntoDb(List<ConsumerRecord<String, String>> buffer) {
        for (ConsumerRecord<String, String> record : buffer) {
            System.out.printf("insertIntoDb:offset = %d, key = %s, value = %s%n", record.offset(), record.key(), record.value());
        }
    }


    private void sleep(int seconds) {
        try {
            TimeUnit.SECONDS.sleep(seconds);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

poll方法

从上一次消费完毕后提交的offset处拉取数据，消费后提交offset有两种方式，手动和自动。

Consumer读取partition中的数据是通过调用发起一个fetch请求来执行的。而从Kafka Consumer来看，它有一个poll方法。但是这个poll方法只是可能会发起fetch请求。原因是：Consumer每次发起fetch请求时，读取到的数据是有限制的，通过配置项max.partition.fetch.bytes来限制。而在执行poll方法时，会根据配置项max.poll.records来限制一次最多pool多少个record。

那么就可能出现这样的情况： 在满足max.partition.fetch.bytes限制的情况下，假如fetch到了100个record，放到本地缓存后，由于max.poll.records限制每次只能poll出15个record。那么KafkaConsumer就需要执行7次poll方法才能将这一次通过网络发起的fetch请求所fetch到的这100个record消费完毕。其中前6次是每次poll中15个record，最后一次是poll出10个record。

在consumer中，还有一个配置项：max.poll.interval.ms，它表示最大的poll数据间隔，默认值是3秒。如果超过这个间隔没有发起pool请求，但heartbeat仍旧在发，就认为该consumer处于 livelock状态。就会将该consumer移出consumer group。所以为了不使 Consumer 自己被移出，Consumer 应该不停的发起poll(timeout)操作。而这个动作 KafkaConsumer Client是不会帮我们做的，这就需要自己在程序中不停的调用poll方法了。

commit offset

当一个consumer因某种原因退出Group时，进行重新分配partition后，同一group中的另一个consumer在读取该partition时，怎么能够知道上一个consumer该从哪个offset的message读取呢？也是是如何保证同一个group内的consumer不重复消费消息呢？上面说了一次走网络的fetch请求会拉取到一定量的数据，但是这些数据还没有被消息完毕，Consumer就挂掉了，下一次进行数据fetch时，是否会从上次读到的数据开始读取，而导致Consumer消费的数据丢失吗？

为了做到这一点，当使用完poll从本地缓存拉取到数据之后，需要client调用commitSync方法（或者commitAsync方法）去commit 下一次该去读取 哪一个offset的message。

而这个commit方法会通过走网络的commit请求将offset在coordinator中保留，这样就能够保证下一次读取（不论进行了rebalance）时，既不会重复消费消息，也不会遗漏消息。

对于offset的commit，Kafka Consumer Java Client支持两种模式：由Kafka Consumer自动提交，或者是用户通过调用commitSync、commitAsync方法的方式手动完成offset的提交。

Consumer配置说明

bootstrap.servers

参考生产者中的bootstrap.servers配置说明

key.descrializer、value.descrializer

Message record 的key, value的反序列化类。

group.id

A unique string that identifies the consumer group this consumer belongs to. This property is required if the consumer uses either the group management functionality by using subscribe(topic) or the Kafka-based offset management strategy.

用于表示该consumer想要加入到哪个group中。默认值是 “”。

heartbeat.interval.ms

The expected time between heartbeats to the consumer coordinator when using Kafka's group management facilities. Heartbeats are used to ensure that the consumer's session stays active and to facilitate rebalancing when new consumers join or leave the group. The value must be set lower than session.timeout.ms, but typically should be set no higher than 1/3 of that value. It can be adjusted even lower to control the expected time for normal rebalances.

心跳间隔。心跳是在consumer与coordinator之间进行的。心跳用来保持consumer的会话，并且在有consumer加入或者离开group时帮助进行rebalance。
这个值必须设置的小于session.timeout.ms，因为：当Consumer由于某种原因不能发Heartbeat到coordinator时，并且时间超过session.timeout.ms时，就会认为该consumer已退出，它所订阅的partition会分配到同一group 内的其它的consumer上。

通常设置的值要低于session.timeout.ms的1/3。默认值是：3000 （3s）

session.timeout.ms

The timeout used to detect consumer failures when using Kafka's group management facility. The consumer sends periodic heartbeats to indicate its liveness to the broker. If no heartbeats are received by the broker before the expiration of this session timeout, then the broker will remove this consumer from the group and initiate a rebalance. Note that the value must be in the allowable range as configured in the broker configuration by group.min.session.timeout.ms and group.max.session.timeout.ms.

Consumer session 过期时间。consumer会发送周期性的心跳表明该consumer是活着的。如果超过session.timeout.ms设定的值仍然没有收到心跳，zebroker会把这个consumer从group中移除，并且重新rebalance。
这个值必须设置在broker configuration中的group.min.session.timeout.ms 与 group.max.session.timeout.ms之间。
其默认值是：10000 （10 s）

enable.auto.commit

If true the consumer's offset will be periodically committed in the background.

设置Consumer 在 commit 方式是否是自动调焦。默认值是true。

auto.commit.interval.ms

The frequency in milliseconds that the consumer offsets are auto-committed to Kafka if enable.auto.commit is set to true.

自动提交间隔。范围：[0,Integer.MAX]，默认值是 5000 （5 s）

auto.offset.reset

What to do when there is no initial offset in Kafka or if the current offset does not exist any more on the server (e.g. because that data has been deleted):

• earliest: automatically reset the offset to the earliest offset
• latest: automatically reset the offset to the latest offset
• none: throw exception to the consumer if no previous offset is found for the consumer's group
• anything else: throw exception to the consumer.

这个配置项，是告诉Kafka Broker在发现kafka在没有初始offset，或者当前的offset是一个不存在的值（如果一个record被删除，就肯定不存在了）时，该如何处理。它有4种处理方式：

• earliest：自动重置到最早的offset。
• latest：看上去重置到最晚的offset。
• none：如果边更早的offset也没有的话，就抛出异常给consumer，告诉consumer在整个consumer group中都没有发现有这样的offset。
• 如果不是上述3种，只抛出异常给consumer。

默认值是latest。

fetch.max.wait.ms

The maximum amount of time the server will block before answering the fetch request if there isn't sufficient data to immediately satisfy the requirement given by fetch.min.bytes.

Fetch请求发给broker后，在broker中可能会被阻塞的（当topic中records的总size小于fetch.min.bytes时），此时这个fetch请求耗时就会比较长。这个配置就是来配置consumer最多等待response多久。

fetch.min.bytes

The minimum amount of data the server should return for a fetch request. If insufficient data is available the request will wait for that much data to accumulate before answering the request. The default setting of 1 byte means that fetch requests are answered as soon as a single byte of data is available or the fetch request times out waiting for data to arrive. Setting this to something greater than 1 will cause the server to wait for larger amounts of data to accumulate which can improve server throughput a bit at the cost of some additional latency.

当consumer向一个broker发起fetch请求时，broker返回的records的大小最小值。如果broker中数据量不够的话会wait，直到数据大小满足这个条件。

取值范围是：[0, Integer.Max]，默认值是1。默认值设置为1的目的是：使得consumer的请求能够尽快的返回。

fetch.max.bytes

The maximum amount of data the server should return for a fetch request. Records are fetched in batches by the consumer, and if the first record batch in the first non-empty partition of the fetch is larger than this value, the record batch will still be returned to ensure that the consumer can make progress. As such, this is not a absolute maximum. The maximum record batch size accepted by the broker is defined via message.max.bytes (broker config) or max.message.bytes (topic config). Note that the consumer performs multiple fetches in parallel.

一次fetch请求，从一个broker中取得的records最大大小。如果在从topic中第一个非空的partition取消息时，如果取到的第一个record的大小就超过这个配置时，仍然会读取这个record，也就是说在这片情况下，只会返回这一条record。broker、topic都会对producer发给它的message size做限制。所以在配置这值时，可以参考broker的message.max.bytes和topic的max.message.bytes的配置。

取值范围是：[0, Integer.Max]，默认值是：52428800 （5 MB）

max.partition.fetch.bytes

一次fetch请求，从一个partition中取得的records最大大小。如果在从topic中第一个非空的partition取消息时，如果取到的第一个record的大小就超过这个配置时，仍然会读取这个record，也就是说在这片情况下，只会返回这一条record。 broker、topic都会对producer发给它的message size做限制。所以在配置这值时，可以参考broker 的message.max.bytes和 topic 的max.message.bytes的配置。

max.poll.interval.ms

The maximum delay between invocations of poll() when using consumer group management. This places an upper bound on the amount of time that the consumer can be idle before fetching more records. If poll() is not called before expiration of this timeout, then the consumer is considered failed and the group will rebalance in order to reassign the partitions to another member.

前面说过要求程序中不间断的调用poll()。如果长时间没有调用poll，且间隔超过这个值时，就会认为这个consumer失败了。

max.poll.records

The maximum number of records returned in a single call to poll().

Consumer每次调用poll()时取到的records的最大数。