Kafka Consumer源码解读（一）

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Apache Kafka 版本 1.1.0

Java版本的Consumer client

先看接口声明：

/*
 * 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.kafka.clients.consumer;

import org.apache.kafka.common.Metric;
import org.apache.kafka.common.MetricName;
import org.apache.kafka.common.PartitionInfo;
import org.apache.kafka.common.TopicPartition;

import java.io.Closeable;
import java.util.Collection;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.TimeUnit;
import java.util.regex.Pattern;

/**
 * @see KafkaConsumer
 * @see MockConsumer
 */
public interface Consumer<K, V> extends Closeable {

    /**
     * @see KafkaConsumer#assignment()
     */
    public Set<TopicPartition> assignment();

    /**
     * @see KafkaConsumer#subscription()
     */
    public Set<String> subscription();

    /**
     * @see KafkaConsumer#subscribe(Collection)
     */
    public void subscribe(Collection<String> topics);

    /**
     * @see KafkaConsumer#subscribe(Collection, ConsumerRebalanceListener)
     */
    public void subscribe(Collection<String> topics, ConsumerRebalanceListener callback);

    /**
     * @see KafkaConsumer#assign(Collection)
     */
    public void assign(Collection<TopicPartition> partitions);

    /**
    * @see KafkaConsumer#subscribe(Pattern, ConsumerRebalanceListener)
    */
    public void subscribe(Pattern pattern, ConsumerRebalanceListener callback);

    /**
    * @see KafkaConsumer#subscribe(Pattern)
    */
    public void subscribe(Pattern pattern);

    /**
     * @see KafkaConsumer#unsubscribe()
     */
    public void unsubscribe();

    /**
     * @see KafkaConsumer#poll(long)
     */
    public ConsumerRecords<K, V> poll(long timeout);

    /**
     * @see KafkaConsumer#commitSync()
     */
    public void commitSync();

    /**
     * @see KafkaConsumer#commitSync(Map)
     */
    public void commitSync(Map<TopicPartition, OffsetAndMetadata> offsets);

    /**
     * @see KafkaConsumer#commitAsync()
     */
    public void commitAsync();

    /**
     * @see KafkaConsumer#commitAsync(OffsetCommitCallback)
     */
    public void commitAsync(OffsetCommitCallback callback);

    /**
     * @see KafkaConsumer#commitAsync(Map, OffsetCommitCallback)
     */
    public void commitAsync(Map<TopicPartition, OffsetAndMetadata> offsets, OffsetCommitCallback callback);

    /**
     * @see KafkaConsumer#seek(TopicPartition, long)
     */
    public void seek(TopicPartition partition, long offset);

    /**
     * @see KafkaConsumer#seekToBeginning(Collection)
     */
    public void seekToBeginning(Collection<TopicPartition> partitions);

    /**
     * @see KafkaConsumer#seekToEnd(Collection)
     */
    public void seekToEnd(Collection<TopicPartition> partitions);

    /**
     * @see KafkaConsumer#position(TopicPartition)
     */
    public long position(TopicPartition partition);

    /**
     * @see KafkaConsumer#committed(TopicPartition)
     */
    public OffsetAndMetadata committed(TopicPartition partition);

    /**
     * @see KafkaConsumer#metrics()
     */
    public Map<MetricName, ? extends Metric> metrics();

    /**
     * @see KafkaConsumer#partitionsFor(String)
     */
    public List<PartitionInfo> partitionsFor(String topic);

    /**
     * @see KafkaConsumer#listTopics()
     */
    public Map<String, List<PartitionInfo>> listTopics();

    /**
     * @see KafkaConsumer#paused()
     */
    public Set<TopicPartition> paused();

    /**
     * @see KafkaConsumer#pause(Collection)
     */
    public void pause(Collection<TopicPartition> partitions);

    /**
     * @see KafkaConsumer#resume(Collection)
     */
    public void resume(Collection<TopicPartition> partitions);

    /**
     * @see KafkaConsumer#offsetsForTimes(java.util.Map)
     */
    public Map<TopicPartition, OffsetAndTimestamp> offsetsForTimes(Map<TopicPartition, Long> timestampsToSearch);

    /**
     * @see KafkaConsumer#beginningOffsets(java.util.Collection)
     */
    public Map<TopicPartition, Long> beginningOffsets(Collection<TopicPartition> partitions);

    /**
     * @see KafkaConsumer#endOffsets(java.util.Collection)
     */
    public Map<TopicPartition, Long> endOffsets(Collection<TopicPartition> partitions);

    /**
     * @see KafkaConsumer#close()
     */
    public void close();

    /**
     * @see KafkaConsumer#close(long, TimeUnit)
     */
    public void close(long timeout, TimeUnit unit);

    /**
     * @see KafkaConsumer#wakeup()
     */
    public void wakeup();

}

Consumer的API可以分为以下几类：

  1. 订阅指定topic列表或者匹配某种模式的topic以及返回所订阅的topic列表和取消订阅

// 订阅指定的topic列表或者匹配某种模式的topic

    
        /**
         * Subscribe to the given list of topics to get dynamically assigned partitions.
         * <b>Topic subscriptions are not incremental. This list will replace the current
         * assignment (if there is one).</b> It is not possible to combine topic subscription with group management
         * with manual partition assignment through {@link #assign(Collection)}.
         *
         * If the given list of topics is empty, it is treated the same as {@link #unsubscribe()}.
         *
         * <p>
         * This is a short-hand for {@link #subscribe(Collection, ConsumerRebalanceListener)}, which
         * uses a no-op listener. If you need the ability to seek to particular offsets, you should prefer
         * {@link #subscribe(Collection, ConsumerRebalanceListener)}, since group rebalances will cause partition offsets
         * to be reset. You should also provide your own listener if you are doing your own offset
         * management since the listener gives you an opportunity to commit offsets before a rebalance finishes.
         *
         * @param topics The list of topics to subscribe to
         * @throws IllegalArgumentException If topics is null or contains null or empty elements
         * @throws IllegalStateException If {@code subscribe()} is called previously with pattern, or assign is called
         *                               previously (without a subsequent call to {@link #unsubscribe()}), or if not
         *                               configured at-least one partition assignment strategy
     */
    public void subscribe(Collection<String> topics);

    /**
     * @see KafkaConsumer#subscribe(Collection, ConsumerRebalanceListener)
     */
    public void subscribe(Collection<String> topics, ConsumerRebalanceListener callback);
        /**
    * @see KafkaConsumer#subscribe(Pattern, ConsumerRebalanceListener)
    */
    public void subscribe(Pattern pattern, ConsumerRebalanceListener callback);

    /**
    * @see KafkaConsumer#subscribe(Pattern)
    */
    public void subscribe(Pattern pattern);

// 返回所订阅的topic列表

    /**
     * @see KafkaConsumer#subscription()
     */
    public Set<String> subscription();

// 取消订阅

    /**
     * @see KafkaConsumer#unsubscribe()
     */
    public void unsubscribe();
  1. 分配消费指定topic、获取分配的Topic分区 // 分配消费指定topic

    /**
     * @see KafkaConsumer#assign(Collection)
     */
    public void assign(Collection<TopicPartition> partitions);

// 获取分配的Topic分区

    /**
     * @see KafkaConsumer#assignment()
     */
    public Set<TopicPartition> assignment();
  1. 拉取数据
    /**
     * @see KafkaConsumer#poll(long)
     */
    public ConsumerRecords<K, V> poll(long timeout);
  1. 提交offsets
    /**
     * @see KafkaConsumer#commitSync()
     */
    public void commitSync();

    /**
     * @see KafkaConsumer#commitSync(Map)
     */
    public void commitSync(Map<TopicPartition, OffsetAndMetadata> offsets);

    /**
     * @see KafkaConsumer#commitAsync()
     */
    public void commitAsync();

    /**
     * @see KafkaConsumer#commitAsync(OffsetCommitCallback)
     */
    public void commitAsync(OffsetCommitCallback callback);

    /**
     * @see KafkaConsumer#commitAsync(Map, OffsetCommitCallback)
     */
    public void commitAsync(Map<TopicPartition, OffsetAndMetadata> offsets, OffsetCommitCallback callback);
  1. 定位、获取offsets
    /**
     * @see KafkaConsumer#seek(TopicPartition, long)
     */
    public void seek(TopicPartition partition, long offset);

    /**
     * @see KafkaConsumer#seekToBeginning(Collection)
     */
    public void seekToBeginning(Collection<TopicPartition> partitions);

    /**
     * @see KafkaConsumer#seekToEnd(Collection)
     */
    public void seekToEnd(Collection<TopicPartition> partitions);

    /**
     * @see KafkaConsumer#position(TopicPartition)
     */
    public long position(TopicPartition partition);
        /**
     * @see KafkaConsumer#committed(TopicPartition)
     */
    public OffsetAndMetadata committed(TopicPartition partition);
        /**
     * @see KafkaConsumer#offsetsForTimes(java.util.Map)
     */
    public Map<TopicPartition, OffsetAndTimestamp> offsetsForTimes(Map<TopicPartition, Long> timestampsToSearch);
        /**
     * @see KafkaConsumer#beginningOffsets(java.util.Collection)
     */
    public Map<TopicPartition, Long> beginningOffsets(Collection<TopicPartition> partitions);

    /**
     * @see KafkaConsumer#endOffsets(java.util.Collection)
     */
    public Map<TopicPartition, Long> endOffsets(Collection<TopicPartition> partitions);
  1. 获取topic及topic分区信息
    /**
     * @see KafkaConsumer#partitionsFor(String)
     */
    public List<PartitionInfo> partitionsFor(String topic);

    /**
     * @see KafkaConsumer#listTopics()
     */
    public Map<String, List<PartitionInfo>> listTopics();
  1. 控制消费行为（暂停、恢复、唤醒、关闭）
    /**
     * @see KafkaConsumer#paused()
     */
    public Set<TopicPartition> paused();

    /**
     * @see KafkaConsumer#pause(Collection)
     */
    public void pause(Collection<TopicPartition> partitions);

    /**
     * @see KafkaConsumer#resume(Collection)
     */
    public void resume(Collection<TopicPartition> partitions);
        /**
     * @see KafkaConsumer#close()
     */
    public void close();

    /**
     * @see KafkaConsumer#close(long, TimeUnit)
     */
    public void close(long timeout, TimeUnit unit);

    /**
     * @see KafkaConsumer#wakeup()
     */
    public void wakeup();
  1. 获取消费指标
    /**
     * @see KafkaConsumer#metrics()
     */
    public Map<MetricName, ? extends Metric> metrics();

一、初始化KafkaConsumer

KafkaConsumer有以下几个构造器： image.png

其中常用的是 KafkaConsumer(Properties)，传入的Properties对象，会转换为内部的ConsumerConfig对象，最终调用的是KafkaConsumer(ConsumerConfig config, Deserializer<K> keyDeserializer, Deserializer<V> valueDeserializer) 这个构造器，因此重点分析一下Properties转换为ConsumerConfig对象，看下有哪些配置参数，然后研究这个构造器里面的逻辑。

首先看下ConsumerConfig这个类，其中可以看到构造KafkaConsumer需要的参数：

参数描述默认值
group.idA 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.
bootstrap.serversA 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).
session.timeout.msThe 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.10000
heartbeat.interval.msThe 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.3000
partition.assignment.strategyThe class name of the partition assignment strategy that the client will use to distribute partition ownership amongst consumer instances when group management is usedorg.apache.kafka.clients.consumer.RangeAssignor
metadata.max.age.msThe period of time in milliseconds after which we force a refresh of metadata even if we haven't seen any partition leadership changes to proactively discover any new brokers or partitions.5 * 60 * 1000
enable.auto.commitIf true the consumer's offset will be periodically committed in the background.true
auto.commit.interval.msThe frequency in milliseconds that the consumer offsets are auto-committed to Kafka if enable.auto.commit is set to true.5000
client.idAn id string to pass to the server when making requests. The purpose of this is to be able to track the source of requests beyond just ip/port by allowing a logical application name to be included in server-side request logging.
max.partition.fetch.bytesmax.partition.fetch.bytes will return. Records are fetched in batches by the consumer. If the first record batch in the first non-empty partition of the fetch is larger than this limit, the batch will still be returned to ensure that the consumer can make progress. The maximum record batch size accepted by the broker is defined via message.max.bytes (broker config) or max.message.bytes (topic config). See 'fetch.max.bytes' for limiting the consumer request size.1MB
send.buffer.bytesThe size of the TCP send buffer (SO_SNDBUF) to use when sending data. If the value is -1, the OS default will be used.128 * 1024
receive.buffer.bytesThe size of the TCP receive buffer (SO_RCVBUF) to use when reading data. If the value is -1, the OS default will be used.64 * 1024
fetch.min.bytesThe 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.1
fetch.max.bytesThe 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.50 * 1024 * 1024
fetch.max.wait.msThe 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.500
reconnect.backoff.msThe base amount of time to wait before attempting to reconnect to a given host. This avoids repeatedly connecting to a host in a tight loop. This backoff applies to all connection attempts by the client to a broker.50
reconnect.backoff.max.msThe maximum amount of time in milliseconds to wait when reconnecting to a broker that has repeatedly failed to connect. If provided, the backoff per host will increase exponentially for each consecutive connection failure, up to this maximum. After calculating the backoff increase, 20% random jitter is added to avoid connection storms.1000L
retry.backoff.msThe amount of time to wait before attempting to retry a failed request to a given topic partition. This avoids repeatedly sending requests in a tight loop under some failure scenarios.100L
auto.offset.resetWhat 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.
latest
check.crcsAutomatically check the CRC32 of the records consumed. This ensures no on-the-wire or on-disk corruption to the messages occurred. This check adds some overhead, so it may be disabled in cases seeking extreme performance.true
metrics.sample.window.msThe window of time a metrics sample is computed over.30000
metrics.num.samplesThe number of samples maintained to compute metrics.30000
metrics.recording.levelThe highest recording level for metrics.INFO
metric.reportersA list of classes to use as metrics reporters. Implementing the org.apache.kafka.common.metrics.MetricsReporter interface allows plugging in classes that will be notified of new metric creation. The JmxReporter is always included to register JMX statistics.
key.deserializerDeserializer class for key that implements the org.apache.kafka.common.serialization.Deserializer interface.
value.deserializerDeserializer class for value that implements the org.apache.kafka.common.serialization.Deserializer interface.
request.timeout.msThe configuration controls the maximum amount of time the client will wait for the response of a request. If the response is not received before the timeout elapses the client will resend the request if necessary or fail the request if retries are exhausted.305000
connections.max.idle.msClose idle connections after the number of milliseconds specified by this config.9 * 60 * 1000
interceptor.classesA list of classes to use as interceptors. Implementing the org.apache.kafka.clients.consumer.ConsumerInterceptor interface allows you to intercept (and possibly mutate) records received by the consumer. By default, there are no interceptors.
max.poll.recordsThe maximum number of records returned in a single call to poll().500
max.poll.interval.msThe 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.300000
exclude.internal.topicsWhether records from internal topics (such as offsets) should be exposed to the consumer. If set to true the only way to receive records from an internal topic is subscribing to it.true
internal.leave.group.on.closeWhether or not the consumer should leave the group on close. If set to false then a rebalance won't occur until session.timeout.ms expires.
isolation.level

Controls how to read messages written transactionally. If set to read_committed, consumer.poll() will only return transactional messages which have been committed. If set to read_uncommitted' (the default), consumer.poll() will return all messages, even transactional messages which have been aborted. Non-transactional messages will be returned unconditionally in either mode.

Messages will always be returned in offset order. Hence, in read_committed mode, consumer.poll() will only return messages up to the last stable offset (LSO), which is the one less than the offset of the first open transaction. In particular any messages appearing after messages belonging to ongoing transactions will be withheld until the relevant transaction has been completed. As a result, read_committed consumers will not be able to read up to the high watermark when there are in flight transactions.

Further, when in read_committed the seekToEnd method will return the LSO

security.protocolProtocol used to communicate with brokers. Valid values are: PLAINTEXT SSL SASL_PLAINTEXT SASL_SSLPLAINTEXT

KafkaConsumer构建步骤如下：

1）设置clientId

先从ConsumerConfig中取key为client.id对应的值，如果没有设置，则生成一个，以 "consumer-" + CONSUMER_CLIENT_ID_SEQUENCE递增序列号；

2）设置groupId

从ConsumerConfig中取key为group.id对应的值

3）设置requestTimeoutMs参数

从ConsumerConfig中取key为request.timeout.ms对应的值

4）设置sessionTimeOutMs参数

从ConsumerConfig中取key为session.timeout.ms对应的值

5）设置fetchMaxWaitMs参数

从ConsumerConfig中取key为fetch.max.wait.ms对应的值

6）判断requestTimeoutMs是否大于sessionTimeOutMs和fetchMaxWaitMs

7）设置time

8）构建Metrics体系

9）设置retryBackoffMs

从ConsumerConfig中取key为retry.backoff.ms对应的值

10）构建ConsumerInterceptor列表

11）设置keyDeserializer

12）设置valueDeserializer

13）配置构建ClusterResourceListeners

14）构建MetaData实例

这步只是新建了一个Metadata对象，并设置了相关参数，但是还没有访问Kafka集群真正去获取元数据

15）根据bootstrap.servers参数生成InetSocketAddress

16)更新元数据Metadata

构造Cluster对象，包括节点、topic、topic分区及其它们之间关系索引，这里也没有真正获取元数据，只根据bootstrap.servers参数补充了Node信息

17）构建 ChannelBuilder

18）获取IsolationLevel

从ConsumerConfig中取key为isolation.level对应的值

19）获取heartbeatIntervalMs参数

20）构建NetworkClient

这是构建KafkaConsumer很关键的一步，在这一步里面，先新建了一个Selector对象（NIO Selector）。

21）构建ConsumerNetworkClient对象

22）获取OffsetResetStrategy

23）构建PartitionAssignor

即创建RangeAssignor实例

24）构建ConsumerCoordinator

25）构建Fetcher

其中关键对象是Fetcher、ConsumerCoordinator、ConsumerNetworkClient、NetworkClient。

二、订阅Topic列表

image.png

consumer.subscribe.png

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