Kafka Streams的WordCount收到消息后是怎么处理的呢？

1 准备工作
- 1.1 Kafka Streams是什么
- 1.2 Kafka 集群
2 创建Kafka Streams应用
- 2.1 官网的WordCount应用
- 2.2 启动WordCount应用
3 往source topic 发消息
4 概念及其他

本文将尝试从Kafka Streams的WordCount应用收到消息后如何处理来分析Kafka Streams的工作原理。

1 准备工作

1.1 Kafka Streams是什么

The easiest way to write mission-critical real-time applications and microservices.

Kafka Streams提供了一个最简单的，开发实时流处理应用的方式。因为：

它是一个jar包而非流处理框架。单独构建Kafka Streams应用只需要一个jar包；与其他项目集成也只需引用这个jar包。
Kafka Streams只依赖Kafka，输入数据和输出数据都存放在Kafka中。

1.2 Kafka 集群

假设我们已经有了Zookeeper和Kafka环境，现在需要创建两个topic：

source topic，命名为TextLinesTopic，分区数为2，WordCount应用消费的topic。
target topic，命名为WordsWithCountsTopic，分区数为2，WordCount应用处理过的数据会发往这个topic。

创建topic的命令为：

bin/kafka-topics.sh --create --zookeeper localhost:2181 --replication-factor 1 --partitions 2 --topic topic-name

# 创建成功后，命令行显示Created topic "topic-name".

创建两个topic后，查看topic列表

bin/kafka-topics.sh --zookeeper localhost:2181 --list

# 看到创建的source topic和target topic

2 创建Kafka Streams应用

2.1 官网的WordCount应用

使用Hello Kafka Streams官网提供的入门的WordCount应用，在页面最下可以看到官网提供的WordCountApplication，如下（有小改动）：

// 略去import

public class WordCountApplication {

  private static final String BOOTSTRAP_SERVERS_CONFIG = "127.0.0.1:9092";

  public static void main(String[] args) {

    Properties props = new Properties();

    props.put(StreamsConfig.APPLICATION_ID_CONFIG, "wordcount-application");
    props.put(StreamsConfig.BOOTSTRAP_SERVERS_CONFIG, BOOTSTRAP_SERVERS_CONFIG);
    props.put(StreamsConfig.DEFAULT_KEY_SERDE_CLASS_CONFIG, Serdes.String().getClass());
    props.put(StreamsConfig.DEFAULT_VALUE_SERDE_CLASS_CONFIG, Serdes.String().getClass());

    StreamsBuilder builder = new StreamsBuilder();
    // stream & source topic
    KStream<String, String> textLines = builder.stream("TextLinesTopic");

    // table
    KTable<String, Long> wordCounts = textLines

        .flatMapValues(textLine -> Arrays.asList(textLine.toLowerCase().split("\W+")))

        .groupBy((key, word) -> word)

        .count(Materialized.<String, Long, KeyValueStore<Bytes, byte[]>>as("counts-store"));

    // target topic
    wordCounts

        .toStream()

        .to("WordsWithCountsTopic", Produced.with(Serdes.String(), Serdes.Long()));

    Topology toplogy = builder.build();
    // 打印toplogy
    System.out.println(toplogy.describe());

    // 构建KafkaStreams
    KafkaStreams streams = new KafkaStreams(toplogy, props);

    Runtime.getRuntime().addShutdownHook(new Thread("streams-shutdown-hook") {
      @Override
      public void run() {
        streams.close();
      }
    });

    streams.start();

  }

}

由上边代码可见，构建一个KafkaStreams至少需要Topology和Properties两个参数（KafkaStreams总共提供了7个public的构造函数，3个已经弃用）。

2.1.1 Topology

WordCount应用通过StreamsBuilder#build创建了一个Topology。在控制台把创建的拓扑打印出来，如下：

Topologies:
   Sub-topology: 0
    Source: KSTREAM-SOURCE-0000000000 (topics: [TextLinesTopic])
      --> KSTREAM-FLATMAPVALUES-0000000001
    Processor:  KSTREAM-FLATMAPVALUES-0000000001 (stores: [])
      --> KSTREAM-KEY-SELECT-0000000002
      <-- KSTREAM-SOURCE-0000000000
    Processor: KSTREAM-KEY-SELECT-0000000002 (stores: [])
      --> KSTREAM-FILTER-0000000005
      <-- KSTREAM-FLATMAPVALUES-0000000001
    Processor: KSTREAM-FILTER-0000000005 (stores: [])
      --> KSTREAM-SINK-0000000004
      <-- KSTREAM-KEY-SELECT-0000000002
    Sink: KSTREAM-SINK-0000000004 (topic: counts-store-repartition)
      <-- KSTREAM-FILTER-0000000005

  Sub-topology: 1
    Source: KSTREAM-SOURCE-0000000006 (topics: [counts-store-repartition])
      --> KSTREAM-AGGREGATE-0000000003
    Processor: KSTREAM-AGGREGATE-0000000003 (stores: [counts-store])
      --> KTABLE-TOSTREAM-0000000007
      <-- KSTREAM-SOURCE-0000000006
    Processor: KTABLE-TOSTREAM-0000000007 (stores: [])
      --> KSTREAM-SINK-0000000008
      <-- KSTREAM-AGGREGATE-0000000003
    Sink: KSTREAM-SINK-0000000008 (topic: WordsWithCountsTopic)
      <-- KTABLE-TOSTREAM-0000000007

由上可见，这个Topology又分为两个子拓扑，分别是子拓扑0和子拓扑1，对应Task0和Task1，具体如下：
WordCount的拓扑

2.1.2 Properties

WordCount应用使用了4个StreamsConfig的配置，分别是：

key	value	describe
StreamsConfig.APPLICATION_ID_CONFIG	wordcount-application	应用名。3个作用：client-id的前缀；相同的应用名组成Kafka Streams集群；内部topic的前缀
StreamsConfig.BOOTSTRAP_SERVERS_CONFIG	127.0.0.1:9092	kafka地址，多个以","分隔
StreamsConfig.DEFAULT_KEY_SERDE_CLASS_CONFIG	Serdes.String().getClass()	key序列和反序列的类
StreamsConfig.DEFAULT_VALUE_SERDE_CLASS_CONFIG	Serdes.String().getClass()	value序列和反序列的类

StreamsConfig所有的配置可查看Kafka Streams Configs

2.1.3 内部topic

WordCount应用启动后，会创建2个内部topic，分区数

wordcount-application-counts-store-repartition，子拓扑0的SinkNode会把经过处理的数据发往这个名为repartition的内部topic，子拓扑1的SourceNode会从这个名为repartition的内部topic取到经过子拓扑0处理过的数据。
wordcount-application-counts-store-changelog，这个topic会记录WordCount的聚合结果，changelog topic使用了kafka的Log compaction功能，可以安全地清除旧数据，以防止topic无限增长。changelog changelog用于Kafka Streams容错处理。

2.2 启动WordCount应用

启动一个WordCount应用，查看启动的日志

...
stream-thread [wordcount-application-6f260d17-7347-4e80-94ad-786753f4d683-StreamThread-1] Starting
stream-client [wordcount-application-6f260d17-7347-4e80-94ad-786753f4d683]Started Streams client
stream-thread [wordcount-application-6f260d17-7347-4e80-94ad-786753f4d683-StreamThread-1] State transition from CREATED to RUNNING
[Consumer clientId=wordcount-application-6f260d17-7347-4e80-94ad-786753f4d683-StreamThread-1-consumer, groupId=wordcount-application] Discovered group coordinator K-PC:9092 (id: 2147483647 rack: null)
[Consumer clientId=wordcount-application-6f260d17-7347-4e80-94ad-786753f4d683-StreamThread-1-consumer, groupId=wordcount-application] Revoking previously assigned partitions []
stream-thread [wordcount-application-6f260d17-7347-4e80-94ad-786753f4d683-StreamThread-1] State transition from RUNNING to PARTITIONS_REVOKED
stream-client [wordcount-application-6f260d17-7347-4e80-94ad-786753f4d683]State transition from RUNNING to REBALANCING
stream-thread [wordcount-application-6f260d17-7347-4e80-94ad-786753f4d683-StreamThread-1] partition revocation took 1 ms.
  suspended active tasks: []
  suspended standby tasks: []
[Consumer clientId=wordcount-application-6f260d17-7347-4e80-94ad-786753f4d683-StreamThread-1-consumer, groupId=wordcount-application] (Re-)joining group
stream-thread [wordcount-application-6f260d17-7347-4e80-94ad-786753f4d683-StreamThread-1-consumer] Assigned tasks to clients as {6f260d17-7347-4e80-94ad-786753f4d683=[activeTasks: ([0_0, 0_1, 1_0, 1_1]) standbyTasks: ([]) assignedTasks: ([0_0, 0_1, 1_0, 1_1]) prevActiveTasks: ([]) prevAssignedTasks: ([]) capacity: 1]}.
[Consumer clientId=wordcount-application-6f260d17-7347-4e80-94ad-786753f4d683-StreamThread-1-consumer, groupId=wordcount-application] Successfully joined group with generation 1
[Consumer clientId=wordcount-application-6f260d17-7347-4e80-94ad-786753f4d683-StreamThread-1-consumer, groupId=wordcount-application] Setting newly assigned partitions [wordcount-application-counts-store-repartition-0, TextLinesTopic-1, TextLinesTopic-0, wordcount-application-counts-store-repartition-1]
stream-thread [wordcount-application-6f260d17-7347-4e80-94ad-786753f4d683-StreamThread-1] State transition from PARTITIONS_REVOKED to PARTITIONS_ASSIGNED
stream-thread [wordcount-application-6f260d17-7347-4e80-94ad-786753f4d683-StreamThread-1] partition assignment took 31 ms.
  current active tasks: [0_0, 0_1, 1_0, 1_1]
  current standby tasks: []
  previous active tasks: []

stream-thread [wordcount-application-6f260d17-7347-4e80-94ad-786753f4d683-StreamThread-1] State transition from PARTITIONS_ASSIGNED to RUNNING
stream-client [wordcount-application-6f260d17-7347-4e80-94ad-786753f4d683]State transition from REBALANCING to RUNNING

可以看到：

streams应用state的改变：CREATED -> RUNNING -> REBALANCING -> RUNNING
streams线程的状态state的改变：CREATED -> RUNNING -> PARTITIONS_REVOKED -> PARTITIONS_ASSIGNED -> RUNNING
最终应用拿到的任务是：0_0, 0_1, 1_0, 1_1，下划线前的数字是子拓扑，下划线后的数字是source topic的分区。

再启动一个WordCount应用（使用同样的APPLICATION_ID_CONFIG，与第一个应用组成集群），控制台日志分别输出：

#第一个WordCount应用日志
...
[Consumer clientId=wordcount-application-6f260d17-7347-4e80-94ad-786753f4d683-StreamThread-1-consumer, groupId=wordcount-application] Revoking previously assigned partitions [wordcount-application-counts-store-repartition-0, TextLinesTopic-1, TextLinesTopic-0, wordcount-application-counts-store-repartition-1]
stream-thread [wordcount-application-6f260d17-7347-4e80-94ad-786753f4d683-StreamThread-1] State transition from RUNNING to PARTITIONS_REVOKED
stream-client [wordcount-application-6f260d17-7347-4e80-94ad-786753f4d683]State transition from RUNNING to REBALANCING
stream-thread [wordcount-application-6f260d17-7347-4e80-94ad-786753f4d683-StreamThread-1] partition revocation took 277 ms.
  suspended active tasks: [0_0, 0_1, 1_0, 1_1]
  suspended standby tasks: []
[Consumer clientId=wordcount-application-6f260d17-7347-4e80-94ad-786753f4d683-StreamThread-1-consumer, groupId=wordcount-application] (Re-)joining group
stream-thread [wordcount-application-6f260d17-7347-4e80-94ad-786753f4d683-StreamThread-1-consumer] Assigned tasks to clients as {b391295c-1bee-493e-9ebf-ac4ba0302190=[activeTasks: ([1_0, 1_1]) standbyTasks: ([]) assignedTasks: ([1_0, 1_1]) prevActiveTasks: ([]) prevAssignedTasks: ([0_0, 0_1, 1_0, 1_1]) capacity: 1], 6f260d17-7347-4e80-94ad-786753f4d683=[activeTasks: ([0_0, 0_1]) standbyTasks: ([]) assignedTasks: ([0_0, 0_1]) prevActiveTasks: ([0_0, 0_1, 1_0, 1_1]) prevAssignedTasks: ([0_0, 0_1, 1_0, 1_1]) capacity: 1]}.
[Consumer clientId=wordcount-application-6f260d17-7347-4e80-94ad-786753f4d683-StreamThread-1-consumer, groupId=wordcount-application] Successfully joined group with generation 7
[Consumer clientId=wordcount-application-6f260d17-7347-4e80-94ad-786753f4d683-StreamThread-1-consumer, groupId=wordcount-application] Setting newly assigned partitions [TextLinesTopic-1, TextLinesTopic-0]
stream-thread [wordcount-application-6f260d17-7347-4e80-94ad-786753f4d683-StreamThread-1] State transition from PARTITIONS_REVOKED to PARTITIONS_ASSIGNED
stream-thread [wordcount-application-6f260d17-7347-4e80-94ad-786753f4d683-StreamThread-1] partition assignment took 229 ms.
  current active tasks: [0_0, 0_1]
  current standby tasks: []
  previous active tasks: [0_0, 0_1, 1_0, 1_1]

stream-thread [wordcount-application-6f260d17-7347-4e80-94ad-786753f4d683-StreamThread-1] State transition from PARTITIONS_ASSIGNED to RUNNING
stream-client [wordcount-application-6f260d17-7347-4e80-94ad-786753f4d683]State transition from REBALANCING to RUNNING


# 第二个WordCount应用的日志
...
stream-thread [wordcount-application-b391295c-1bee-493e-9ebf-ac4ba0302190-StreamThread-1] Starting
stream-client [wordcount-application-b391295c-1bee-493e-9ebf-ac4ba0302190]Started Streams client
stream-thread [wordcount-application-b391295c-1bee-493e-9ebf-ac4ba0302190-StreamThread-1] State transition from CREATED to RUNNING
[Consumer clientId=wordcount-application-b391295c-1bee-493e-9ebf-ac4ba0302190-StreamThread-1-consumer, groupId=wordcount-application] Discovered group coordinator K-PC:9092 (id: 2147483647 rack: null)
[Consumer clientId=wordcount-application-b391295c-1bee-493e-9ebf-ac4ba0302190-StreamThread-1-consumer, groupId=wordcount-application] Revoking previously assigned partitions []
stream-thread [wordcount-application-b391295c-1bee-493e-9ebf-ac4ba0302190-StreamThread-1] State transition from RUNNING to PARTITIONS_REVOKED
stream-client [wordcount-application-b391295c-1bee-493e-9ebf-ac4ba0302190]State transition from RUNNING to REBALANCING
stream-thread [wordcount-application-b391295c-1bee-493e-9ebf-ac4ba0302190-StreamThread-1] partition revocation took 1 ms.
  suspended active tasks: []
  suspended standby tasks: []
[Consumer clientId=wordcount-application-b391295c-1bee-493e-9ebf-ac4ba0302190-StreamThread-1-consumer, groupId=wordcount-application] (Re-)joining group
[Consumer clientId=wordcount-application-b391295c-1bee-493e-9ebf-ac4ba0302190-StreamThread-1-consumer, groupId=wordcount-application] Successfully joined group with generation 7
[Consumer clientId=wordcount-application-b391295c-1bee-493e-9ebf-ac4ba0302190-StreamThread-1-consumer, groupId=wordcount-application] Setting newly assigned partitions [wordcount-application-counts-store-repartition-0, wordcount-application-counts-store-repartition-1]
stream-thread [wordcount-application-b391295c-1bee-493e-9ebf-ac4ba0302190-StreamThread-1] State transition from PARTITIONS_REVOKED to PARTITIONS_ASSIGNED
stream-thread [wordcount-application-b391295c-1bee-493e-9ebf-ac4ba0302190-StreamThread-1] partition assignment took 35 ms.
  current active tasks: [1_0, 1_1]
  current standby tasks: []
  previous active tasks: []

stream-thread [wordcount-application-b391295c-1bee-493e-9ebf-ac4ba0302190-StreamThread-1] State transition from PARTITIONS_ASSIGNED to RUNNING
stream-client [wordcount-application-b391295c-1bee-493e-9ebf-ac4ba0302190]State transition from REBALANCING to RUNNING

可以看到，第二个应用启动后组成了集群，新的应用加入后，两个应用的任务进行了REBALANCE，第一个应用拿到了0_0, 0_1，第二个应用拿到了1_0, 1_1。REBALANCE后第一个应用的子拓扑0执行topic分区0和分区1的任务，子拓扑1没有拿到任务；第二个应用的子拓扑1执行topic分区0和分区1的任务，子拓扑0没有拿到任务。如下图：

这时，如果一条消息发往source topic的1分区：

消息会被第一个WordCount应用子拓扑0的SourceNode（KSTREAM-SOURCE-0000000000）拿到
子拓扑0处理后由子拓扑0的SinkNode（KSTREAM-SINK-0000000004）发回到kafka的reparation分区
然后被第二个WordCount应用的子拓扑1（2个WordCount应用通过任务REBALANCE后，的SourceNode（KSTREAM-SOURCE-0000000006）拿到
经过子拓扑1处理由SinkNode（KSTREAM-SINK-0000000008）把最终结果发至target topic。

3 往source topic 发消息

往source topic（TextLinesTopic）发送一条消息，消息的key和value都是"hello world"。

1 消息被子拓扑0的SourceNode（KSTREAM-SOURCE-0000000000）拿到。
2 原始消息被Kstream#flatMapValues处理，对应子拓扑0的ProcessorNode（KSTREAM-FLATMAPVALUES-0000000001）。这个算子（Processor）会把消息的value转为小写格式在进行切分操作（以非字母数字下划线切分），这些操作并不改变原消息的key。因此，key和value都是"hello world"的消息经过Kstream#flatMapValues处理后，变成key是"hello world"，value分别是"hello"和"world"的两条消息。
3 然后由Kstream#groupBy处理。这个算子等价于先后调用Kstream#selectKey和Kstream#groupByKey，分别对应子拓扑0的ProcessorNode（KSTREAM-KEY-SELECT-0000000002）和ProcessorNodeKSTREAM-FILTER-0000000005）。
- 3.1 Kstream#selectKey，改变消息的key。消息变为key和value分别是“hello”和“world”的两条消息。
- 3.2 Kstream#groupByKey，按key进行分组，相同的key会被分到一组（Kafka Streams进行聚合操作前必须进行分组操作）。
4 消息通过SinkNode（KSTREAM-SINK-0000000004）将子拓扑0处理完的消息发往reparation topic。

接下来的操作由子拓扑1完成。

1 子拓扑1的SourceNode（KSTREAM-SOURCE-0000000006）从reparation topic拿到分组后的消息。
2 由Kstream#count处理，对应子拓扑1的ProcessorNode（KSTREAM-AGGREGATE-0000000003），count操作后消息类型变成KTable。
- 2.1 local state store，将结果存进rocksdb，存储在本地。
- 2.2 将结果发往changelog topic，用于容错处理。
3 ProcessorNode（KTABLE-TOSTREAM-0000000007）把消息类型从KTable转化为KStream。
4 通过SinkNode（KSTREAM-SINK-0000000008）将子拓扑1 count的消息发往target topic。

由于时间有限，很多Kafka Streams的概念，比如窗口等本文没有涉及。

4 概念及其他

4.1 topology

Kafka Streams 通过拓扑定义处理逻辑，拓扑由点和边组成。

点，分为3类：SourceNode、SinkNode和ProcessorNode。
- SourceNode和SinkNode分别是拓扑的起止。SourceNode从Kafka的source topic中取消息给ProcessorNode处理，SinkNode把ProcessorNode处理完的结果发往Kafka的target topic；
- Processor负责处理流数据。分为Kafka Streams DSL和Processor API两类，前者提供常用的数据操作，如map、filter、join、aggregations；后者可以由开发者自己定制开发。
边，数据流向。

4.2 state

Stream thread的states

              +-------------+
        +<--- | Created (0) |
        |     +-----+-------+
        |           |
        |           v
        |     +-----+-------+
        +<--- | Running (1) | <----+
        |     +-----+-------+      |
        |           |              |
        |           v              |
        |     +-----+-------+      |
        +<--- | Partitions  |      |
        |     | Revoked (2) | <----+
        |     +-----+-------+      |
        |           |              |
        |           v              |
        |     +-----+-------+      |
        |     | Partitions  |      |
        |     | Assigned (3)| ---->+
        |     +-----+-------+
        |           |
        |           v
        |     +-----+-------+
        +---> | Pending     |
              | Shutdown (4)|
              +-----+-------+
                    |
                    v
              +-----+-------+
              | Dead (5)    |
              +-------------+

Kafka Streams的states

                +--------------+
        +<----- | Created (0)  |
        |       +-----+--------+
        |             |
        |             v
        |       +--------------+
        +<----- | Running (2)  | -------->+
        |       +----+--+------+          |
        |            |  ^                 |
        |            v  |                 |
        |       +----+--+------+          |
        |       | Re-          |          v
        |       | Balancing (1)| -------->+
        |       +-----+--------+          |
        |             |                   |
        |             v                   v
        |       +-----+--------+     +----+-------+
        +-----> | Pending      |<--- | Error (5)  |
                | Shutdown (3) |     +------------+
                +-----+--------+
                     |
                      v
                +-----+--------+
                | Not          |
                | Running (4)  |
                +--------------+

4.3 KTable与KStream

KStream，数据流，所有数据通过insert only的方式加入到这个数据流中。

Ktable，数据集，像是数据库中的表，数据以update only的方式加入。

4.4 Kafka发送消息时，如何确定分区

源码KafkaProducer#doSend在发送消息时，通过partition方法获得分区。

    private Future<RecordMetadata> doSend(ProducerRecord<K, V> record, Callback callback) {
      ...
            int partition = partition(record, serializedKey, serializedValue, cluster);
      ...
    }

kafka提供了一个默认的partition实现DefaultPartitioner#partition

    /**
     * Compute the partition for the given record.
     *
     * @param topic The topic name
     * @param key The key to partition on (or null if no key)
     * @param keyBytes serialized key to partition on (or null if no key)
     * @param value The value to partition on or null
     * @param valueBytes serialized value to partition on or null
     * @param cluster The current cluster metadata
     */
    public int partition(String topic, Object key, byte[] keyBytes, Object value, byte[] valueBytes, Cluster cluster) {
        List<PartitionInfo> partitions = cluster.partitionsForTopic(topic);
        int numPartitions = partitions.size();
        if (keyBytes == null) {
            int nextValue = nextValue(topic);
            List<PartitionInfo> availablePartitions = cluster.availablePartitionsForTopic(topic);
            if (availablePartitions.size() > 0) {
                int part = Utils.toPositive(nextValue) % availablePartitions.size();
                return availablePartitions.get(part).partition();
            } else {
                // no partitions are available, give a non-available partition
                return Utils.toPositive(nextValue) % numPartitions;
            }
        } else {
            // hash the keyBytes to choose a partition
            return Utils.toPositive(Utils.murmur2(keyBytes)) % numPartitions;
        }
    }

    private int nextValue(String topic) {
        AtomicInteger counter = topicCounterMap.get(topic);
        if (null == counter) {
            counter = new AtomicInteger(ThreadLocalRandom.current().nextInt());
            AtomicInteger currentCounter = topicCounterMap.putIfAbsent(topic, counter);
            if (currentCounter != null) {
                counter = currentCounter;
            }
        }
        return counter.getAndIncrement();
    }

上述源码可以看到：

消息key是null，topic可用分区大于0时，使用根据topic获取的nextValue的值和可用分区数进行取模操作。
消息key是null，topic可用分区小于等于0时，获取根据topic获取的nextValue的值和总分区数进行取模操作（give a non-available partition，给了个不可用的分区）。
消息key不是null时，对key进行hash后和topic的分区数取模。可以保证topic的分区数不变的情况下，相同的key每次都发往同一个分区。

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原文地址：https://www.cnblogs.com/yizhishi/p/12205770.html