alpakka-kafka(1)-producer

  alpakka项目是一个基于akka-streams流处理编程工具的scala/java开源项目，通过提供connector连接各种数据源并在akka-streams里进行数据处理。alpakka-kafka就是alpakka项目里的kafka-connector。对于我们来说：可以用alpakka-kafka来对接kafka，使用kafka提供的功能。或者从另外一个角度讲：alpakka-kafka就是一个用akka-streams实现kafka功能的scala开发工具。

alpakka-kafka提供了kafka的核心功能：producer、consumer，分别负责把akka-streams里的数据写入kafka及从kafka中读出数据并输入到akka-streams里。用akka-streams集成kafka的应用场景通常出现在业务集成方面：在一项业务A中产生一些业务操作指令写入kafka，然后通过kafka把指令传送给另一项业务B，业务B从kafka中获取操作指令并进行相应的业务操作。如：有两个业务模块：收货管理和库存管理，一方面收货管理向kafka写入收货记录。另一头库存管理从kafka中读取收货记录并更新相关库存数量记录。注意，这两项业务是分别操作的。在alpakka中，实际的业务操作基本就是在akka-streams里的数据处理（transform），其实是典型的CQRS模式：读写两方互不关联，写时不管受众是谁，如何使用、读者不关心谁是写方。这里的写和读两方分别代表kafka里的producer和consumer。

本篇我们先介绍alpakka-kafka的producer功能及其使用方法。如前所述：alpakka是用akka-streams实现了kafka-producer功能。alpakka提供的producer也就是akka-streams的一种组件，可以与其它的akka-streams组件组合形成更大的akka-streams个体。构建一个producer需要先完成几个配件类构成：

1、producer-settings配置：alpakka-kafka在reference.conf里的akka.kafka.producer配置段落提供了足够支持基本运作的默认producer配置。用户可以通过typesafe config配置文件操作工具来灵活调整配置

2、de/serializer序列化工具：alpakka-kafka提供了String类型的序列化/反序列化函数，可以直接使用

4、bootstrap-server：一个以逗号分隔的kafka-cluster节点ip清单文本

下面是一个具体的例子：

  implicit val system = ActorSystem("kafka_sys")
  val bootstrapServers = "localhost:9092"
  val config = system.settings.config.getConfig("akka.kafka.producer")
  val producerSettings =
    ProducerSettings(config, new StringSerializer, new StringSerializer)
      .withBootstrapServers(bootstrapServers)

这里使用ActorSystem只是为了读取.conf文件里的配置，还没有使用任何akka-streams组件。akka.kafka.producer配置段落在alpakka-kafka的reference.conf里提供了默认配置，不需要在application.conf里重新定义。

alpakka-kafka提供了一个最基本的producer，非akka-streams组件，sendProducer。下面我们示范一下sendProducer的使用和效果：

import akka.actor.ActorSystem
import akka.kafka.scaladsl.{Consumer, SendProducer}
import org.apache.kafka.clients.producer.{ProducerRecord, RecordMetadata}
import akka.kafka._
import org.apache.kafka.common.serialization._
import scala.concurrent.duration._
import scala.concurrent.{Await, Future}

object SendProducerDemo extends App {
  implicit val system = ActorSystem("kafka_sys")
  implicit val executionContext = system.dispatcher
  val bootstrapServers = "localhost:9092"
  val config = system.settings.config.getConfig("akka.kafka.producer")
  val producerSettings =
    ProducerSettings(config, new StringSerializer, new StringSerializer)
      .withBootstrapServers(bootstrapServers)
  val producer = SendProducer(producerSettings)
  val topic = "greatings"
  val lstfut: Seq[Future[RecordMetadata]] =
    (100 to 200).reverse
      .map(_.toString)
      .map(value => new ProducerRecord[String, String](topic, s"hello-$value"))
      .map(msg => producer.send(msg))

  val futlst = Future.sequence(lstfut)
  Await.result(futlst, 2.seconds)


  scala.io.StdIn.readLine()
  producer.close()
  system.terminate()
}

以上示范用sendProducer向kafka写入100条hello消息。使用的是集合遍历，没有使用akka-streams的Source。为了检验具体效果，我们可以使用kafka提供的一些手工指令，如下：

w> ./kafka-topics --create --topic greatings --bootstrap-server localhost:9092
Created topic greatings.
w> ./kafka-console-consumer --topic greatings  --bootstrap-server localhost:9092
hello-100
hello-101
hello-102
hello-103
hello-104
hello-105
hello-106
...

既然producer代表写入功能，那么在akka-streams里就是Sink或Flow组件的功能了。下面这个例子是producer Sink组件plainSink的示范：

import akka.Done
import akka.actor.ActorSystem
import akka.kafka.scaladsl._
import akka.kafka._
import akka.stream.scaladsl._
import org.apache.kafka.clients.producer.ProducerRecord
import org.apache.kafka.common.serialization._

import scala.concurrent._
import scala.concurrent.duration._

object plain_sink extends App {
  implicit val system = ActorSystem("kafka_sys")
  val bootstrapServers = "localhost:9092"
  val config = system.settings.config.getConfig("akka.kafka.producer")
  val producerSettings =
    ProducerSettings(config, new StringSerializer, new StringSerializer)
      .withBootstrapServers(bootstrapServers)

  implicit val executionContext = system.dispatcher
  val topic = "greatings"
  val done: Future[Done] =
    Source(1 to 100)
      .map(_.toString)
      .map(value => new ProducerRecord[String, String](topic, s"hello-$value"))
      .runWith(Producer.plainSink(producerSettings))

  Await.ready(done,3.seconds)

  scala.io.StdIn.readLine()
  system.terminate()
}

这是一个典型的akka-streams应用实例，其中Producer.plainSink就是一个akka-streams Sink组件。

以上两个示范都涉及到构建一个ProducerRecord类型并将之写入kafka。ProducerRecord是一个基本的kafka消息类型：

   public ProducerRecord(String topic, K key, V value) {
        this(topic, null, null, key, value, null);
    }

topic是String类型，key, value 是 Any 类型的。 alpakka-kafka在ProducerRecord之上又拓展了一个复杂点的消息类型ProducerMessage.Envelope类型：

sealed trait Envelope[K, V, +PassThrough] {
    def passThrough: PassThrough
    def withPassThrough[PassThrough2](value: PassThrough2): Envelope[K, V, PassThrough2]
  }


  final case class Message[K, V, +PassThrough](
      record: ProducerRecord[K, V],
      passThrough: PassThrough
  ) extends Envelope[K, V, PassThrough] {
    override def withPassThrough[PassThrough2](value: PassThrough2): Message[K, V, PassThrough2] =
      copy(passThrough = value)
  }

ProducerMessage.Envelope增加了个PassThrough参数，用来与消息一道传递额外的元数据。alpakka-kafka streams组件使用这个消息类型作为流元素，最终把它转换成一或多条ProducerRecord写入kafka。如下： 

object EventMessages {
//一对一条ProducerRecord
   def createMessage[KeyType,ValueType,PassThroughType](
      topic: String,
      key: KeyType,
      value: ValueType,
      passThrough: PassThroughType): ProducerMessage.Envelope[KeyType,ValueType,PassThroughType] = {
     val single = ProducerMessage.single(
       new ProducerRecord[KeyType,ValueType](topic,key,value),
       passThrough
     )
     single
   }
//一对多条ProducerRecord
  def createMultiMessage[KeyType,ValueType,PassThroughType] (
       topics: List[String],
       key: KeyType,
       value: ValueType,
       passThrough: PassThroughType): ProducerMessage.Envelope[KeyType,ValueType,PassThroughType] = {
    import scala.collection.immutable
    val msgs = topics.map { topic =>
      new ProducerRecord(topic,key,value)
    }.toSeq
    val multi = ProducerMessage.multi(
      msgs,
      passThrough
    )
    multi
  }
//只传递通过型元数据
  def createPassThroughMessage[KeyType,ValueType,PassThroughType](
       topic: String,
       key: KeyType,
       value: ValueType,
       passThrough: PassThroughType): ProducerMessage.Envelope[KeyType,ValueType,PassThroughType] = {
    ProducerMessage.passThrough(passThrough)
  }

}

flexiFlow是一个alpakka-kafka Flow组件，流入ProducerMessage.Evelope，流出Results类型： 

  def flexiFlow[K, V, PassThrough](
      settings: ProducerSettings[K, V]
  ): Flow[Envelope[K, V, PassThrough], Results[K, V, PassThrough], NotUsed] = { ... }

Results类型定义如下：

  final case class Result[K, V, PassThrough] private (
      metadata: RecordMetadata,
      message: Message[K, V, PassThrough]
  ) extends Results[K, V, PassThrough] {
    def offset: Long = metadata.offset()
    def passThrough: PassThrough = message.passThrough
  }

也就是说flexiFlow可以返回写入kafka后kafka返回的操作状态数据。我们再看看flexiFlow的使用案例： 

import akka.kafka.ProducerMessage._
import akka.actor.ActorSystem
import akka.kafka.scaladsl._
import akka.kafka.{ProducerMessage, ProducerSettings}
import akka.stream.scaladsl.{Sink, Source}
import org.apache.kafka.clients.producer.ProducerRecord
import org.apache.kafka.common.serialization.StringSerializer

import scala.concurrent._
import scala.concurrent.duration._

object flexi_flow extends App {
  implicit val system = ActorSystem("kafka_sys")
  val bootstrapServers = "localhost:9092"
  val config = system.settings.config.getConfig("akka.kafka.producer")
  val producerSettings =
    ProducerSettings(config, new StringSerializer, new StringSerializer)
      .withBootstrapServers(bootstrapServers)

  // needed for the future flatMap/onComplete in the end
  implicit val executionContext = system.dispatcher
  val topic = "greatings"

  val done = Source(1 to 100)
    .map { number =>
      val value = number.toString
      EventMessages.createMessage(topic,"key",value,number)
    }
    .via(Producer.flexiFlow(producerSettings))
    .map {
      case ProducerMessage.Result(metadata, ProducerMessage.Message(record, passThrough)) =>
        s"${metadata.topic}/${metadata.partition} ${metadata.offset}: ${record.value}"

      case ProducerMessage.MultiResult(parts, passThrough) =>
        parts
          .map {
            case MultiResultPart(metadata, record) =>
              s"${metadata.topic}/${metadata.partition} ${metadata.offset}: ${record.value}"
          }
          .mkString(", ")

      case ProducerMessage.PassThroughResult(passThrough) =>
        s"passed through"
    }
    .runWith(Sink.foreach(println(_)))

  Await.ready(done,3.seconds)

  scala.io.StdIn.readLine()
  system.terminate()
}

object EventMessages {
   def createMessage[KeyType,ValueType,PassThroughType](
      topic: String,
      key: KeyType,
      value: ValueType,
      passThrough: PassThroughType): ProducerMessage.Envelope[KeyType,ValueType,PassThroughType] = {
     val single = ProducerMessage.single(
       new ProducerRecord[KeyType,ValueType](topic,key,value),
       passThrough
     )
     single
   }
  def createMultiMessage[KeyType,ValueType,PassThroughType] (
       topics: List[String],
       key: KeyType,
       value: ValueType,
       passThrough: PassThroughType): ProducerMessage.Envelope[KeyType,ValueType,PassThroughType] = {
    import scala.collection.immutable
    val msgs = topics.map { topic =>
      new ProducerRecord(topic,key,value)
    }.toSeq
    val multi = ProducerMessage.multi(
      msgs,
      passThrough
    )
    multi
  }
  def createPassThroughMessage[KeyType,ValueType,PassThroughType](
       topic: String,
       key: KeyType,
       value: ValueType,
       passThrough: PassThroughType): ProducerMessage.Envelope[KeyType,ValueType,PassThroughType] = {
    ProducerMessage.passThrough(passThrough)
  }

}

producer除向kafka写入与业务相关的业务事件或业务指令外还会向kafka写入当前消息读取的具体位置offset，所以alpakka-kafka的produce可分成两种类型：上面示范的plainSink, flexiFlow只向kafka写业务数据。还有一类如commitableSink还包括了把消息读取位置offset写入commit的功能。如下：

val control =
  Consumer
    .committableSource(consumerSettings, Subscriptions.topics(topic1, topic2))
    .map { msg =>
      ProducerMessage.single(
        new ProducerRecord(targetTopic, msg.record.key, msg.record.value),
        msg.committableOffset
      )
    }
    .toMat(Producer.committableSink(producerSettings, committerSettings))(DrainingControl.apply)
    .run()

control.drainAndShutdown()

如上所示，committableSource从kafka读取业务消息及读取位置committableOffsset，然后Producer.committableSink把业务消息和offset再写入kafka。

下篇讨论我们再具体介绍consumer。