Akka Streams：流中的状态

Question

我想使用 Akka Streams 读取多个大文件来处理每一行。想象一下，每个键都由一个 (identifier -> value) 组成。如果找到新的标识符，我想将它和它的值保存在数据库中；否则，如果在处理行流时已经找到标识符，我只想保存该值。为此，我认为我需要某种递归状态流来保留已经在 Map 中找到的标识符。我想我会在这个流程中收到一对 (newLine, contextWithIdentifiers).

我刚刚开始研究 Akka Streams。我想我可以管理自己来做无状态处理的事情，但我不知道如何保留 contextWithIdentifiers。如果能指出正确的方向，我将不胜感激。

Answer 1

也许statefulMapConcat之类的东西可以帮到你：

import akka.actor.ActorSystem
import akka.stream.ActorMaterializer
import akka.stream.scaladsl.{Sink, Source}
import scala.util.Random._
import scala.math.abs
import scala.concurrent.ExecutionContext.Implicits.global

implicit val system = ActorSystem()
implicit val materializer = ActorMaterializer()

//encapsulating your input
case class IdentValue(id: Int, value: String)
//some random generated input
val identValues = List.fill(20)(IdentValue(abs(nextInt()) % 5, "valueHere"))

val stateFlow = Flow[IdentValue].statefulMapConcat{ () =>
  //state with already processed ids
  var ids = Set.empty[Int]
  identValue => if (ids.contains(identValue.id)) {
    //save value to DB
    println(identValue.value)
    List(identValue)
  } else {
    //save both to database
    println(identValue)
    ids = ids + identValue.id
    List(identValue)
  }
}

Source(identValues)
  .via(stateFlow)
  .runWith(Sink.seq)
  .onSuccess { case identValue => println(identValue) }

Answer 2

几年后，如果您只需要 1 对 1 映射（而不是 1-to-N），这里是我编写的一个实现：

import akka.stream.stage.{GraphStage, GraphStageLogic}
import akka.stream.{Attributes, FlowShape, Inlet, Outlet}

object StatefulMap {
  def apply[T, O](converter: => T => O) = new StatefulMap[T, O](converter)
}

class StatefulMap[T, O](converter: => T => O) extends GraphStage[FlowShape[T, O]] {
  val in = Inlet[T]("StatefulMap.in")
  val out = Outlet[O]("StatefulMap.out")
  val shape = FlowShape.of(in, out)

  override def createLogic(inheritedAttributes: Attributes): GraphStageLogic = new GraphStageLogic(shape) {
    val f = converter
    setHandler(in, () => push(out, f(grab(in))))
    setHandler(out, () => pull(in))
  }
}

测试（和演示）：

  behavior of "StatefulMap"

  class Counter extends (Any => Int) {
    var count = 0

    override def apply(x: Any): Int = {
      count += 1
      count
    }
  }

  it should "not share state among substreams" in {
    val result = await {
      Source(0 until 10)
        .groupBy(2, _ % 2)
        .via(StatefulMap(new Counter()))
        .fold(Seq.empty[Int])(_ :+ _)
        .mergeSubstreams
        .runWith(Sink.seq)
    }
    result.foreach(_ should be(1 to 5))
  }

Akka Streams：流中的状态

Akka Streams: State in a flow

scala

stateful

stream

akka

akka-stream