ActorFlow.ask

Use the “Ask Pattern” to send each stream element as an ask to the target actor (of the new actors API), and expect a reply that will be emitted downstream.

Actor interop operators

Dependency

This operator is included in:

sbt

val PekkoVersion = "1.1.2+29-e21fa9eb-SNAPSHOT"
libraryDependencies += "org.apache.pekko" %% "pekko-stream-typed" % PekkoVersion

Maven

<properties>
  <scala.binary.version>2.13</scala.binary.version>
</properties>
<dependencyManagement>
  <dependencies>
    <dependency>
      <groupId>org.apache.pekko</groupId>
      <artifactId>pekko-bom_${scala.binary.version}</artifactId>
      <version>1.1.2+29-e21fa9eb-SNAPSHOT</version>
      <type>pom</type>
      <scope>import</scope>
    </dependency>
  </dependencies>
</dependencyManagement>
<dependencies>
  <dependency>
    <groupId>org.apache.pekko</groupId>
    <artifactId>pekko-stream-typed_${scala.binary.version}</artifactId>
  </dependency>
</dependencies>

Gradle

def versions = [
  ScalaBinary: "2.13"
]
dependencies {
  implementation platform("org.apache.pekko:pekko-bom_${versions.ScalaBinary}:1.1.2+29-e21fa9eb-SNAPSHOT")

  implementation "org.apache.pekko:pekko-stream-typed_${versions.ScalaBinary}"
}

Signature

ActorFlow.askActorFlow.ask

Description

Use the Ask pattern to send a request-reply message to the target ref actor. If any of the asks times out it will fail the stream with an AskTimeoutExceptionAskTimeoutException.

The ask operator requires

• the actor ref,
• a makeMessage function to create the message sent to the actor from the incoming element, and the actor ref accepting the actor’s reply message
• a timeout.

See also:

• Flow.ask for the classic actors variant

Examples

The ActorFlow.ask sends a message to the actor. The actor expects Asking messages which contain the actor ref for replies of type Reply. When the actor for replies receives a reply, the ActorFlow.ask stream stage emits the reply and the map extracts the message String.

Scala

copysourceimport org.apache.pekko
import pekko.stream.scaladsl.{ Flow, Sink, Source }
import pekko.stream.typed.scaladsl.ActorFlow
import pekko.actor.typed.ActorRef
import pekko.actor.typed.scaladsl.Behaviors
import pekko.util.Timeout

final case class Asking(s: String, replyTo: ActorRef[Reply])
final case class Reply(msg: String)

final case class AskingWithStatus(s: String, replyTo: ActorRef[StatusReply[String]])

    val ref = spawn(Behaviors.receiveMessage[Asking] { asking =>
      asking.replyTo ! Reply(asking.s + "!!!")
      Behaviors.same
    })

implicit val timeout: Timeout = 1.second

val askFlow: Flow[String, Reply, NotUsed] =
  ActorFlow.ask(ref)(Asking.apply)

// explicit creation of the sent message
val askFlowExplicit: Flow[String, Reply, NotUsed] =
  ActorFlow.ask(ref)(makeMessage = (el, replyTo: ActorRef[Reply]) => Asking(el, replyTo))

val in: Future[immutable.Seq[String]] =
  Source(1 to 50).map(_.toString).via(askFlow).map(_.msg).runWith(Sink.seq)

Java

copysourceimport org.apache.pekko.actor.typed.ActorRef;
import org.apache.pekko.actor.typed.ActorSystem;
import org.apache.pekko.pattern.StatusReply;
import org.apache.pekko.stream.javadsl.Flow;
import org.apache.pekko.stream.javadsl.Sink;
import org.apache.pekko.stream.javadsl.Source;
import org.apache.pekko.stream.typed.javadsl.ActorFlow;

class Asking {
  final String payload;
  final ActorRef<Reply> replyTo;

  public Asking(String payload, ActorRef<Reply> replyTo) {
    this.payload = payload;
    this.replyTo = replyTo;
  }
}

static class AskingWithStatus {
  final String payload;
  final ActorRef<StatusReply<String>> replyTo;

  public AskingWithStatus(String payload, ActorRef<StatusReply<String>> replyTo) {
    this.payload = payload;
    this.replyTo = replyTo;
  }
}

class Reply {
  public final String msg;

  public Reply(String msg) {
    this.msg = msg;
  }
}

final ActorRef<Asking> actorRef = // ???
final ActorRef<AskingWithStatus> actorWithStatusRef = // ???
Duration timeout = Duration.ofSeconds(1);

// method reference notation
Flow<String, Reply, NotUsed> askFlow = ActorFlow.ask(actorRef, timeout, Asking::new);

// explicit creation of the sent message
Flow<String, Reply, NotUsed> askFlowExplicit =
    ActorFlow.ask(actorRef, timeout, (msg, replyTo) -> new Asking(msg, replyTo));

Flow<String, String, NotUsed> askFlowExplicitWithStatus =
    ActorFlow.askWithStatus(
        actorWithStatusRef, timeout, (msg, replyTo) -> new AskingWithStatus(msg, replyTo));

Source.repeat("hello").via(askFlow).map(reply -> reply.msg).runWith(Sink.seq(), system);

Reactive Streams semantics