akka.streams.Source that you can emit values (similar to monix.BehaviorSubject)

Question

I'm looking for akka.stream.scaladsl.Source construction method that will allow me to simply emit next value from different place of code (e.g. watching on system events).

I need somethig similar to Promise. Promise emits single value to Future. I need to emit multiple values to Source.
like monix.reactive.subjects.BehaviorSubject.onNext(_)
I don't care about backpressure too much.

currently I've implemented this using monix & akka-streams (code below) but I expect that there should be akka-streams only sollution:

import akka.stream.scaladsl.{Flow, Sink, Source}
import monix.reactive.subjects.BehaviorSubject
import monix.execution.Scheduler.Implicits.global

val bs = BehaviorSubject("") //monix subject is sink and source at the same time

//this is how it is currently implemented
def createSource() = { 
    val s1 = Source.fromPublisher(bs.toReactivePublisher) //here we create source from subject
    Flow.fromSinkAndSourceCoupled[String, String](Sink.ignore, s1)
}

//somewhere else in code... some event happened
//this is how it works in monix.
val res:Future[Ack] = bs.onNext("Event #1471 just happened!") //here we emit value

Maybe this topic would be useful: https://stackoverflow.com/questions/36397424/how-to-use-an-akka-streams-sourcequeue-with-playframework — Marcus Linke, Mar 06 '20 at 09:23
Yes... this was helpful. The `Source.queue(...)` solves a problem. Thanks a lot. — Scalway, Mar 06 '20 at 13:09

score 2 · Accepted Answer · answered Mar 05 '20 at 13:48

Perhaps you are looking for Actor Source

An example from the docs:

import akka.actor.typed.ActorRef
import akka.stream.OverflowStrategy
import akka.stream.scaladsl.{ Sink, Source }
import akka.stream.typed.scaladsl.ActorSource

trait Protocol
case class Message(msg: String) extends Protocol
case object Complete extends Protocol
case class Fail(ex: Exception) extends Protocol

val source: Source[Protocol, ActorRef[Protocol]] = ActorSource.actorRef[Protocol](completionMatcher = {
  case Complete =>
}, failureMatcher = {
  case Fail(ex) => ex
}, bufferSize = 8, overflowStrategy = OverflowStrategy.fail)

val ref = source
  .collect {
    case Message(msg) => msg
  }
  .to(Sink.foreach(println))
  .run()

ref ! Message("msg1")

This way you would be able to send messages to actor via actor system, and these messages will be emitted from the ActorSource down the stream.

looks promising. Still testing. It need additional dependency. — Scalway, Mar 05 '20 at 14:29
I've not used it (other problems), but it answers my question well. Thanks. — Scalway, Mar 06 '20 at 13:12

score 0 · Answer 2 · answered Mar 05 '20 at 12:55

0

Source abstraction, as name says, provide API to work with data source. Instead, you need to take a look at abstraction which consumes data - Sink. And Sink.foreach operation is what you are looking for, most likely: https://doc.akka.io/docs/akka/current/stream/operators/Sink/foreach.html

In you case, the code would look something like:

import akka.stream.scaladsl.{Sink, Source}

val s1 = Source.// your WS akka stream source
s1.runWith(Sink.foreach(write))

Hope this helps!

answered Mar 05 '20 at 12:55

Ivan Kurchenko

4,043
1
11
28

Not what i wanted. I really need to create Source... not Sink. I just need to create it in such way that items could be emitted from other parts of code freely. Something like Promise and Future. – Scalway Mar 05 '20 at 13:32

Anand Sai · Answer 3 · 2020-03-05T14:59:16.447

0

I think what you are looking for is sink.foreach.it invoke a given procedure for each element received.I think code will look like:

s1.runWith(Sink.foreach(write))

Essentially, what is being done is that for stream a source, sink tries to write every element of that stream.

EDIT

I think you are looking for maybe. It creates a source that emits once the materialized Promise is completed with a value.Check out this documentation

EDIT

futureSource may also work.It streams the elements of the given future source once it successfully completes it.

Let me know if it helps!!

edited Mar 05 '20 at 14:59

answered Mar 05 '20 at 13:07

Anand Sai

1,566
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Not what I need. I've improved question. – Scalway Mar 05 '20 at 13:36
I have updated the answer. Please let me know if it helps!! – Anand Sai Mar 05 '20 at 13:39
Nice try... but still. It will emit only single value. I expect to emit them continuously... Whenever something interesting in system happens. – Scalway Mar 05 '20 at 13:44
So, let's say that once the future completes, you want to stream that data. Am I right? – Anand Sai Mar 05 '20 at 14:09
Future emits one item... I need something that could emit more than one. – Scalway Mar 05 '20 at 15:33
I have updated the answer. Using `futureSource` might work for you. – Anand Sai Mar 05 '20 at 15:37

score 0 · Answer 4 · answered Mar 05 '20 at 15:35

0

https://doc.akka.io/docs/akka/current/stream/operators/Source/fromIterator.html or https://doc.akka.io/docs/akka/current/stream/operators/Source/fromPublisher.html is what you need, depending on where does your Source consume data from.

answered Mar 05 '20 at 15:35

yiksanchan

1,890
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akka.streams.Source that you can emit values (similar to monix.BehaviorSubject)

4 Answers4