Pushing elements externally to a reactive stream in fs2

Question

I have an external (that is, I cannot change it) Java API which looks like this:

public interface Sender {
    void send(Event e);
}

I need to implement a Sender which accepts each event, transforms it to a JSON object, collects some number of them into a single bundle and sends over HTTP to some endpoint. This all should be done asynchronously, without send() blocking the calling thread, with some fixed-size buffer and dropping new events if the buffer is full.

With akka-streams this is quite simple: I create a graph of stages (which uses akka-http to send HTTP requests), materialize it and use the materialized ActorRef to push new events to the stream:

lazy val eventPipeline = Source.actorRef[Event](Int.MaxValue, OverflowStrategy.fail)
  .via(CustomBuffer(bufferSize))  // buffer all events
  .groupedWithin(batchSize, flushDuration)  // group events into chunks
  .map(toBundle)  // convert each chunk into a JSON message
  .mapAsyncUnordered(1)(sendHttpRequest)  // send an HTTP request
  .toMat(Sink.foreach { response =>
    // print HTTP response for debugging
  })(Keep.both)

lazy val (eventsActor, completeFuture) = eventPipeline.run()

override def send(e: Event): Unit = {
  eventsActor ! e
}

Here CustomBuffer is a custom GraphStage which is very similar to the library-provided Buffer but tailored to our specific needs; it probably does not matter for this particular question.

As you can see, interacting with the stream from non-stream code is very simple - the ! method on the ActorRef trait is asynchronous and does not need any additional machinery to be called. Each event which is sent to the actor is then processed through the entire reactive pipeline. Moreover, because of how akka-http is implemented, I even get connection pooling for free, so no more than one connection is opened to the server.

However, I cannot find a way to do the same thing with FS2 properly. Even discarding the question of buffering (I will probably need to write a custom Pipe implementation which does additional things that we need) and HTTP connection pooling, I'm still stuck with a more basic thing - that is, how to push the data to the reactive stream "from outside".

All tutorials and documentation that I can find assume that the entire program happens inside some effect context, usually IO. This is not my case - the send() method is invoked by the Java library at unspecified times. Therefore, I just cannot keep everything inside one IO action, I necessarily have to finalize the "push" action inside the send() method, and have the reactive stream as a separate entity, because I want to aggregate events and hopefully pool HTTP connections (which I believe is naturally tied to the reactive stream).

I assume that I need some additional data structure, like Queue. fs2 does indeed have some kind of fs2.concurrent.Queue, but again, all documentation shows how to use it inside a single IO context, so I assume that doing something like

val queue: Queue[IO, Event] = Queue.unbounded[IO, Event].unsafeRunSync()

and then using queue inside the stream definition and then separately inside the send() method with further unsafeRun calls:

val eventPipeline = queue.dequeue
  .through(customBuffer(bufferSize))
  .groupWithin(batchSize, flushDuration)
  .map(toBundle)
  .mapAsyncUnordered(1)(sendRequest)
  .evalTap(response => ...)
  .compile
  .drain

eventPipeline.unsafeRunAsync(...)  // or something

override def send(e: Event) {
  queue.enqueue(e).unsafeRunSync()
}

is not the correct way and most likely would not even work.

So, my question is, how do I properly use fs2 to solve my problem?

Answer 1

Consider the following example:

import cats.implicits._
import cats.effect._
import cats.effect.implicits._
import fs2._
import fs2.concurrent.Queue

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

object Answer {
  type Event = String

  trait Sender {
    def send(event: Event): Unit
  }

  def main(args: Array[String]): Unit = {
    val sender: Sender = {
      val ec = ExecutionContext.global
      implicit val cs: ContextShift[IO] = IO.contextShift(ec)
      implicit val timer: Timer[IO] = IO.timer(ec)

      fs2Sender[IO](2)
    }

    val events = List("a", "b", "c", "d")
    events.foreach { evt => new Thread(() => sender.send(evt)).start() }
    Thread sleep 3000
  }

  def fs2Sender[F[_]: Timer : ContextShift](maxBufferedSize: Int)(implicit F: ConcurrentEffect[F]): Sender = {
    // dummy impl
    // this is where the actual logic for batching
    //   and shipping over the network would live
    val consume: Pipe[F, Event, Unit] = _.evalMap { event =>
      for {
        _ <- F.delay { println(s"consuming [$event]...") }
        _ <- Timer[F].sleep(1.seconds)
        _ <- F.delay { println(s"...[$event] consumed") }
      } yield ()
    }

    val suspended = for {
      q <- Queue.bounded[F, Event](maxBufferedSize)
      _ <- q.dequeue.through(consume).compile.drain.start
      sender <- F.delay[Sender] { evt =>
        val enqueue = for {
          wasEnqueued <- q.offer1(evt)
          _ <- F.delay { println(s"[$evt] enqueued? $wasEnqueued") }
        } yield ()
        enqueue.toIO.unsafeRunAsyncAndForget()
      }
    } yield sender

    suspended.toIO.unsafeRunSync()
  }
}

The main idea is to use a concurrent Queue from fs2. Note, that the above code demonstrates that neither the Sender interface nor the logic in main can be changed. Only an implementation of the Sender interface can be swapped out.