[akka-user][deprecated] How to create a Source that restarts if it was not empty

'Mr B' via Akka User List

2018-09-03 06:49:56 UTC

Hi,

I came up with a solution (below) however it is sub optimal - it has to
materialise the graph every time to get Sink.lastOption - ideally what I
want is to avoid materialisation on each iteration:

"Loop on Source on Sink.optionLast" in {
def dynamicSource = Source(1 to Random.nextInt(4)) // represents a
cursor from a database that we want to re-run if not empty

def maybeLastFromMaterialisedGraph: Future[Option[Int]] = {
val source = dynamicSource
val sink = Sink.foreach(println)
val sink2 = Sink.lastOption[Int]
val dataFlow =
source.alsoToMat(sink)(Keep.right)
.toMat(sink2)(Keep.both)
val (fIgnore, fInt) = dataFlow.run()
fInt
}

// TODO: make this tail recursive
def loop(eventuallyMaybeInt: Future[Option[Int]]): Future[Option[Int]]
= {
eventuallyMaybeInt.flatMap { o =>
o match {
case None => eventuallyMaybeInt
case _ => loop(maybeLastFromMaterialisedGraph)
}
}
}

val i = await(loop(maybeLastFromMaterialisedGraph))
println(s"Sink.last i=$i")
}

Post by 'Mr B' via Akka User List
Hi,
I am an akka streams newbie and was wondering how to create a Source that
restarts if it was not empty (Source would be dynamic eg the result of a
database fetch)?
My initial thoughts were to broadcast the source to say Sink.optionFirst
and if the materialised value of Sink.optionFirst is Some(element), then
restart the source, if it is None then complete as normal. However I am
struggling with how I would do this.
Thanks

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Post by 'Mr B' via Akka User List

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