Akka Streams how to write a GraphStage with OrElse

Question

I have the following requirement. I am writing a GraphStage which needs to lookup a SQL Table (in a different db) . If that lookup fails, only then it should lookup a second table (in a different db) and if that lookup also fails, then lookup in the 3rd table (in a different table). if all lookups fail, then use a default

I googled and found this

http://doc.akka.io/japi/akka/current/akka/stream/scaladsl/OrElse.html

and also this thead

Alternative flows based on condition for akka stream

But broadcast and partition is not what I am looking for. I don't want to lookup both the tables simultaneously. what I want is that if one flow returns a None, only then the second flow is used to fetch the value.

Right now I have done something like this

val flow = Flow[Foo].map{foo => 
    lookup1(foo.id) orElse lookup2(foo.id) getOrElse default
}

But this makes the flow above very monolithic. it would be nice if I can break the flow above into 3 separate ones, and then connect them via orelse clause in my graphstage.

`OrElse` seems like a private piece of API, and the `OrElse` class is private to `akka.stream` package. I think your solution is the best you can do. — Vladimir Matveev, Sep 19 '17 at 16:12
Ah, so the lookup is not in a second table, but a second database. — Viktor Klang, Sep 19 '17 at 21:04

score 1 · Answer 1 · answered Sep 19 '17 at 17:20

Using flatMapConcat and orElse might help you getting your code more generic in terms of the number of source you want to combine. See example below

  val altFlows: List[Flow[Foo, Option[Bar], NotUsed]] = ???
  val default : Bar = ???

  Flow[Foo].flatMapConcat { foo ⇒
    val altSources = altFlows.map(Source.single(foo).via(_).collect{ case Some(x) ⇒ x })
    val default    = Source.single(default)

    (altSources :+ default).reduce(_ orElse _)
  }

Akka Streams how to write a GraphStage with OrElse

1 Answers1