Conditionally skip flow using akka streams

Question

I'm using akka streams and I have a segment of my graph that I need to conditionally skip because the flow can't handle certain values. Specifically, I have a flow that takes a string and makes http requests, but the server can't handle the case when the string is empty. But I need to just return an empty string instead. Is there a way of doing this without having to go through the http request knowing it will fail? I basically have this:

val source = Source("1", "2", "", "3", "4")
val httpRequest: Flow[String, HttpRequest, _]
val httpResponse: Flow[HttpResponse, String, _]
val flow = source.via(httpRequest).via(httpResponse)

The only thing I can think of doing is catching the 400 error in my httpResponse flow and returning a default value. But I'd like to be able to avoid the overhead of hitting the server for a request I know is going to fail beforehand.

Answer 1

You could use flatMapConcat:

(Warning: was never compiled, but you'll get the gist of it)

val source = Source("1", "2", "", "3", "4")
val httpRequest: Flow[String, HttpRequest, _]
val httpResponse: Flow[HttpResponse, String, _]
val makeHttpCall: Flow[HttpRequest, HttpResponse, _]
val someHttpTransformation = httpRequest via makeHttpCall via httpResponse
val emptyStringSource = Source.single("")
val cleanerSource = source.flatMapConcat({
  case "" => emptyStringSource
  case other => Source.single(other) via someHttpTransformation
})