akka/akka-core

Improve Sink.combine to return materialized values of all Sinks

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#23,958 opened on Nov 9, 2017

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 (19 comments) (2 reactions) (0 assignees)Scala (13,277 stars) (3,547 forks)batch import
1 - triagedhelp wantedt:stream

Description

The current implementation of Sink.combine materializes to NotUsed for quite obvious reasons: There is no way of getting the materialization types of the rest parameter.

Since Sink.combine is a super handy method to easily create FanOut/FanIn etc. streams, I kinda expected it to give me the materialized values of all sinks in a tuple.

In my specific case, I implemented that bit myself in:

def combine[T, U, M1, M2](first: Sink[U, M1], second: Sink[U, M2])(
  strategy: Int ⇒ Graph[UniformFanOutShape[T, U], NotUsed]): Sink[T, (M1, M2)] = {
  Sink.fromGraph(GraphDSL.create(first, second)((_, _)) { implicit b => (s1, s2) =>
    import GraphDSL.Implicits._
    val d = b.add(strategy(2))

    d ~> s1
    d ~> s2

    SinkShape(d.in)
  })
}

I can imagine the overrides one would need to be relatively low. combine20 would probably be overkill and arguably, having combine2 one can just reapply that method to get a huge stacked tuple back in the end.

In any case: Do you think this would be a useful thing to have?

Contributor guide

Tech stack
scala
Domain
backenddata
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
3-5 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Akka StreamsScala type systemGraphDSL
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20
Research direction
The current Sink.combine returns NotUsed. To improve it, one needs to add overloaded methods for combine2 to combineN that return tuples of materialized values. Study the existing GraphDSL.create overloads that accept multiple shapes to see how materialized values are combined. Refer to the issue discussion for additional constraints.
Improve Sink.combine to return materialized values of all Sinks · akka/akka-core#23958 | Good First Issue