Question about some of the documentation in akka streams

Akka Akka Streams & Alpakka
1

In working with graphs you have an example of a partial graph that doesn’t make a ton of sense to me.

val pickMaxOfThree = GraphDSL.create() { implicit b ⇒
  import GraphDSL.Implicits._

  val zip1 = b.add(ZipWith[Int, Int, Int](math.max _))
  val zip2 = b.add(ZipWith[Int, Int, Int](math.max _))
  zip1.out ~> zip2.in0

  UniformFanInShape(zip2.out, zip1.in0, zip1.in1, zip2.in1)
}

wouldn’t zip1.in2 and zip2.in2 be unbound in this example? and why are two ZipWiths needed at all, wouldn’t the first pick the max of the three inputs already?

2

Those ZipWiths are look like this: ZipWith[In0, In1, Out](f: (In0, In1) => Out)

So they don’t have in3.

The graph looks like this:

UniformIn1+-------+
                 zip1+--+
UniformIn2+-------+     +
                       zip2+----+UniformOut
UniformIn3+-------------+
3

Ah ok. But you could just have a ZipWith[Int,Int,Int,Int]{ case (i0,i1,i2) => math.max(math.max(i0,i1),i2) } right?

4

Yes, ZipWith has generated versions for many inputs – though the purpose of that specific example is more to teach about how to combine things in the advanced graph DSL and the predefined shapes rather than showing ZipWith specifically – but yeah, you’re absolutely right.

It could be a good @@@ note to add on that docs section, would you PR such addition @tg44?
The file is here https://github.com/johanandren/akka/blob/master/akka-docs/src/main/paradox/stream/stream-graphs.md

5

Yapp! Today I found another “not so clear” part of the docs too, I will fix that too!

2 Likes
6

Awesome, thanks a lot :slight_smile:

7

Thanks for asking this question. I was confused about the same example, and the drawing our made @tg44 helped a lot! Is there a way to add such drawings to the documentation?

8

We would want nice diagrams but we have yet to decide how, there is an issue discussing how we can add nice diagrams without it becoming too work intense to maintain/get to work here: https://github.com/akka/akka/issues/24933

2 Likes
9

Thanks @johanandren!