CODEWITHSUNDEEP
scalaakkaakka-stream
Wang Liqin
Wang Liqin

Reputation: 25

Akka Streams - Understanding when and how materialisation works

An app that I am developing requires/gives the users the ability to create and define arbitrary streams at runtime. I understand that in Akka streams in particular

Materialisation = Execute or Run

My questions

1) Should materialisation for a stream be done only once? i.e if it is already materialised then can I use the value for subsequent runs?

2) As said above, maybe I misunderstood the term materialisation. If a stream has to run, it is materialised each time?

I am confused because in the docs, it says materialisation actually creates the resources needed for stream execution. So my immediate understanding is that it has to be done only once. Just like a JDBC connection to a database. Can someone please explain in a non-akka terminology.

Upvotes: 0

Views: 507

Answers (1)

Jeffrey Chung
Jeffrey Chung

Reputation: 19497

Yes, a stream can be materialized multiple times. And yes, if a stream is run multiple times, it is materialized each time. From the documentation:

Since a stream can be materialized multiple times, the materialized value will also be calculated anew for each such materialization, usually leading to different values being returned each time. In the example below we create two running materialized instance of the stream that we described in the runnable variable, and both materializations give us a different Future from the map even though we used the same sink to refer to the future:

// connect the Source to the Sink, obtaining a RunnableGraph
val sink = Sink.fold[Int, Int](0)(_ + _)

val runnable: RunnableGraph[Future[Int]] =
  Source(1 to 10).toMat(sink)(Keep.right)

// get the materialized value of the FoldSink
val sum1: Future[Int] = runnable.run()
val sum2: Future[Int] = runnable.run()

// sum1 and sum2 are different Futures!

Think of a stream as a reusable blueprint that can be run/materialized multiple times. A materializer is required to materialize a stream, and Akka Streams provides a materializer called ActorMaterializer. The materializer allocates the necessary resources (actors, etc.) and executes the stream. While it is common to use the same materializer for different streams and multiple materializations, each materialization of a stream triggers the resource allocation needed to run the stream. In the example above, sum1 and sum2 use the same blueprint (runnable) and the same materializer, but they are the results of distinct materializations that incurred distinct resource allocations.

Upvotes: 1

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