Akka Future - Parallel versus Concurrent?

Question

From the well-written Akka Concurrency:

As I understand, the diagram points out, both numSummer and charConcat will run on the same thread.

Is it possible to run each Future in parallel, i.e. on separate threads?

Answer 1

The picture on the left is them running in parallel.

The point of the illustration is that the Future.apply method is what kicks off the execution, so if it doesn't happen until the first future's result is flatMaped (as in the picture on the right), then you don't get the parallel execution.

(Note that by "kicked off", i mean the relevant ExecutionContext is told about the job. How it parallelizes is a different question and may depend on things like the size of its thread pool.)

Equivalent code for the left:

val numSummer = Future { ... }  // execution kicked off
val charConcat = Future { ... }  // execution kicked off
numSummer.flatMap { numsum =>
  charConcat.map { string =>
    (numsum, string)
  }
}

and for the right:

Future { ... }  // execution kicked off
  .flatMap { numsum =>
    Future { ... }  // execution kicked off (Note that this does not happen until
                    // the first future's result (`numsum`) is available.)
      .map { string =>
        (numsum, string)
      }
  }