scala futures execution in parallel using for-loop - scala

Question

package p1
import scala.util.Failure
import scala.util.Success
import scala.concurrent.Await
import scala.concurrent.Future
import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.duration._
object modCheck extends App {
def getStudentRoolNo(name: String) = Future {
println("getStudentRoolNo")
name match {
case "name1" => 1
case "name2" => 2
case _ => throw new Exception("No doesnt exist")
}
}
def getRank(roolNo: Int) = Future {
println("getRank")
Thread.sleep(500)
roolNo match {
case 1 => "1"
case 2 => "2"
case _ => throw new Exception("No roolNo exist")
}
}
def getDetails(roolNo: Int) = Future {
println("getDetails")
roolNo match {
case 1 => "details 1"
case 2 => "Details 2"
case _ => throw new Exception("No details exist")
}
}
def getStudentRecord(name: String) = {
for {
rollNo <- getStudentRoolNo(name)
rank <- getRank(rollNo)
details <- getDetails(rollNo)
} yield (rank + details)
}
getStudentRecord("name1").onComplete {
case Success(ground) => println(s"got my Details $ground")
case Failure(ex) => println("Exception!" + ex)
}
Thread.sleep(2000)
}
I want to execute functions getrank and getDetails in parallel in below code(once the getStudentRollNo is returned). How can I achieve this?
I Tried below way, It seems it still executing in sequentially
Please let me know , How to execute in parallel

Answer 1

Future starts computation when it's created.
for (a <- x; b <- y) yield ??? is desugared to x.flatMap(a => y.map(b => ???))
flatMap() and map() execute it's argument after a Future is completed.
getDetails() can start before completion of getRank() by separating creation of Future and flatMap() invocation.
for {
rollNo <- getStudentRoolNo(name)
rankFuture = getRank(rollNo)
detailsFuture = getDetails(rollNo)
rank <- rankFuture
details <- detailsFuture
} yield (rank + details)

Answer 2

As you have probable guessed, your current code does not make calls to getRank and getDetails in parallel, since its inside for-comprehension. Its a syntactic sugar for map operation. To achieve parallelism you need to create two futures outside for-comprehension.
val student = getStudentRollNo(name)
val detailsFuture = student map {s => getRank(rollNo) }
val rankFuture = student map {s => getDetails(rollNo) }
for {
rank <- rankFuture
details <- detailsFuture
} yield (rank + details)

Answer 3

You can use a zip in your for-comprehension to run two Futures in parallel:
def getStudentRecord(name: String) = {
for {
rollNo <- getStudentRoolNo(name)
rankRollNo <- getRank(rollNo) zip getDetails(rollNo)
} yield rankRollNo
}
In the above getRank and getDetails are run simultaneously and the result is a string tuple. If you want to yield a single string, you will need to separate rankRollNo into its separate components:
yield rankRollNo._1 + rankRollNo._2