Futures

Creating a Future

import scala.concurrent.Future
import scala.concurrent.ExecutionContext.Implicits.global

object FutureDivider {
    def divide(a: Int, b: Int): Future[Int] = Future {
        // Note that this is integer division.
        a / b
    }
}

Quite simply, the divide method creates a Future that will resolve with the quotient of a over b.

Consuming a Successful Future

The easiest way to consume a successful Future— or rather, get the value inside the Future— is to use the map method. Suppose some code calls the divide method of the FutureDivider object from the “Creating a Future” example. What would the code need to look like to get the quotient of a over b?

object Calculator {
    def calculateAndReport(a: Int, b: Int) = {
        val eventualQuotient = FutureDivider divide(a, b)
        
        eventualQuotient map {
            quotient => println(quotient)
        }
    }
}

Consuming a Failed Future

Sometimes the computation in a Future can create an exception, which will cause the Future to fail. In the “Creating a Future” example, what if the calling code passed 55 and 0 to the divide method? It’d throw an ArithmeticException after trying to divide by zero, of course. How would that be handled in consuming code? There are actually a handful of ways to deal with failures.

Handle the exception with recover and pattern matching.

object Calculator {
    def calculateAndReport(a: Int, b: Int) = {
        val eventualQuotient = FutureDivider divide(a, b)
    
        eventualQuotient recover {
            case ex: ArithmeticException => println(s"It failed with: ${ex.getMessage}")
        }
    }
}

Handle the exception with the failed projection, where the exception becomes the value of the Future:

object Calculator {
    def calculateAndReport(a: Int, b: Int) = {
        val eventualQuotient = FutureDivider divide(a, b)
    
        // Note the use of the dot operator to get the failed projection and map it.
        eventualQuotient.failed.map {
            ex => println(s"It failed with: ${ex.getMessage}")
        }
    }
}

Putting the Future Together

The previous examples demonstrated the individual features of a Future, handling success and failure cases. Usually, however, both features are handled much more tersely. Here’s the example, written in a neater and more realistic way:

object Calculator {
    def calculateAndReport(a: Int, b: Int) = {
        val eventualQuotient = FutureDivider divide(a, b)
        
        eventualQuotient map {
            quotient => println(s"Quotient: $quotient")
        } recover {
            case ex: ArithmeticException => println(s"It failed with: ${ex.getMessage}")
        }
    }
}

Sequencing and traversing Futures

In some cases it is necessary to calculate a variable amount of values on separate Futures. Assume to have a List[Future[Int]], but instead a List[Int] needs to be processed. Then the question is how to turn this instance of List[Future[Int]] into a Future[List[Int]]. For this purpose there is the sequence method on the Future companion object.

def listOfFuture: List[Future[Int]] = List(1,2,3).map(Future(_))
def futureOfList: Future[List[Int]] = Future.sequence(listOfFuture)

In general sequence is a commonly known operator within the world of functional programming that transforms F[G[T]] into G[F[T]] with restrictions to F and G.

There is an alternate operator called traverse, which works similar but takes a function as an extra argument. With the identity function x => x as a parameter it behaves like the sequence operator.

def listOfFuture: List[Future[Int]] = List(1,2,3).map(Future(_))
def futureOfList: Future[List[Int]] = Future.traverse(listOfFuture)(x => x)

However, the extra argument allows to modify each future instance inside the given listOfFuture. Furthermore, the first argument doesn’t need to be a list of Future. Therefore it is possible to transform the example as follows:

def futureOfList: Future[List[Int]] = Future.traverse(List(1,2,3))(Future(_))

In this case the List(1,2,3) is directly passed as first argument and the identity function x => x is replaced with the function Future(_) to similarly wrap each Int value into a Future. An advantage of this is that the intermediary List[Future[Int]] can be omitted to improve performance.

Combine Multiple Futures – For Comprehension

The for comprehension is a compact way to run a block of code that depends on the successful result of multiple futures.

With f1, f2, f3 three Future[String]’s that will contain the strings one, two, three respectively,

val fCombined = 
    for {
        s1 <- f1
        s2 <- f2
        s3 <- f3
    } yield (s"$s1 - $s2 - $s3")

fCombined will be a Future[String] containing the string one - two - three once all the futures have completed successfully.

Note that an implicit ExectionContext is assumed here.

Also, keep in mind that for comprehension is just a syntactic sugar for a flatMap method, so Future objects construction inside for body would eliminate concurrent execution of code-blocks enclosed by futures and lead to sequential code. You see it on example:

val result1 = for {
  first <- Future {
    Thread.sleep(2000)
    System.currentTimeMillis()
  }
  second <- Future {
    Thread.sleep(1000)
    System.currentTimeMillis()
  }
} yield first - second

val fut1 = Future {
  Thread.sleep(2000)
  System.currentTimeMillis()
}
val fut2 = Future {
  Thread.sleep(1000)
  System.currentTimeMillis()
}
val result2 = for {
  first <- fut1
  second <- fut2
} yield first - second

Value enclosed by result1 object would be always negative while result2 would be positive.

For more details about the for comprehension and yield in general, see https://docs.scala-lang.org/tutorials/FAQ/yield.html