This document provides an agenda for a Scala programming conference taking place in Madrid from November 27-28, 2015. The conference will cover topics including Akka actors, futures, and transitioning from Java to Scala. One session is titled "Akka voló sobre el nido del Future" (Akka flew over the Future's nest) presented by Javier Santos and David Vallejo. Another session is titled "Caminando de Java a Scala en menos de 2 horas" (Walking from Java to Scala in less than 2 hours).

1. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Akka voló sobre el nido del Future
Javier Santos
David Vallejo

2. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
About us
David Vallejo @dvnavarro
« It’s not a bug – it’s an
undocumented feature.»
Anonymous
Javier Santos @jpaniego
«Hay dos formas de programar sin
errores; solo la tercera funciona»
Alan J Perlis

3. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Introduction: A functional world
● Purely functional: whole world could be implemented
in a single functional line
● Functional features
○ Robust
○ Debuggable
○ Predictable
○ Pluggable

4. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Introduction: A functional world
● Example
type Context = (Input,Output,Error)
type Action = Context => (Context,Event)
def main(
actions: Iterable[Action],
context: Context):(Context,Seq[Event]) = {
((context,Seq.empty[Event]) /: actions) {
case ((context,events),action) =>
action.apply(context) match {
case (context,event) => (context, events :+ event)
}
}
}

5. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid

6. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Introduction: A functional world
● Problem? World actions are not precalculated.
What happens to I/O?
● World can be considered as
○ asynchronous: Things may happen at same time
○ reactive: Things may happen due to
■ actions: that trigger …
■ ...events: action consequences.

7. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Introduction: Runnable & Callable
● java.lang.Runnable
trait Runnable {
def run(): Unit
}
● java.util.concurrent.Callable
trait Callable[V] {
def call(): V
}

8. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Introduction: Threads
● Scala’s concurrency model is built on Java’s
● java.lang.Thread
val myThread = new Thread(new Runnable {
def run(){
println(“hi”)
}
}
myThread.start()
● Thread improvements (ExecutorService ~ Thread pool)
● Anyway, Threads abstraction level is too low

9. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
What about Scala and threads?

10. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Future: Overview
● By default, non-blocking operations
● They will hold a T value at some point
● So a future may be uncompleted(it has no value yet) or
completed
● Completion will be treated as a scala.util.Try value
It will have two possible values:
○ Success(t: T)
○ Failure(t: Throwable)

11. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Future: Overview
● Future example
import scala.concurrent._
import ExecutionContext.Implicits.global
val firstPrimeNumbers: Future[List[Int]] = Future {
List(1,2,3,5,7,11,13)
//what if 'calculateFirstPrimeNumbers(100000000)'…
}
res0: Future[List[Int]]

12. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Future: Overview
● Failing Future example
import scala.concurrent._
import ExecutionContext.Implicits.global
val thisWillFail: Future[Int] = Future(2 / 0)
res0: Future[Int]

13. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Future: ExecutionContext
● A future, once it’s completed, it never changes of value
● An ExecutionContext
○ executes tasks submitted to them.
○ They can be seen as thread pools.
○ Most of future ops require an implicit
ExecutionContext.

14. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Future: Expecting results
● Expecting results.
○ Blocker way (discouraged but sometimes
mandatory).
○ Non-blocker way: using callbacks

15. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Future: Blocking - Await
● Blocking: Await.result / Await.ready
import scala.concurrent._
import scala.concurrent.duration._
import scala.concurrent.ExecutionContext.Implicits.global
val f: Future[Int] = Future{
Thread.sleep(10000)
2
}
println(Await.result(f,12.seconds))
//2

16. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Future: Blocking - Await
● Blocking: Await (Problem: Not enough time)
import scala.concurrent._
import scala.concurrent.duration._
import scala.concurrent.ExecutionContext.Implicits.global
val f: Future[Int] = Future{
Thread.sleep(10000)
2
}
println(Await.result(f,5.seconds))
java.util.concurrent.TimeoutException: Futures timed out after
[5 seconds]

17. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Future: Non-blocking - Callbacks
● Non-Blocking: callbacks
import scala.concurrent._
import scala.concurrent.ExecutionContext.Implicits.global
val f: Future[Int] = Future{
Thread.sleep(10000)
2
}
f.onComplete( n => println(n) )
//at some point, “Success(2)” will appear
Non-blocking

18. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Future: Callbacks
● Callbacks will be executed asynchronously when future
is completed
● type Callback = Try[T] => Unit
● Try[T] ~ Either[Throwable,T]
○ Left(throwable) ~ Failure(throwable)
○ Right(t) ~ Success(t)

19. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Future: Callbacks
● onComplete
f.onComplete( (t: Try[Int]) => println(t) )
//Success(2)
● onSuccess
f.onSuccess( n => println(n) )
//2
● onFailure
f.onFailure( throwable => println(throwable.getMessage) )
//it will never print an error (because it equals Success(2))

20. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Future: Operations
● map
def getFirstMillionOfPrimes(): Future[List[Int]] = ???
getFirstMillionOfPrimes().map(
(list: List[Int]) => list.head)
res0: Future[Int]

21. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Future: Operations
● flatMap
def getFirstMillionOfPrimes(): Future[List[Int]] = ???
def concatenate(l: List[Int]): Future[String] = ???
getFirstMillionOfPrimes().flatMap(
(list: List[Int]) => concatenate(list))
res0: Future[String]

22. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Future: Monad behavior
● Composition
○ Future is like a monad
■ Point Function: Future.apply
■ Bind Function: flatMap
■ Some math properties and so...
○ Since Future has map,flatMap,filter methods;
it can be composed easily in a for-comprehension

23. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid

24. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Future: Composition
● Problem
var result: String = “”
val f1: Future[Unit] = Future{
result += “Hi ”
}
val f2: Future[Unit] = Future{
result += “ everyone”
}
● result value? “Hi everyone”? “ everyoneHi”?

25. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid

26. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Future: Composition
● for-comprehension is your friend
for {
primes <- getFirstMillionPrimes()
primesString <- concatenate(primes)
} yield primesString
res0: Future[String]
Future[List[Int]]
Future[String]

27. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Future: Recovering from failure
● recover
val f: Future[Int] = Future{
1 / 0
}.recover{
case e: ArithmeticException => 0
}

28. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Future: Recovering from failure
● recoverWith
val f: Future[Int] = Future{
1 / 0
}.recoverWith{
case e: ArithmeticException => Future(0)
}

29. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Future: Recovering from failure
● fallbackTo
val f1: Future[Int] = Future{
1 / 0
}
val f2: Future[Int] = Future(0)
val f = f1 fallbackTo f2

30. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Promise: Overview
● Futures can be created by
○ Future.apply
○ Promises
● You can think about it like
○ Future ~ Read future value
○ Promise ~ Write future value
● Promises are single-assigned (just once. Immutable as
futures)

31. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Promise: Baggage & claim
Menu
order
Menu
Being
Cooked
Ticket
Menu ready

32. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Promise: Baggage & claim
Begin
completion
Future[T]
p.success
or
p.failure
Try[T]
Promise[T]

33. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Promise: example
val r: T = produceSomething()
p success r
doSomethingElse()
startDoingSomething()
f onSuccess {
case r: T => handleResult()
}
val p = promise[T]
val f = p.future
Producer Consumer

34. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Promise: completion
● complete
val p = promise[Int]
p.complete(Try(2))
● completeWith
val p = promise[Int]
p.completeWith(Future(2))

35. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Future alternatives
● Scalaz futures
● Scala’s async library

36. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Tones of slides and however...
Futures/Promises
Akka actors

37. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid

38. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Akka: main concepts
● Actors, actors everywhere
● Asynchronous messages
● Event-driven processes
● Distributed systems

39. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Akka: Actors
● Black box
● State without race condition
● Messages to update the state

40. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Akka: Actors
class MyFirstActor extends Actor {
override def receive = {
case _ => println("Hello World!")
}
}

41. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Akka: Send a message
● Tell message
actor ! "Hi"
● Ask message
actor ? "Are you there"

42. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Akka: Ask pattern
implicit val timeout = Timeout(5 seconds)
val f: Future[String] =
(actorRef ? AreYouThere).mapTo[String]

43. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Akka: An example

44. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Akka: Services user
service
contact
service
address
service
System
GetUser(id)
GetContacts(name)
GetAddress(name)
Option[User]
List[Contact]
Address
UserInfo

45. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Akka: Ask Pattern & For
Comprehension
class UserServiceActor extends Actor {
val users: Map[String, User] =
Map("1" -> User("John"), "2" -> User("Jack"))
override def receive = {
case GetUser(userId: String) =>
sender ! users.get(userId)
}
}

46. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Akka: Ask Pattern & For
Comprehension
def getInfo(userId: String): Future[UserInfo] =
for {
Some(user) <- (usersActor ? GetUser(userId))
.mapTo[Option[User]]
contacts <- (contactsActor.?(GetContacts(user.name))
(Timeout(5 seconds),context.self))
.mapTo[List[Contact]]
address <- (addressesActor ? GetAddress(user.name))
.mapTo[Address]
} yield UserInfo(user, contacts, address)

47. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid

48. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
scalera.es
@scalerablog

49. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid

50. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Primeros pasos con Akka:
Olvídate de los threads
27 NOVEMBER
TRACK D
18:00

51. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Caminando de Java a Scala en
menos de 2 horas
28 NOVEMBER
TRACK B
9:30

52. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid

53. MADRID · NOV 27-28 · 2015
Scala Programming @ Madrid
Akka voló sobre el nido del Future
Javier Santos
David Vallejo