Akka and futures

Akka Actors And Futures

Meetu Maltiar
Principal Consultant
Email: meetu@knoldus.com
Twitter:@meetumaltiar

Akka 2.0
Akka name comes from Sami mythology is actually
name of a goddess of wisdom and beauty.

Akka incidentally means sister in Telugu!!

The Problem
It is way too hard to build
=> correct highly concurrent systems
=> truly scalable systems
=> self-healing, fault-tolerant systems

What is Akka?
Right abstraction with actors for concurrent, fault-tolerant
and scalable applications

For Fault-Tolerance uses “let it crash” model

Abstraction for transparent distribution for load

Introducing Actors
Actor is an entity encapsulating behavior, state and a
mailbox to receive messages

For a message received by Actor a thread is allocated to it

Then Actors behavior is applied to the message and
potentially some state is changed or messages is passed to
other Actors

Introducing Actors..
There is elasticity between message processing and addition
of new messages. New messages can be added while actor
execution is happening.

When processing of messages is completed thread is
deallocated from the actor. It can be reallocated a thread at a
later time

Create Application
import akka.actor.ActorSystem

val system =
ActorSystem("firstApp")

My First Actor
import akka.actor.{ Actor, Props }

class MyFirstActor extends Actor {
def receive = {
case msg => println("Hello!!")
}
}

Create Actors
import akka.actor.{ ActorSystem, Props }

val system = ActorSystem("firstApp")
val myFirstActor =
system.actorOf(Props[MyFirstActor])

MyFirstActor is an ActorRef
Create a top level actor

Stop Actors
system stop myFirstActor

Also stops all actors in hierarchy

Send: !
myFirstActor ! “Hello”

fire-forget

Ask: ?
import akka.pattern.ask

implicit val timeout = Timeout(50000 milliseconds)

val future = myActor ? "hello"

Await.result(future, timeout.duration).asInstanceOf[Int]

Returns a Future[Any]

Reply
import akka.actor.Actor

class LongWorkingActor extends Actor {
def receive = {
case number: Int =>
sender ! (“Hi I received ” + number)
}
}

Routers
RoundRobin
Random
SmallestMailBox
BroadCast
ScatterGatherFirstCompleted

Routers...

val router =
system.actorOf(
Props[RouterWorkerActor].
withRouter(RoundRobinRouter(nrOfInstances = 5)))

Actor Path
val actorRef =
system.actorFor("akka://actorPathApp/user/pa
rent/child")

val parent = context.actorFor("..")

val sibling = context.actorFor("../sibling")

val refPath = actorRef.path

Akka Futures
A Future is a data structure

Used to retrieve of some concurrent
operation

This operation is performed by an Actor or a
dispatcher directly

The result can be accessed synchronously or
asynchronously

Execution Context
Futures need ExecutionContext to execute
callback and operations

If we have ActorSystem in scope Future will
use default dispatcher as ExecutionContext

We can use factory methods provided by
ExecutionContext companion object to wrap
Executors and ExecutorServices

Use With Actors
There are two ways to get a reply from an
Actor. First one is (myActor ! Msg)

The second way is through a Future. Using
Actors “?” method will return a Future

The simplest way to use Await method call,
though not recommended as the thread blocks
till result is obtained.

Future With Akka
andAwait
import akka.actor._
import akka.pattern.ask
import akka.util.duration._
import akka.util.Timeout
import akka.dispatch.Await

object FutureWithAwaitApp extends App {
implicit val timeout = Timeout(50000 milliseconds)
val system = ActorSystem("future")
val echoActor = system.actorOf(Props[EchoActor])
val future = echoActor ? "Hello World"
val result = Await.result(future,
timeout.duration).asInstanceOf[String]
println(result)
}

Use Futures Directly
import akka.dispatch._
import akka.util.duration._
import akka.actor.ActorSystem

object MonadicFutureApplication extends App {
implicit val system = ActorSystem("future")

val f1 = Future { "Hello" + "World" }
val f2 = f1 map { x => x.length }

val result = Await.result(f2, 1 second)
println(result)
}

Composing Futures
object MultiMonadicFutureApplication extends App {
implicit val system = ActorSystem("future")

val f1 = Future { "Hello" + "World" }
val f2 = Future { 3 }

val f3 = f1 flatMap { x =>
f2 map { y =>
x.length * y
}
}

val result = Await.result(f3, 1 second)
}

Code Samples

https://github.com/meetumaltiar/AkkaKnolX

References

Viktor Klang talk on Akka 2.0 at NE Scala symposium

Akka website akka.io

Akka and futures

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