Async programming allows non-blocking execution by invoking logic asynchronously when resources become available. This avoids blocking threads and improves server performance. Modern frameworks support async programming with futures/promises and actors. Futures allow composing asynchronous operations through transformations like mapping. Actors encapsulate state and process messages asynchronously without blocking threads. The example shows implementing an async controller in Play Framework that uses a future to call an external API and an actor to generate random numbers.

1. Async
React
instead of waiting for better times
Johan Andrén
johan.andren@mejsla.se
@apnylle

2. In the olden days...
... there was a way to write
web applications
that bound 1 thread per request
oh, wait!
let me fix that for
me

3. Most modern web
frameworks
do bind 1 thread per request

4. do bind 1 thread per request
t1
Request
Thread
pool
t1
ReSponse

5. What does that thread spend
most of its time doing?
Hint:

6. Wait
Request
YOur logic
Blocked
yOur logic
ReSponse
WS

7. Why is this a problem?
• threadpool depletion
• server choked with 5% cpu usage
• unrelated requests affected

8. Async!
nodejs
All kinds
of Cool Shit
you never heard
about

9. WUT!?
Let someone invoke our logic
when a resource is available
What is a resource?
• network
• disk
• internal state

10. Don´t call us
We´ll call you
Our logic
WS
Async
HTTP-client
Our logic

11. What do we need?
• callback based clients/drivers/libs
• framework support
but doesn’t this lead to...

12. ”Callback Hell”?
1 GMaps.geocode({!
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address: fromAddress,!
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callback: function( results, status ) {!
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if ( status == "OK" ) {!
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fromLatLng = results[0].geometry.location;!
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GMaps.geocode({!
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address: toAddress,!
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callback: function( results, status ) {!
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if ( status == "OK" ) {!
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toLatLng = results[0].geometry.location;!
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map.getRoutes({!
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origin: [ fromLatLng.lat(), fromLatLng.lng() ],!
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destination: [ toLatLng.lat(), toLatLng.lng() ],!
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travelMode: "driving",!
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unitSystem: "imperial",!
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callback: function( e ){!
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console.log("ANNNND FINALLY here's the directions..
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// do something with e!
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}!
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});!
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}!
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}!
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});!
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}!

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}!
26 });
Not with better abstractions!
• futures / promises
• actors

14. Future[T]
Empty
Completed
(
Failed )
t

15. Composition
Future[A]
Future[B]
Map
When A arrives
Apply this A
B transformation

16. Play action
1 package controllers!
2 !
3 import play.api.mvc._!
4 !
5 object LuckyNumberController extends Controller {!
6 !
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def giveMeLuckyNumber = Action {!
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!
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Ok(views.html.luckyNumber(42))!
10!
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}!
12 !
13 }

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package controllers!
!
import scala.concurrent.Future!
import play.api.libs.ws._!
import play.api.libs.concurrent.Execution.Implicits.defaultContext!
import play.api.mvc._!
!
object LuckyNumberController extends Controller {!
!
def giveMeLuckyNumber = Action {!
!
val fResponse: Future[Response] = !
WS.url("http://lucky-number.api/lucky-number").get!
!
!
!
!
!
!
!
!
}!
!
}
Will start execute here

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package controllers!
!
import scala.concurrent.Future!
import play.api.libs.ws._!
import play.api.libs.concurrent.Execution.Implicits.defaultContext!
import play.api.mvc._!
!
object LuckyNumberController extends Controller {!
!
def giveMeLuckyNumber = Action {!
!
val fResponse: Future[Response] = !
WS.url("http://lucky-number.api/lucky-number").get!
!
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!
!
!
!
!
!
}!
!
}
THe future response

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package controllers!
!
import scala.concurrent.Future!
import play.api.libs.ws._!
import play.api.libs.concurrent.Execution.Implicits.defaultContext!
import play.api.mvc._!
!
object LuckyNumberController extends Controller {!
!
def giveMeLuckyNumber = Action {!
!
val fResponse: Future[Response] = WS.url("http://lucky-number.api/lucky-number").get!
!
val fNumber: Future[Int] = fResponse.map { response: Response =>!
Integer.parseInt(response.body)!
}!
!
!
!
!
}!
!
}
When it arrives: do this
Response
Int Transformation

20. Future[T]
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package controllers!
!
import scala.concurrent.Future!
import play.api.libs.ws._!
import play.api.libs.concurrent.Execution.Implicits.defaultContext!
import play.api.mvc._!
!
object LuckyNumberController extends Controller {!
!
def giveMeLuckyNumber = Action {!
!
val fResponse: Future[Response] = WS.url("http://lucky-number.api/lucky-number").get!
!
val fNumber: Future[Int] = fResponse.map { response: Response =>!
Integer.parseInt(response.body)!
}!
!
!
!
!
}!
!
}
(On this Execution context)

21. Future[T]
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package controllers!
!
import scala.concurrent.Future!
import play.api.libs.ws._!
import play.api.libs.concurrent.Execution.Implicits.defaultContext!
import play.api.mvc._!
!
object LuckyNumberController extends Controller {!
!
def giveMeLuckyNumber = Action {!
!
val fResponse: Future[Response] = WS.url("http://lucky-number.api/lucky-number").get!
!
val fNumber: Future[Int] = fResponse.map { response: Response =>!
Integer.parseInt(response.body)!
}!
!
fNumber.map { number =>!
Ok(views.html.luckyNumber(number))!
}!
}!
!
}
When it arrives: do this
Transformation

22. Future[T]
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package controllers!
!
import scala.concurrent.Future!
import play.api.libs.ws._!
import play.api.libs.concurrent.Execution.Implicits.defaultContext!
import play.api.mvc._!
!
object LuckyNumberController extends Controller {!
!
def giveMeLuckyNumber = Action.async {!
!
val fResponse: Future[Response] = WS.url("http://lucky-number.api/lucky-number").get!
!
val fNumber: Future[Int] = fResponse.map { response: Response =>!
Integer.parseInt(response.body)!
}!
!
fNumber.map { number =>!
Ok(views.html.luckyNumber(number))!
}!
}!
!
}
Play allows us to return a
Future[Result]

23. If there was an exception
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Future[T]
package controllers!
!
import scala.concurrent.Future!
import play.api.libs.ws._!
import play.api.libs.concurrent.Execution.Implicits.defaultContext!
import play.api.mvc._!
!
object LuckyNumberController extends Controller {!
!
def giveMeLuckyNumber = Action.async {!
!
val fResponse: Future[Response] = WS.url("http://lucky-number.api/lucky-number").get!
!
val fNumber: Future[Int] = fResponse.map { response: Response =>!
Integer.parseInt(response.body)!
}!
!
fNumber.map { number =>!
Ok(views.html.luckyNumber(number))!
}!
}!
!
}

24. Future[T]
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package controllers!
!
import play.api.libs.ws._!
import play.api.libs.concurrent.Execution.Implicits.defaultContext!
import play.api.mvc._!
!
object LuckyNumberController extends Controller {!
!
def giveMeLuckyNumber = Action.async {!
for {!
response <- WS.url("http://lucky-number.api/lucky-number").get!
} yield {!
val number = Integer.parseInt(response.body)!
Ok(views.html.luckyNumber(number))!
}!
}!
!
}

25. Actors
State
Inbox
Behaviour

26. • no work - no thread
• react - no waiting
• can have private state

27. Actor
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package controllers!
!
import akka.actor.Actor!
!
case object GiveMeNumber!
case class HereYaGo(number: Int)!
!
class LuckyNumberGenerator extends Actor {!
!
var nextNumber = 42!
!
def receive = {!
case GiveMeNumber =>!
sender ! HereYaGo(nextNumber)!
nextNumber += 1!
!
}!
}

28. Actor
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package controllers!
!
import akka.actor.Actor!
!
case object GiveMeNumber!
case class HereYaGo(number: Int)!
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class LuckyNumberGenerator extends Actor {!
!
var nextNumber = 42!
!
def receive = {!
case GiveMeNumber =>!
sender ! HereYaGo(nextNumber)!
nextNumber += 1!
!
}!
}
state (mutable)

29. Actor
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package controllers!
!
import akka.actor.Actor!
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case object GiveMeNumber!
case class HereYaGo(number: Int)!
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class LuckyNumberGenerator extends Actor {!
!
var nextNumber = 42!
!
def receive = {!
case GiveMeNumber =>!
sender ! HereYaGo(nextNumber)!
nextNumber += 1!
!
}!
}
for each message

30. Controller (calling actor)
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package controllers!
!
import akka.actor._!
import akka.pattern.ask!
import play.api.libs.concurrent.Execution.Implicits.defaultContext!
import play.api.mvc._!
!
class LuckyNumberController(numberGenerator: ActorRef) extends Controller {!
!
def giveMeLuckyNumber = Action.async {!
for {!
response <- (numberGenerator ? GiveMeNumber).mapTo[HereYaGo]!
} yield {!
Ok(views.html.luckyNumber(response.number))!
}!
}!
!
}
Di (someone gave it to us)

31. Controller (calling actor)
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package controllers!
!
import akka.actor._!
import akka.pattern.ask!
import play.api.libs.concurrent.Execution.Implicits.defaultContext!
import play.api.mvc._!
!
class LuckyNumberController(numberGenerator: ActorRef) extends Controller {!
!
def giveMeLuckyNumber = Action.async {!
for {!
response <- (numberGenerator ? GiveMeNumber).mapTo[HereYaGo]!
} yield {!
Ok(views.html.luckyNumber(response.number))!
}!
}!
!
}
Sends message, gives us
Future[Any] (reply)

32. Controller (calling actor)
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package controllers!
!
import akka.actor._!
import akka.pattern.ask!
import play.api.libs.concurrent.Execution.Implicits.defaultContext!
import play.api.mvc._!
!
class LuckyNumberController(numberGenerator: ActorRef) extends Controller {!
!
def giveMeLuckyNumber = Action.async {!
for {!
response <- (numberGenerator ? GiveMeNumber).mapTo[HereYaGo]!
} yield {!
Ok(views.html.luckyNumber(response.number))!
}!
}!
!
}
We know what we will get back

33. Controller (calling actor)
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package controllers!
!
import akka.actor._!
import akka.pattern.ask!
import play.api.libs.concurrent.Execution.Implicits.defaultContext!
import play.api.mvc._!
!
class LuckyNumberController(numberGenerator: ActorRef) extends Controller {!
!
def giveMeLuckyNumber = Action.async {!
for {!
response <- (numberGenerator ? GiveMeNumber).mapTo[HereYaGo]!
} yield {!
Ok(views.html.luckyNumber(response.number))!
}!
}!
!
}
Make a webpage out of that

34. What to look out for
• really heavy computations
• blocking by miss-take
• jdbc :(

35. I wanna try it!
www.playframework.com
typesafe.com

36. Qs?
K Thx Bye!
Johan Andrén
johan.andren@mejsla.se
@apnylle