This document introduces Akka, an open-source toolkit for building distributed, concurrent, and resilient message-driven applications for Java and Scala. It discusses how application requirements have changed to require clustering, concurrency, elasticity, and resilience. Akka uses an actor model with message-driven actors that can be distributed and made fault-tolerant. The document provides examples of creating and communicating between actors using messages, managing failures with supervision, and load balancing with routers.

1. Akka
Rahul Shukla
rahul.shukla@synerzip.com

2. Why ?

3. Application requirements have changed

4. New requirements

5. Apps in the 60s-90s were written for Apps today are written for
Single machines Clusters of machines
Single core processors Multicore processors
Expensive RAM Cheap RAM
Expensive disk Cheap disk
Slow networks Fast networks
Few concurrent users Lots of concurrent users
Small data sets Large data sets
Latency in seconds Latency in milliseconds

6. As a matter of necessity,businesses are going
Reactive

8. Responsive
● Real-time, engaging, rich and collaborative
○ Create an open and ongoing dialog with users
○ More efficient workflow; inspires a feeling of connectedness
○ Fully Reactive enabling push instead of pull

9. Elastic
● Scalability and elasticity to embrace the Cloud
○ Leverage all cores via asynchronous programming
○ Clustered servers support joining and leaving of nodes
○ More cost-efficient utilization of hardware

10. Resilient
● Failure is embraced as a natural state in the app lifecycle
○ Resilience is a first-class construct
○ Failure is detected, isolated, and managed
○ Applications self heal

11. Message-Driven
● Loosely coupled architecture, easier to extend, maintain, evolve
○ Asynchronous and non-blocking
○ Concurrent by design, immutable state
○ Lower latency and higher throughput

12. What?
Introducing Akka

13. The name comes from the goddess in the Sami
(native swedes) mythology that represented all
the wisdom and beauty in the world.
It is also the name of a beautiful mountain in
Laponia in the north part of Sweden

14. ● Message - Driven
● Concurrency
● Scalability (Elastic)
● Fault Tolerance (Resilient)
Vision

15. Manage System Overload

16. Scale Up & Scale Out

17. Program at higher level

18. ● Never think in terms of shared state, state visibility, threads,
locks, concurrent collections, thread notifications etc.
● Low level concurrency plumbing BECOMES SIMPLE
WORKFLOW - you only think about how messages flow in
the system
● You get high CPU utilization, low latency, high throughput
and scalability - FOR FREE as part of the model
● Proven and superior model for detecting and recovering
from errors

19. Distributable by Design

20. ● Actors are location transparent & distributable by design
● Scale UP and OUT for free as part of the model
● You get the PERFECT FABRIC for the CLOUD
○ elastic & dynamic
○ fault-tolerant & self-healing
○ adaptive load-balancing, cluster rebalancing & actor
migration
○ build extremely loosely coupled and dynamic systems
that can change and adapt at runtime

21. How
Can we achieve this ?

22. Actor
What is an Actor ?

23. ● Akka's unit of code organization is called an Actor
● Actors helps you create concurrent, scalable and fault-
tolerant applications
● Like Java EE servlets and session beans, Actors is a model
for organizing your code that keeps many “policy
decisions” separate from the business logic
● Actors may be new to many in the Java community, but
they are a tried-and-true concept (Hewitt 1973) used for
many years in telecom systems with 9 nines uptime

24. What can I use Actors for ?

25. ● In different scenarios, an Actor may be an alternative to: -
○ a thread
○ an object instance or component
○ a callback or listener
○ a singleton or service
○ a router, load-balancer or pool
○ a Java EE Session Bean or Message-Driven Bean

26. Carl Hewitt’s definition

27. - The fundamental unit of computation that embodies:
- Processing
- Storage
- Communication
- 3 axioms - When an Actor receives a message it can:
- Create new Actors
- Send messages to Actors it knows
- Designate how it should handle the next message it
receives

28. Let’s Code

29. Define an actor
Scala : import akka.actor._
import bank.Bank.Balance
object Bank {
case class Balance(number: String)
def props(name: String): Props = { return Props(Bank(name)) }
}
case class Bank(name: String) extends AbstractLoggingActor {
def balance(number: String) = { //Do Something }
override def receive = {
case Balance(number) => balance(number)
}
}

30. Define an actor
Java : public class Bank extends AbstractLoggingActor {
public static Props props(String name) {
return Props.create(Bank.class, () -> new Bank(name));
}
@Override
public PartialFunction<Object, BoxedUnit> receive() {
return ReceiveBuilder
.match(Balance.class, b -> balance(b.number))
.build();}
private void balance(String number) {
//DO Something
}
}

31. Create an actor
• CREATE - creates a new instance of an Actor
• Extremely lightweight (2.7 Million per Gb RAM)
• Very strong encapsulation - encapsulates:
- state
- behavior
- message queue
• State & behavior is indistinguishable from each other
• Only way to observe state is by sending an actor a
message and see how it reacts

32. Create an actor
Scala : val system = ActorSystem("Bank")
val bank = system.actorOf(Bank.props("Central Bank"),”CBNK”)
Java :
ActorSystem system = ActorSystem.create("Bank");
ActorRef bank = system.actorOf(Bank.props("Central Bank"),”CBNK”);

33. Actors hierarchies
Bank
User System
/Bank
CBNK /Bank/user/CBNK
/Bank/system/Bank/user

34. SEND
• SEND - sends a message to an Actor
• Asynchronous and Non-blocking - Fire-forget
• Everything happens Reactively
- An Actor is passive until a message is sent to it,which
triggers something within the Actor
- Messages is the Kinetic Energy in an Actor system
- Actors can have lots of buffered Potential Energy but
can't do anything with it until it is triggered by a
message
• EVERYTHING is asynchronous and lockless

35. SEND
Scala : bank ! Bank.Balance(“12345”)
Java : bank.tell(new Bank.Balance(“12345”), ActorRef.noSender());

36. SEND

37. Anatomy of an Actor
● Each actor is represented by an ActorRef
● Each actor is has a mailbox and a dispatcher
● Only one message at a time is passed to the actor

38. REPLY
Scala : override def receive = {
case Balance(number) => sender ! "Message"
}
Java :
@Override
public PartialFunction<Object, BoxedUnit> receive() {
return ReceiveBuilder
.match(Balance.class, b -> sender().tell(“Message”,self()))
.build();}

39. SUPERVISE (Fault Tolerance)
SUPERVISE - manage another Actor’s failures
● Error handling in actors is handle by letting Actors
monitor (supervise) each other for failure
● This means that if an Actor crashes, a notification will
be sent to his supervisor, who can react upon the
failure
● This provides clean separation of processing and
error handling

40. Supervise
Scala : override def supervisorStrategy = OneForOneStrategy(maxNrOfRetries = 10,
withinTimeRange = 1 minute) {
case _: Account.OverdrawException => Resume
case _: Exception => Restart
}
Java :
@Override
public SupervisorStrategy supervisorStrategy() {
return new OneForOneStrategy(DeciderBuilder
.match(Account.OverdrawException.class, c -> SupervisorStrategy.
resume())
.match(Exception.class, c -> SupervisorStrategy.escalate())
.build());
}

41. Manage Failure
Scala : @throws[Exception](classOf[Exception])
override def preStart(): Unit = ???
@throws[Exception](classOf[Exception])
override def postRestart(reason: Throwable): Unit = ???
@throws[Exception](classOf[Exception])
override def postStop(): Unit = ???

42. Manage Failure
Java : @Override
public void preRestart(Throwable reason, Option<Object> message) throws
Exception {
// Handle here
}
@Override
public void postRestart(Throwable reason) throws Exception {
// Handle here
}
@Override
public void postStop() throws Exception {
// Handle here
}

43. Supervisor Strategies
● Akka ships with two highly configurable supervisor
strategies:
○ OneForOneStrategy: Only the faulty child is
affected when it fails
○ AllForOneStrategy: All children are affected when
one child fails

44. Supervisor Strategy Directives
● Resume: Simply resume message processing
● Restart:
○ Transparently replace affected actor(s) with new
instance(s)
○ Then resume message processing
● Stop: Stop affected actor(s)
● Escalate: Delegate the decision to the supervisor's
parent

45. Load Balancing (Elastic)

46. Concurrency vs. Parallelism
● Two or more tasks are concurrent, if the order in which
they get executed in time is not predetermined
● In other words, concurrency introduces non-determinism
● Concurrent tasks may or may not get executed in parallel
● Concurrent programming is primarily concerned with the
complexity that arises due to non-deterministic control
flow
● Parallel programming aims at improving throughput and
making control flow deterministic

48. Routers
Scala : context.actorOf(RoundRobinPool(5).props(SomeActor.props()),"some-actor")
Java :
getContext().actorOf(new RoundRobinPool(5).props(SomeActor.props()),"
some-actor");

49. Routing Strategies provided by Akka
● RandomRoutingLogic
● RoundRobinRoutingLogic
● SmallestMailboxRoutingLogic
● ConsistentHashingRoutingLogic
● BroadcastRoutingLogic
● ScatterGatherFirstCompletedRoutingLogic
● To write your own routing strategy, extend RoutingLogic:
● Attention: The implementation must be thread-safe!

50. Thanks