In this you will learn about Akka – What is Akka, some design principles behind it, and why was it developed. So let’s begin.

1. Presented By: Purva Agrawal
Introduction To
AKKA

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3. Our Agenda
01 Introduction-What is Akka
02 Akka Actors And Creating
Actors
03 Actor System and Life
Cycle of an Actor
04 Testing and Fault
Tolerance
05 Demo

4. Introduction to AKKA
Akka is a toolkit and runtime for building highly concurrent, distributed, and fault-tolerant message-driven
applications on the JVM (Java Virtual Machine).
Akka is written in Scala. It implements the Actor Based Model.
AKKA is reactive,i.e,it follows the reactive architecture.
What is Akka?
Why named Akka?
● Name of a beautiful Swedish mountain up in the northern part of Sweden called Laponia.
● Name of a goddess in the Sami mythology. She is the goddess that stands for all the beauty and good in the
world.

5. Reactive Architecture And its Principles

6. AKKA ACTORS
● Actors were invented in 1973 by Carl Hewitt. Akka is an implementation of the Actor Model.
● An actor is an entity which communicates to other actor by message passing. Actor
has it's own state and behavior. As in object-oriented programming everything is an
object same like everything is an actor in actor-based system.
In other words, we can say that an actor is an object that encapsulates state and
behavior.
Creating Actors
We can create actor by extending Actor trait and implementing it's receive method.
This method is called each time a message is received by the actor.
The receive method does pattern matching on the received message and decide what
to do.

7. Actor System
The ActorSystem is a root actor in actors structure. An ActorSystem is a hierarchical group of
actors which share common configuration, e.g. dispatchers, deployments, remote capabilities and
addresses. It is also the entry point for creating or looking up actors.
ActorSystem provides an actorOf() method which is used to create actor instance.

8. Actor Life Cycle
So, here is a glimpse of how an Akka Actor Lifecycle works. The actor lifecycle begins as soon as
the actor is created.
The stages of an Akka Actor Lifecycle are as follows –
● preStart()
● Start
● Stop
● postStop()
● terminated

9. Anatomy of an Actor
Each Actor is represented by an ActorRef. So, we never get access to an Actor instance directly.
The Actor reference lets us send messages to the Actor.
Each Actor has a mailbox and a dispatcher. So, the ActorRef enqueues incoming messages in
the mailbox. After that, the dispatcher schedules message processing.
Only one message at a time is passed to an Actor.

10. Creating Child Actors
Akka provides facility to create child actor. You can create child actor by using implicit context
reference. ActorSystem is used to create root-level or top-level actor. Akka provides you
context so that you can create child actor also.
var childActor = context.actorOf(Props[Child],"Child");
Actor Communication
In Akka, actors communicate with each other by sending and receiving messages. The
messages can be sent in many patterns like
● Fire and Forget
● Request and Response
Akka provides two predefined methods tell() and ask() for message exchange.

11. Replying to Messages
You can reply to a message by using sender() method. This method gives you ActorRef which is
a instance of sender Actor. You can also store this reference. Replying to a message is simple,
you just need to use sender() ! ReplyMsg.
If there is no sender i.e. a message which is sent without an actor or future context, the default
sender is a 'dead-letter' actor reference.
What is Dead Letter ?
● Messages which cannot be delivered will be delivered to an actor called deadLetters.
● Implement this by importing akka.actor.DeadLetter package.

12. Scheduling Messages in Akka
● Often, a need exists to schedule the sending of a message to an actor:
○ a single time after some delay (send a message for once after a period of time and then we never send that
message)
○ periodically with a given interval (e.g after every 5sec, 5min, etc.)
● Two Akka facilities exist to achieve this:
○ The Akka Scheduler service
○ The akka.actors.Timers trait
Testing Actors
In Akka generally we have two kinds of tests, synchronous and asynchronous.
● ‘Unit tests‘ are synchronous, you directly test the receive method without requiring an actor system.
● ‘Integration tests‘ are asynchronous, and generally test more than one actor working together.
Akka comes with a dedicated module akka-testkit for supporting tests. To use Actor Testkit in our project we need to
add the following dependency,
val AkkaVersion = "2.6.10"
libraryDependencies += "com.typesafe.akka" %% "akka-testkit" % AkkaVersion % Test

13. Akka Actor LifeCycle with Failure

14. Supervising Actors in Akka
In case of failure, rather than forcing it back on the caller(customer), we prefer to handle it internally. Within
Akka, it is done using a technique called Supervision.
The parent decides from the below directives, depending on the nature of failure :
● Resume: Simply resume message processing.
● Restart: Transparently replace affected actor(s) with new instance(s) and then resume message
processing.
● Stop: Stop affected Actor(s) permanently.
● Escalate: Escalate the failure by failing itself and propagate failure to its parent.
Supervision Strategy
There are two type of supervision strategies to supervise any actor:
1. One-For-One – Only the faulty child is affected when it fails.
2. One-For-All – All children are affected when one child fails.

15. DEMO

16. Thank You !