The document discusses implementing domain events with Akka. It introduces domain events and their benefits, such as loose coupling and scalability. It then provides an overview of Akka and how it can be used to build reactive applications. The presentation demonstrates how to model domain events and implement event handling using Akka actors and event buses. Finally, it briefly discusses reactive application principles and examples of systems using this approach.

1. Implementing Domain Event with
Akka
Septeni Tech Talk - Jan 2016
TungNT @ Septeni Technology

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Agenda
● Introduction to Domain Event
● Introduction to Akka
● Implementation
● Brief introduction to Reactive application

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Domain Event

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What is Domain Event?
“Captures the memory of something interesting
which affects the domain”
- Martin Fowler -

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Monolith problems
● Thread-based
● Tight coupling
~> hard to scale, especially rich domain

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“If you can't split it, you can't scale it”

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Event-driven

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“It really became clear to me in the last couple of
years that we need a new building block and that
is the Domain Event.”
- Eric Evans -

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Event modeling
How can we determine if something that
happens in the domain is important to the
Domain Experts?

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● When domain experts talk:
– “When...”
– “If that happens...”
– “Inform me if...” and “Notify me if...”
– “An occurrence of...”

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Event modeling
How can we determine if something that
happens in the domain is important to the
Domain Experts?

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Event name modeling
● Name and their properties depend on Ubiquitous
Language in Bounded Context
● Event usually derived from the command that was
executed:
Domain Event name = Aggregate name + Command name + past tense
– Ex:
● UserRegistered
● OrderCompleted
● ProjectStarted

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Event properties
● Immutable
– No shared state
● Ansynchronous
● Has no action/behaviors
– just a event pushed to the application

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Benefit
● High performance
● Distributed
● Maintainable
● Durable
● Loose coupling
– add new feature without changes to
existing code
– easy to scale ~> scaling up or down as
needed.

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Introduction
Akka is a toolkit and runtime for building highly
concurrent, distributed, resilient, message-
driven applications on the JVM
- Akka's homepage -

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Akka highlight
● Tool to build reactive application
● Concurrent, distribution by design
● High performance
– 50 million msg/sec on a single machine
– ~2.5 million actors per GB of heap.
● Elastic & Decentralized
– Load balancing
– Routing
– Partitioning and sharding.
● Written by Scala

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Implementation

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Infra layer diagram

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Event Abstraction
Event interface
Event subcriber interface

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Message Broker Provider Factory
Factory to get multi provider

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Akka EventBus
Singleton object to broadcast event and register
event handler

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Akka Actor
Foward message to message consummer

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Domain layer diagram

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Domain Event Bus
Get Akka message provider

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Define event
Define an domain event inteface
Event class
Event Subscriber abstract class

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Event handler
Event consummer with handler
Register subcriber to event bus

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Sample Structure

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Implementation's Drawback
● Consistency
● Deadletters processor
● Duplicate events in bus
● Event versioning
● Distributed transaction
● ...
But easily resolved by:
● Event Sourced
– Akka persistence
● CQRS Architecture
● Akka Cluster, DeadLetter libs
● Apache Kafka – written by Scala, too :)

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Reactive Application

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Manifesto
● Reponsive – react to users
● Resillent – react to failure
● Elastic – react to load
● Message-driven – react to
events

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Twitter during the WorldCup 2010

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● 143,199 New Tweets per second (TPS).
● Typical day: more than 500 million Tweets sent
● Average 5,700 TPS.

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10x Performance
● Monolith (Ruby on Rails): 200 – 300 req / sec / host
● Reactive (Scala): 10 - 20K req / sec / host.

37. Few more examples
https://www.typesafe.com/resources/case-studies-and-stories

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Resources
● Implementing Domain-Driven Design –
Vaughn Vernon
http://www.amazon.com/Implementing-Domain-Driven-Design-Vaughn-Vernon/dp/03218345

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Resources(2)
● Reactive Messaging Patterns with the
Actor Model: Applications and Integration
in Scala and Akka – Vaughn Vernon
http://www.amazon.com/Reactive-Messaging-Patterns-Actor-Model/dp/0133846830

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References
● Akka: http://akka.io/
● Reactive Manifesto: http://www.reactivemanifesto.org/
● Implementation:
https://github.com/NgTung/domain_event_sample.git
● Others:
– Twitter:
https://blog.twitter.com/2013/new-tweets-per-second-record-and-
– Dwango Niconico:
http://www.typesafe.com/resources/case-studies-and-stories/how

41. Thank you!