Implementing Domain Event with Akka

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!