Akka Persistence, Event Sourcing, Domain Events, Commands, CQRS, Command side of Akka Persistence

1. Akka Persistence | Event Sourcing
Sidharth Khattri
Senior Software Consultant
Knoldus Software

2. Agenda
● What is Event Sourcing
● What are Domain Events
● What are Commands
● What is CQRS
● When to use Event Sourcing
● What is Akka Persistence
● Command side of Akka Persistence

3. Agenda
● What is Event Sourcing
● What are Domain Events
● What are Commands
● What is CQRS
● When to use Event Sourcing
● What is Akka Persistence
● Command side of Akka Persistence

4. What is Event Sourcing?
● storing all changes of a system as event log
● changes here means Domain Events
● changes are persisted in Event Store
● event store is append only storage
● no updates to Event Store, they're immutable
● event log can be replayed to construct current state

5. Agenda
● What is Event Sourcing
● What are Domain Events
● What are Commands
● What is CQRS
● When to use Event Sourcing
● What is Akka Persistence
● Command side of Akka Persistence

6. What are Domain Events?
● something that already took place in a system
● named as past-participle verbs
● statement of facts
● data transfer objects DTOs
● immutable, because they're facts
● represents state change in a system

7. Agenda
● What is Event Sourcing
● What are Domain Events
● What are Commands
● What is CQRS
● When to use Event Sourcing
● What is Akka Persistence
● Command side of Akka Persistence

8. What are Commands?
● something that triggers a Domain Event
● named as imperative verbs
● are requests but not statement of facts
● data transfer objects DTOs
● immutable, because they're are triggers
● can be rejected

9. Agenda
● What is Event Sourcing
● What are Domain Events
● What are Commands
● What is CQRS
● When to use Event Sourcing
● What is Akka Persistence
● Command side of Akka Persistence

10. What is CQRS?
● CQRS stands for command query responsibility segregation
● commands as write sides and query as read sides
● commands involve domain model interaction
● queries involve reporting module interaction

11. What is CQRS?
● aggregate roots receives Commands and publish Events
● aggregate is a group of associated objects
● an aggregate has one root, which is an Entity
● aggregate root is the only object accessible from outside world

12. Architecture Without CQRS

13. Architecture With CQRS

14. Agenda
● What is Event Sourcing
● What are Domain Events
● What are Commands
● What is CQRS
● When to use Event Sourcing
● What is Akka Persistence
● Command side of Akka Persistence

15. When to use Event Sourcing?
● when keeping audit/event logs have great significance
● for building scalable systems
● for predicting future needs based on event logs
● for better debugging

16. Agenda
● What is Event Sourcing
● What are Domain Events
● What are Commands
● What is CQRS
● When to use Event Sourcing
● What is Akka Persistence
● Command side of Akka Persistence

17. What is Akka Persistence?
● provides reactive way of facilitating DDD/CQRS event
sourced architecture
● is implicitly event-driven

18. Agenda
● What is Event Sourcing
● What are Domain Events
● What are Commands
● What is CQRS
● When to use Event Sourcing
● What is Akka Persistence
● Command side of Akka Persistence

19. What is PersistenceActor?
● PersistenceActor is a trait which supports Event Sourcing
● it can act as an Aggregate Root (AR) for DDD
● its behavior is defined by receiveRecover and receiveCommand
● receiveCommand acts as command handler
● receiveRecover helps in replaying events

20. Command side of Akka Persistence
● Command
case class CreateUser(id: String, firstName: String, lastName: String, email: String)
● Event
case class UserCreated(id: String, firstName: String, lastName: String, email: String)
Imperative verb
Immutable
Past-participle verb
Immutable

21. object Protocols {
case class Command(data: String)
case class Event(data: String)
}
final case class ExampleState(events: List[String] = Nil) {
def updated(event: Event): ExampleState = copy(event.data :: events)
def size: Int = events.length
override def toString: String = events.reverse.toString()
}
Persistence example protocols
For maintaining
Persistence example state

22. class PersistenceExample extends PersistenceActor with ActorLogging {
override def persistenceId: String = "persistence-example-1"
var state = ExampleState()
def updateState(event: Event): Unit = state = state.updated(event)
def numEvents: Int = state.size
val receiveRecover: Receive = {
case event: Event => updateState(event)
}
val receiveCommand: Receive = {
case Command(data) =>
persist(Event(s"$data-${numEvents + 1}")) { event =>
updateState(event)
}
}
}

23. PersistenceActor Details
● persistenceId method
→ id of entity for which events should be replayed
→ remains same across different actor incarnations
● persist method
→ persists events asynchronously
→ handler is only called on successful persistence
→ between call to persist and execution of handler no new commands are received
→ event handlers can update state, notify listeners and reply senders

24. PersistenceActor Details
override def recovery = Recovery(toSequenceNr = 457)
● recovery method
→ journals are automatically recovered on start and restart
→ new messages sent to persistence actor during recovery do not interfere with
replayed messages
→ recovery can be disabled by using Recovery.none()
→ initialization after recovery can be done can be done by handling
RecoveryCompleted

25. References
● http://doc.akka.io/docs/akka/2.4.0/scala/persistence.html
● http://martinfowler.com/eaaDev/EventSourcing.html
● http://cqrs.nu/faq
● https://github.com/ironfish/akka-persistence-mongo-samples
● http://pkaczor.blogspot.in/2014/04/reactive-ddd-with-akka.html

26. Thank You