Event Sourcing with Kotlin

You've heard about Event Sourcing, it sounds like a useful technique that can potentially make some really hard problems a lot easier. But where do you start? There are many talks about the theory and application of Event Sourcing, but what does it actually look like in code? And why is the Kotlin language such an excellent match for writing event sourced applications?

Presented at: KotlinDevDay 2022

Transcript

1. Who needs frameworks! Event Sourcing with Kotlin, Nico Krijnen
2. Nico Krijnen @nicokrijnen nkrijnen nicokrijnen
3. Who needs frameworks! Event Sourcing with Kotlin,
4. Concepts – Domain – Code – Frameworks What to expect...
5. Commands – Events – CQRS – Event Sourcing Commands Concepts
6. fun registerUser(email: String, password: String) { ... }
7. // as function call fun registerUser(email: String, password: String) { ... } // using command data class RegisterUser( val email: String, val password: String, ) fun handle(command: RegisterUser) { ... }
8. Naming ❌ ✅ CreateUser RegisterUser UpdateAddress MoveToAddress UpdateProduct InventoryIncrease DeleteUser DeactiveUserRegistration Don't use CRUD Capture intent 
 in language of the domain
9. Events Concepts Commands – Events – CQRS – Event Sourcing
10. data class UserRegisteredEvent( val userId: UUID, val email: String, val password: String, ) data class MovedToAddressEvent( val person: UUID, val newAddress: Address, val atTime: Instant, ) data class InventoryIncreasedEvent( val product: UUID, val amount: Int, ) data class UserRegistrationDeactivatedEvent( val userId: UUID, )
11. data class UserRegisteredEvent( val userId: UUID, val email: String, val password: String, ) data class MovedToAddressEvent( val person: UUID, val newAddress: Address, val atTime: Instant, ) data class InventoryIncreasedEvent( val product: UUID, val amount: Int, ) data class UserRegistrationDeactivatedEvent( val userId: UUID, ) data class RegisterUserCommand( val userId: UUID, val email: String, val password: String, ) data class MoveToAddressCommand( val person: UUID, val newAddress: Address, val atTime: Instant, ) data class IncreaseInventoryCommand( val product: UUID, val amount: Int, ) data class DeactivateUserRegistrationCommand( val userId: UUID, ) What is the difference between 
 a command and an event?
12. Their intent. What is the difference between 
 a command and an event?
13. Event ≠ Event Event Driven ≠ Event Sourced ≠ Event Notification ≠ Integration Events 
 ≠ Event carried state transfer ≠ Domain Events ≠ UI Events
14. Command Query Responsibility Segregation Concepts Commands – Events – CQRS – Event Sourcing
15. Services Model UI
16. UI Command model Query model Write services Read services
17. UI Eventually Consistent Command Query Command model Query model Projection Event Write side Read side
18. UI RDBMS Elasticsearch Command Query Command model Query model Projection Event Write side Read side Query model Projection DynamoDB Query
19. UI RDBMS Elasticsearch Command Query Command model Query model Projection Event Write side Read side Query model Projection DynamoDB Query UI Command model Query model Services Model UI Write services Read services
20. Event Sourcing Concepts Commands – Events – CQRS – Event Sourcing
21. Once tasted have you Back, you cannot go
22. Traditional Capture events instead of directly updating a consolidated view Example: moving to a new house George moved from Apeldoorn to Deventer Traditional
23. Traditional Model data as events Capture events instead of directly updating a consolidated view Example: moving to a new house George moved from Apeldoorn to Deventer Traditional Event sourced odel data as events vents instead of directly updating a consolidated view moving to a new house oved from Apeldoorn to Deventer Traditional Event sourced
24. When should you use EventSourcing?
25. When should you use EventSourcing? • When time/history is important...
26. When should you use EventSourcing? • When time/history is important... maybe • Adjust behavior after the fact...
27. When should you use EventSourcing? • When time/history is important... maybe • Adjust behavior after the fact... probably • When you need an audit log...
28. When should you use EventSourcing? • When time/history is important... maybe • Adjust behavior after the fact... probably • When you need an audit log... probably not
29. Concepts – Domain – Code – Frameworks Understand your domain
30. Domain Driven Design
31. Event Storming Event storming
32. Event Storming
34. Let's explore Renting Lego Sets
35. Lego set booked Rent fee payed Lego set shipped Lego set low on stock Lego set returned Photos of Lego build shared
36. Lego set booked Rent fee payed Lego set shipped Lego set low on stock Lego set returned Photos of Lego build shared Book Lego set Payment Provider Ship Lego set Low stock policy Share 
 Lego build photos Return Lego set
37. Lego set booked Rent fee payed Lego set shipped Lego set low on stock Lego set returned Photos of Lego build shared Book Lego set Payment Provider Ship Lego set Low stock policy Share 
 Lego build photos Return Lego set Lego Store Lego Builder Lego Builder Lego Store
38. Lego set booked Rent fee payed Lego set shipped Lego set low on stock Lego set returned Photos of Lego build shared Book Lego set Payment Provider Ship Lego set Low stock policy Share 
 Lego build photos Return Lego set Lego Store Lego Builder Lego Builder Lego Store Lego set inventory Lego set inventory Lego build gallery Lego set inventory
39. Concepts – Domain – Code – Frameworks CODE
40. Write model 15/05/2022, 15:16 Page 1 of 1 file:///Users/nico/repos/_workshops/kdd2-event-sourcing/src/media/docs/flow.drawio.svg handleCommand API POST /legoset/{nr}/book handleMutation handle(command) ⓸ LegoSetService BookLegoSetCommand existingEvents ⓶ put ⓺ get ⓵ EventStore LegoSetAggregate BookLegoSetCommand newEvents ⓹ constructor(events) ⓷ λ List<LegoSetEvent> List<LegoSetEvent> List<LegoSetEvent> List<LegoSetEvent> KeyValueStore
41. Write model handleCommand API POST /legoset/{nr}/book LegoSetService BookLegoSetCommand
42. Write model handleCommand API POST /legoset/{nr}/book handleMutation handle(command) ⓸ LegoSetService BookLegoSetCommand existingEvents ⓶ put ⓺ get ⓵ EventStore BookLegoSetCommand newEvents ⓹ λ List<LegoSetEvent> List<LegoSetEvent> List<LegoSetEvent> KeyValueStore
43. Write model 15/05/2022, 15:16 eCommand handleMutation handle(command) ⓸ LegoSetService oSetCommand existingEvents ⓶ EventStore LegoSetAggregate BookLegoSetCommand newEvents ⓹ constructor(events) ⓷ λ List<LegoSetEvent> List<LegoSetEvent> List<LegoSetEvent> List<LegoSetEvent>
44. Write model handleCommand API POST /legoset/{nr}/book handleMutation handle(command) ⓸ LegoSetService BookLegoSetCommand existingEvents ⓶ put ⓺ get ⓵ EventStore BookLegoSetCommand newEvents ⓹ λ List<LegoSetEvent> List<LegoSetEvent> List<LegoSetEvent> KeyValueStore
45. Show me the code!
46. EventSourcing +
47. Events are immutable!
48. data class LegoSetBookedEvent( val userId: String, val userName: String, val cardTypeId: Int, val cardNumber: String, val cardSecurityNumber: String, val cardHolderName: String, val cardExpiration: Instant, val order: Order, )
49. data class LegoSetBookedEvent( val userId: String, val userName: String, val cardTypeId: Int, val cardNumber: String, val cardSecurityNumber: String, val cardHolderName: String, val cardExpiration: Instant, val order: Order, ) public record LegoSetBookedEvent( @NotNull String userId, @NotNull String userName, int cardTypeId, @NotNull String cardNumber, @NotNull String cardSecurityNumber, @NotNull String cardHolderName, @NotNull Instant cardExpiration, @NotNull Order order ) { public LegoSetBookedEvent { Objects.requireNonNull(userId, "userId"); Objects.requireNonNull(userName, "userName"); Objects.requireNonNull(cardNumber, "cardNumber"); Objects.requireNonNull(cardSecurityNumber, "cardSecurityNumber"); Objects.requireNonNull(cardHolderName, "cardHolderName"); Objects.requireNonNull(cardExpiration, "cardExpiration"); Objects.requireNonNull(order, "order"); } }
50. public class LegoSetBookedEvent { @NotNull private final String userId; @NotNull private final String userName; private final int cardTypeId; @NotNull private final String cardNumber; @NotNull private final String cardSecurityNumber; @NotNull private final String cardHolderName; @NotNull private final Instant cardExpiration; @NotNull private final Order order; public LegoSetBookedEvent( @NotNull String userId, @NotNull String userName, int cardTypeId, @NotNull String cardNumber, @NotNull String cardSecurityNumber, @NotNull String cardHolderName, @NotNull Instant cardExpiration, @NotNull Order order ) { Objects.requireNonNull(userId, "userId"); Objects.requireNonNull(userName, "userName"); Objects.requireNonNull(cardNumber, "cardNumber"); Objects.requireNonNull(cardSecurityNumber, "cardSecurityNumber"); Objects.requireNonNull(cardHolderName, "cardHolderName"); Objects.requireNonNull(cardExpiration, "cardExpiration"); Objects.requireNonNull(order, "order"); this.userId = userId; this.userName = userName; this.cardTypeId = cardTypeId; this.cardNumber = cardNumber; this.cardSecurityNumber = cardSecurityNumber; this.cardHolderName = cardHolderName; this.cardExpiration = cardExpiration; this.order = order; } @NotNull public final String getUserId() { return this.userId; } @NotNull public final String getUserName() { return this.userName; } public final int getCardTypeId() { return this.cardTypeId; } @NotNull public final String getCardNumber() { return this.cardNumber; } @NotNull public final String getCardSecurityNumber() { return this.cardSecurityNumber; } @NotNull public final String getCardHolderName() { return this.cardHolderName; } @NotNull public final Instant getCardExpiration() { return this.cardExpiration; } @NotNull public final Order getOrder() { return this.order; } @NotNull public String toString() { ... } public int hashCode() { ... } public boolean equals(@Nullable Object var1) { ... } } data class LegoSetBookedEvent( val userId: String, val userName: String, val cardTypeId: Int, val cardNumber: String, val cardSecurityNumber: String, val cardHolderName: String, val cardExpiration: Instant, val order: Order, ) public record LegoSetBookedEvent( @NotNull String userId, @NotNull String userName, int cardTypeId, @NotNull String cardNumber, @NotNull String cardSecurityNumber, @NotNull String cardHolderName, @NotNull Instant cardExpiration, @NotNull Order order ) { public LegoSetBookedEvent { Objects.requireNonNull(userId, "userId"); Objects.requireNonNull(userName, "userName"); Objects.requireNonNull(cardNumber, "cardNumber"); Objects.requireNonNull(cardSecurityNumber, "cardSecurityNumber"); Objects.requireNonNull(cardHolderName, "cardHolderName"); Objects.requireNonNull(cardExpiration, "cardExpiration"); Objects.requireNonNull(order, "order"); } } ≦ 13
51. data class LegoSetBookedEvent( val userId: String, val userName: String, val cardTypeId: Int, val cardNumber: String, val cardSecurityNumber: String, val cardHolderName: String, val cardExpiration: Instant, val order: Order, )
52. data class LegoSetAddedToCatalogEvent( override val legoSet: LegoSetNumber, override val atTime: Instant = Instant.now(), ) : LegoSetEvent() data class LegoSetBookedEvent( override val legoSet: LegoSetNumber, val builder: BuilderId, override val atTime: Instant = Instant.now(), ) : LegoSetEvent() data class LegoSetReturnedEvent( override val legoSet: LegoSetNumber, val builder: BuilderId, override val atTime: Instant = Instant.now(), ) : LegoSetEvent() data class LegoSetInventoryDecreasedEvent( override val legoSet: LegoSetNumber, val amount: Int, override val atTime: Instant = Instant.now(), ) : LegoSetEvent() data class LegoSetInventoryIncreasedEvent( override val legoSet: LegoSetNumber, val amount: Int, override val atTime: Instant = Instant.now(), ) : LegoSetEvent() Events.kt
53. sealed interface LegoSetCommand { val legoSet: LegoSetNumber } data class AddLegoSetToCatalogCommand( override val legoSet: LegoSetNumber ) : LegoSetCommand data class BookLegoSetCommand( override val legoSet: LegoSetNumber, val builder: BuilderId, ) : LegoSetCommand data class ReturnLegoSetCommand( override val legoSet: LegoSetNumber, val builder: BuilderId, ) : LegoSetCommand
54. sealed interface LegoSetCommand { val legoSet: LegoSetNumber } fun handleCommand(command: LegoSetCommand) = when (command) { is AddLegoSetToCatalogCommand -> add(command) is BookLegoSetCommand -> bookSet(command) is ReturnLegoSetCommand -> returnSet(command) } private fun add(command: AddLegoSetToCatalogCommand) = ... private fun bookSet(command: BookLegoSetCommand) = ... private fun returnSet(command: ReturnLegoSetCommand) = ...
55. private fun List<LegoSetEvent>.amountInStock(): Int = this.fold(0) { result, event -> when (event) { is InventoryAffectingEvent -> result + event.amountChanged else -> result } }
56. private val available: Boolean by lazy { amountInStock > 0 } private val amountInStock: Int by lazy { events.amountInStock() } private fun List<LegoSetEvent>.amountInStock(): Int = this.fold(0) { result, event -> when (event) { is InventoryAffectingEvent -> result + event.amountChanged else -> result } }
57. private fun List<LegoSetEvent>.amountInStock(): Int = this.fold(0) { result, event -> when (event) { is InventoryAffectingEvent -> result + event.amountChanged else -> result } } private val available: Boolean by lazy { amountInStock > 0 } private val amountInStock: Int by lazy { events.amountInStock() } private fun bookSet(command: BookLegoSetCommand): ... { if (!available) throw LegoSetNotAvailableException(command.legoSet) return listOf(LegoSetBookedEvent(command.legoSet, command.builder)) }
58. So what about No frameworks Concepts – Domain – Code – Frameworks
59. That looked simple enough! • Define commands and events • Pipeline for handling write model mutations • Aggregates to contain domain logic • Projections to build read models
60. But did you think of... • Dealing with replays • Dealing with eventual consistency • Consistency of write model and event publication
61. That sounds like I need a framework! • Let it solve all the hard stuff, you focus on your domain logic • Flexibility to choose or switch underlying technologies
62. It's a tradeoff • Frameworks have an opinion, do you agree? • Frameworks can be intrusive • Frameworks introduce abstractions you may not need
63. Recap
64. Recap • Do we like Event Sourcing with Kotlin?
65. Recap • Do we like Event Sourcing with Kotlin? YES!
66. Recap • Do we like Event Sourcing with Kotlin? YES! • Should you use Event Sourcing?
67. Recap • Do we like Event Sourcing with Kotlin? YES! • Should you use Event Sourcing? MAYBE?
68. Recap • Do we like Event Sourcing with Kotlin? YES! • Should you use Event Sourcing? MAYBE? • Framework? Or is plain Kotlin code enough?
69. Recap • Do we like Event Sourcing with Kotlin? YES! • Should you use Event Sourcing? MAYBE? • Framework? Or is plain Kotlin code enough? PROBABLY NOT! (and YES for your domain)
70. Event Sourcing with Kotlin,
71. Nico Krijnen @nicokrijnen nkrijnen nicokrijnen Event Sourcing with Kotlin,