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CQRS with Event Source in Functional sauce served by Kotlin


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Slides of my talk at DDD eXchange 2018 London.

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CQRS with Event Source in Functional sauce served by Kotlin

  1. 1. @Ramtop CQRS with Event Source in Functional sauce served by Kotlin Uberto
  2. 2. @Ramtop CQRS + Event Source Very powerful but a bit intimidating… Can we do better?
  3. 3. @Ramtop Functional Programming ● Immutability ● Purity ● Higher order functions ● Transformations and preserving properties
  4. 4. @Ramtop Functional vs OO Programming
  5. 5. @Ramtop Command ● A Command is a request for changing the internal state of the system ● A Command can “fail” if is not congruent with the current state of the System ● Each Command is executed in an atomic context
  6. 6. @Ramtop Query ● A query ask for a snapshot of the state ● Queries typically need different data and denormalization from domain model. So we separate the models in CQRS. ● Queries are eventually consistent with the domain
  7. 7. @Ramtop Values-Entities ● They represent the state of the domain ● In functional programming everything is immutable, so there is no identity ● To keep state changes we have Algebraic Data Types
  8. 8. @Ramtop Aggregates ● An aggregate has all the information for a transaction unit ● Aggregates is what Events fold to ● They are composed by Entities
  9. 9. @Ramtop Event ● Events are the “atoms” of System state change ● Nothing can change without an event, every event can change only one entity ● Entity + Event => Entity
  10. 10. @Ramtop Actors ● More powerful and easy to use concurrency model than Threads/Locks ● They communicate asynchronously and potentially remotely ● Useful for Bounded Contexts and Services
  11. 11. @Ramtop
  12. 12. @Ramtop Antica Pizzeria ● Real application is about finance products booking but a pizzeria has a surprising similar domain ● We want to do implement the backend for a ChatBot that will assist booking and enquires about orders.
  13. 13. @Ramtop Kotlin Syntax ● No semicolon! ● String? Is a different type from String ● myFun{println(it)} is equivalent to myFun(x -> println(x)) ● val xy is declaring xy immutable var xy is declaring xy mutable ● User(“Joe”) is equivalent to Java new User(“Joe”)
  14. 14. @Ramtop Commands
  15. 15. @Ramtop Commands
  16. 16. @Ramtop Queries
  17. 17. @Ramtop Actors (where we can put all this stuff)
  18. 18. @Ramtop Commands emit Events (it’s the only way to change)
  19. 19. @Ramtop Events
  20. 20. @Ramtop Queries listen to Events (to keep up with the changes)
  21. 21. @Ramtop Let's fold Events (to create Entities)
  22. 22. @Ramtop Entities as EventComposable
  23. 23. @Ramtop Execute Commands
  24. 24. @Ramtop Logic inside Commands
  25. 25. @Ramtop Application
  26. 26. @Ramtop We can now process C&Q (Fun with Kotlin ExtFun)
  27. 27. @Ramtop Real Code Now!
  28. 28. @Ramtop QAUberto Barbini @Ramtop
  29. 29. @Ramtop
  30. 30. @Ramtop