The Three Little Pigs with DDD and clean architecture

My tech talk, fairy tale sources: 1, 2, 3, 4.

Java, Groovy + Spock, Kotlin, Maven, Spring

The main focus should be on The Three Little Pigs, but to show an alternative, more pragmatic approach there is also Little Red Riding Hood module, utilizing package-private access more, close to what's presented in this great tech talk by Jakub Nabrdalik.

Web app

App starts as an ordinary web app for

spring.main.web-application-type=servlet

The Three Little Pigs

Available operations:

1. Build house: POST localhost:8080/houses

    {
        "owner": "VERY_LAZY"
    }

   Possible values for owner:
   • VERY_LAZY
   • LAZY
   • NOT_LAZY
   • NOT_LAZY_ANYMORE
2. Verify the house state: GET localhost:8080/houses/{id}
3. Blow house down: DELETE localhost:8080/houses/{id}

There is a dedicated Postman collection with all these operations already defined: pigs3/adapters/src/main/resources/3Pigs.postman_collection.json.

Little Red Riding Hood

There is a dedicated Postman collection with all the operations ready to use: redhood/src/main/resources/RedHood.postman_collection.json.

Console app

When

spring.main.web-application-type=none

app prints the whole The Three Little Pigs story in the console.

---

EventStorming

An example session with myself, for The Three Little Pigs. There was no need to run Process Level ES as we already had a single BoundedContext, and it was doable to jump straight into Design Level.

For the second module I also run Big Picture ES which helped to realize that the main things are interactions and wolf's intentions.

Big Picture

Design Level - commands, rules & actors

Design Level - naming aggregates

Possible improvements

• House could have mechanisms for rebuilding
• Story can be extended - currently there is nothing about wolf climbing through the chimney and pigs lighting the fire
  • New House method, e.g. litFire
  • New BigBadWolfService method, e.g. comeDownTheChimneyOf
  • Event, e.g. WolfStartedClimbing instead of WolfResignedFromAttacking, new event from House, e.g. WolfEscaped (when burns in the fireplace)
  • WolfStartedClimbing should call both litFire and comeDownTheChimneyOf in a proper order
  • WolfEscaped should result in knowledge sharing
• Full Event Sourcing - House can be built just from events, no snapshots in the current form
• Rely fully on DomainEventPublisher - although @DomainEvents annotation looks nice, it relies on ApplicationEventPublisher which has known limitations, especially without additional tooling like Spring Modulith
• Instead of "technical" packages corresponding with Clean Architecture and DDD, there could be more process-oriented packages, like building (or construcitng) and destroying, where the domain logic would lay, similar as presented in this great tech talk by Victor Rentea