Heroes of Domain-Driven Design (Ruby)

Shows how to use Domain-Driven Design, Event Storming, Event Modeling and Event Sourcing in Heroes of Might & Magic III domain.

👉 Let's explore the Heroes of Domain-Driven Design blogpost series

• There you will get familiar with the whole Software Development process: from knowledge crunching with domain experts, designing solution using Event Modeling, to implementation using DDD Building Blocks.

This project probably won't be fully-functional HOMM3 engine implementation, because it's done for educational purposes. If you'd like to talk with me about mentioned development practices fell free to contact on linkedin.com/in/mateusznakodach/.

I'm focused on domain modeling on the backend, but I've also played around with Rails app frontend using Hotwire.

🚀 How to run the project locally?

1. cd heroesofddd_rails_application
2. docker compose up
3. bundle install
4. rails db:drop db:create db:migrate db:seed - (re)creates database and seed with example data
5. rails server

Example use case:

• Go to the url: http://127.0.0.1:3000/heroes/games/fcc8f601-76cb-4b5a-972d-b7431303f69a/creature_recruitment/dwellings/cecc4307-e940-4ef2-8436-80c475729938
• Recruit Angels and click "Next day" in order to wait for the astrologers proclamation of the week symbol.

🧱 Modules

Modules (mostly designed using Bounded Context heuristic) are designed and documented on EventModeling below. Each slice in a module is in certain color which shows the progress:

• green -> completed
• yellow -> implementation in progress
• red -> to do
• grey -> design in progress

List of modules you can see in lib/heroes directory of the Rails application.

heroes/
├── astrologers
├── calendar
├── creature_recruitment

Each domain-focused module follows Vertical-Slice Architecture of three possible types: write, read and automation following Event Modeling nomenclature. Aggregates are implemented using Decider pattern.

👾 Creature Recruitment

Slices:

• Write: BuildDwelling -> DwellingBuilt | test
• Write: IncreaseAvailableCreatures -> AvailableCreaturesChanged | test
• Write: RecruitCreature -> CreatureRecruited | test
• Read: (DwellingBuilt, AvailableCreaturesChanged, CreatureRecruited) -> DwellingReadModel | test

Aggregates:

• Dwelling

🧙 Astrologers

Slices:

• Write: ProclaimWeekSymbol -> WeekSymbolProclaimed | test
• Automation: DayStarted(where day==1) -> ProclaimWeekSymbol | test
• Automation: (WeekSymbolProclaimed, all game dwellings derived from DwellingBuilt events) -> IncreaseAvailableCreatures for each dwelling in the game where creature == symbol | test

Aggregates:

• WeekSymbol

📅 Calendar

Slices:

• Write: StartDay -> DayStarted | test
• Write: FinishDay -> DayFinished | test
• Read: DayStarted -> CurrentDateReadModel | test

Aggregates:

• Calendar

🤖 Working with AI Large Language Models:

If you'd like to use the whole source code as your prompt context generate codebase file by: npx ai-digest --whitespace-removal

Domain Model purity

Domain Events are decoupled from infrastructure RailsEventStore events. Every domain event is registered with corresponding functions which maps from domain to storage structure and vice-versa (as shown below).

WeekSymbolProclaimed = Class.new(RubyEventStore::Event) do
    def self.from_domain(domain_event)
      ::EventStore::Heroes::Astrologers::WeekSymbolProclaimed.new(
        data: {
          month: domain_event.month,
          week: domain_event.week,
          week_of: domain_event.week_of,
          growth: domain_event.growth
        }
      )
    end
    
    def self.to_domain(store_event)
      data = store_event.data.deep_symbolize_keys
      ::Heroes::Astrologers::WeekSymbolProclaimed.new(
        month: data[:month],
        week: data[:week],
        week_of: data[:week_of],
        growth: data[:growth],
      )
    end
end

🧪 Testing

Tests using Real postgres Event Store, follows the approach:

• write slice: given(events) -> when(command) -> then(events)
• read slice: given(events) -> then(read model)
• automation: when(event, state?) -> then(command)

Tests are focused on observable behavior which implicitly covers the DDD Aggregates, so the domain model can be refactored without changes in tests.

Example: write slice

def test_given_dwelling_with_3_creature_when_recruit_2_creature_then_success
  # given
  given_domain_event(@stream_name, DwellingBuilt.new(@dwelling_id, @creature_id, @cost_per_troop))
  given_domain_event(@stream_name, AvailableCreaturesChanged.new(@dwelling_id, @creature_id, 3))

  # when
  recruit_creature = RecruitCreature.new(@dwelling_id, @creature_id, 2)
  execute_command(recruit_creature, @app_context)

  # then
  expected_cost = Heroes::SharedKernel::Resources::Cost.resources([ :GOLD, 6000 ], [ :GEM, 2 ])
  expected_event = CreatureRecruited.new(@dwelling_id, @creature_id, 2, expected_cost)
  then_domain_event(@stream_name, expected_event)
end

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💼 Hire me

If you'd like to hire me for Domain-Driven Design and/or Event Sourcing projects I'm available to work with: Kotlin, Java, C# .NET, Ruby and JavaScript/TypeScript (Node.js or React). Please reach me out on LinkedIn linkedin.com/in/mateusznakodach/.