Eventuality

This is simply a POC and my way of writing down my thoughts about how I would want to implement a CQRS system. The focus here is on components/tools and functions that can be interchanged rather than a full framework.

components

• Domain Model (Aggregate)

  This is the object that represents the state of our model. Only it can change its own state as a result of Commands. Each state change results in an Event.

• EventStore

  A storage facility for events/history/facts. This can be in memory or backed by a database. What I have here is in memory.

• EventBus

  The medium through which facts of state change are shared to interested observers.

• EventListeners

  These are the observers of facts of state changes. They are functions to be invoked when the domain model has changed its state.

• Repository

  A component through which we create and access the domain model.

• Commands

  Objects representing an intent of state change by the user on the domain model.

• CommandHandlers

  Functions with the purpose of communicating the intended state change to the domain model.

• Events

  Objects representing a fact of state change in the domain model.

Based on these definitions of the components, the system should work like this:

CommandHandlers are effectively CH(Command) -> [...Event]

Actions the domain model can execute are A() -> [...Event]

This means every intention to change the state of the domain model results in n events (where n = 0 is a failure and n > 0 is success).

Examples of usage

Of course, I'll use a Todo application because that's the app any system can build.

Eventuality = require 'eventuality'

Todo = Eventuality.defineAggregate
  name: 'Todo'
  state:
    description: null
    completed: false
  methods:
    complete: ->
      @state.completed = true
      Eventuality.Event(aggregateId: @id, name: 'TodoCompletedEvent', payload: {completed: true}, state: @state)

TodoCommands =
  CreateTodo: ({id, description }) -> name: 'CreateTodo', message: {id, description}
  MarkAsCompleted: ({ id }) -> name: 'MarkAsCompleted', message: {id}

TodoEventStore = Eventuality.EventStore()

TodoRepository = Eventuality.Repository 'Todo', Todo, TodoEventStore

TodoCommandHandlers =
  CreateTodo: (attrs) -> TodoRepository.add attrs
  MarkAsCompleted: ({ id }) ->
    TodoRepository.load(id).then (todo) ->
      todo.complete()

TodoEventBus = Eventuality.EventBus()

TodoCreatedEventListenerToUpdateDB = (event) -> # Update database...
TodoCreatedEventListenerToLogStuff = (event) -> # Do some logging...

TodoEventBus.registerListeners
  TodoCreatedEvent: [
    TodoCreatedEventListenerToUpdateDB
    TodoCreatedEventListenerToLogStuff
  ]
  TodoCompletedEvent: [
    (event) ->
      # More stuff to be done
  ]

It can(doesn't have to) all come together like so...

TodoFlow = Eventuality.Flow
  eventStore: TodoEventStore
  eventBus: TodoEventBus
  commandHandlers: TodoCommandHandlers

## Later when the user wants to do things...
TodoFlow.dispatch TodoCommands.CreateTodo id: 'todo1', description: 'Create a todo'
TodoFlow.dispatch TodoCommands.MarkAsCompleted id: 'todo1'