metacosm

Description

Metacosm is an awesome microframework for building reactive systems.

The idea is to enable quick prototyping of command-query separated or event-sourced systems.

One core concept is that we use commands to update "write-only" models, which trigger events that update "read-only" view models that are used by queries.

Models only transform their state in response to commands, so their state can be reconstructed by replaying the stream of commands.

Features

One interesting feature here is a sort of mock in-memory AR component called Registrable that is used for internal tests (note: this has been extracted to PassiveRecord)

Examples

A Fizzbuzz implementation contrived enough to show off many of the features of the framework.

  require 'metacosm'
  include Metacosm

  class Counter < Model
    def initialize
      @counter = 0
      super
    end

    def fizz!
      emit fizz
    end

    def buzz!
      emit buzz
    end

    def increment!(inc)
      @counter += inc
      emit(counter_incremented)
    end

    protected
    def fizz
      FizzEvent.create
    end

    def buzz
      BuzzEvent.create
    end

    def counter_incremented
      CounterIncrementedEvent.create(
        value: @counter,
        counter_id: @id
      )
    end
  end

  class CounterView < View
    attr_accessor :value, :counter_id
    def update_value(new_value)
      @value = new_value
      self
    end
  end

  class IncrementCounterCommand < Command
    attr_accessor :increment, :counter_id
  end

  class IncrementCounterCommandHandler
    def handle(increment:,counter_id:)
      counter = Counter.find(counter_id)
      counter.increment!(increment)
    end
  end

  class CounterIncrementedEvent < Event
    attr_accessor :value, :counter_id
  end

  class CounterIncrementedEventListener < EventListener
    def receive(value:,counter_id:)
      update_counter_view(counter_id, value)

      fizz_buzz!(counter_id, value)
      puts(value) unless fizz?(value) || buzz?(value)
    end

    def update_counter_view(counter_id, value)
      counter_view = CounterView.where(counter_id: counter_id).first_or_create
      counter_view.update value: value
    end

    private
    def fizz_buzz!(counter_id, n)
      fire(FizzCommand.create(counter_id: counter_id)) if fizz?(n)
      fire(BuzzCommand.create(counter_id: counter_id)) if buzz?(n)
    end

    def fizz?(n); n % 3 == 0 end
    def buzz?(n); n % 5 == 0 end
  end

  class FizzCommand < Command
    attr_accessor :counter_id
  end

  class FizzCommandHandler
    def handle(counter_id:)
      counter = Counter.find(counter_id)
      counter.fizz!
    end
  end

  class BuzzCommand < Command
    attr_accessor :counter_id
  end

  class BuzzCommandHandler
    def handle(counter_id:)
      counter = Counter.find(counter_id)
      counter.buzz!
    end
  end

  class FizzEvent < Event
  end

  class FizzEventListener < EventListener
    def receive
      puts "fizz"
    end
  end

  class BuzzEvent < Event
  end

  class BuzzEventListener < EventListener
    def receive
      puts "buzz"
    end
  end

Given all this prelude we can run a fizzbuzz "simulation":

  sim = Simulation.current
  counter_model = Counter.create
  counter_view = CounterView.find_by(counter_id: counter_model.id)

  counter_view.value # => 0

  increment_counter_command = IncrementCounterCommand.create(
    increment: 1, counter_id: counter_model.id
  )

  sim.apply(increment_counter_command)

  counter_view.value # => 1

  100.times { sim.apply(increment_counter_command) }

  sim.events.take(10)
  # => [CounterCreatedEvent (id: 1, counter_id: 1),
  #  CounterIncrementedEvent (id: 1, value: 1, counter_id: 1),
  #  CounterIncrementedEvent (id: 2, value: 2, counter_id: 1),
  #  CounterIncrementedEvent (id: 3, value: 3, counter_id: 1),
  #  FizzEvent (id: 1),
  #  CounterIncrementedEvent (id: 4, value: 4, counter_id: 1),
  #  CounterIncrementedEvent (id: 5, value: 5, counter_id: 1),
  #  BuzzEvent (id: 1),
  #  CounterIncrementedEvent (id: 6, value: 6, counter_id: 1)]

Requirements

Install

$ gem install metacosm

Synopsis

$ metacosm

Copyright

See LICENSE for details.