Playing around with event sourcing principles

event_sourcing.rb

require 'yaml'

class Storage # Normally should be Redis or Kafka?
  def self.events
    @events ||= []
  end
end

# Aggregates (models)

class Order
  attr_reader :items

  def initialize(order_id)
    @order_id = order_id
    @items = []
  end

  def order_total
    @items.inject(0) do |sum, item|
      sum + item.price
    end
  end

  # Commands

  def add_item(item_uid, price)
    apply_event ItemAddedEvent.new(item_uid, price)
  end

  def remove_item(item_uid)
    apply_event ItemRemovedEvent.new(item_uid)
  end

  # Applying events

  def apply_event(event)
    Storage.events << YAML.dump(event) # store event (should be Redis/Kafka)

    # apply it
    if event.is_a? ItemAddedEvent
      @items << Item.new(event.item_uid, event.price)
      # INSERT INTO # (immediate consistency)
    elsif event.is_a? ItemRemovedEvent
      @items.delete_if{|item| item.uid == event.item_uid}
      # DELETE # (immediate consistency)
    end
  end

  # Rebuilding

  def rebuild
    events = Storage.events.map{|event| YAML.load(event)}
    events.each{|event| apply_event event}
  end
end

class Item
  attr_reader :uid, :price

  def initialize(uid, price)
    @uid = uid
    @price = price
  end
end

# Events

class ItemAddedEvent
  attr_reader :item_uid, :price

  def initialize(item_uid, price)
    @item_uid = item_uid
    @price = price
    @time_added = Time.now
  end
end

class ItemRemovedEvent
  attr_reader :item_uid

  def initialize(item_uid)
    @item_uid = item_uid
  end
end

# In Action

order = Order.new('0001')
order.add_item 'item_uid-1', 1.0 # should be +1.0 ? With de-duplication?
order.add_item 'item_uid-2', 2.0
order.add_item 'item_uid-3', 3.0
puts "Order total with 3 items: #{order.order_total}"
order.remove_item 'item_uid-2'
puts "Order total with 2 items: #{order.order_total}"

order = Order.new('0001')
order.rebuild
puts "Order total after rebuild: #{order.items.count} items, $#{order.order_total}"

# The write side and read side won't use the same data store, but can reside in the same database (but different tables)
# http://karmajunkie.com/event-sourcing-talk/#1