Talk about Event Sourcing in Ruby by Anton Paisov, given during 14th #pivorak Lviv Ruby MeetUp, Ukraine. Details: "That's an introduction into Event Sourcing with examples written in Ruby + why UD (from CRUD) are often bad for business".

1. Ruby
meets
Event Sourcing
by @antonpaisov

2. Event Sourcing
There’s a Person who has an Address
We need to change the address

3. Event Sourcing
There’s a Person who has an Address
We need to change the address
CRUD: we just remove old one and add a new one

4. Event Sourcing
There’s a Person who has an Address
We need to change the address
CRUD: we just remove old one and add a new one
ES: publish event about this change and then apply it

5. Back to CRUD

6. CRUD may hurt business

7. CRUD may hurt business
Approaching development from business perspective

8. CRUD may hurt business
Approaching development from business perspective
Business avoids updates

9. CRUD may hurt business
Approaching development from business perspective
Business avoids updates
Business never deletes

10. CRUD may hurt business
Approaching development from business perspective
Business avoids updates
Business never deletes
Business loves audit

11. What’s the most popular
example of ES system
among developers?

12. GIT!

13. +

14. Audit

15. Debug

16. Historic State

17. Alternative Histories

18. Distribution support
(Microservices)

19. In-Memory processing
http://bit.ly/memory-img

20. IMPORTANT:
Event Sourcing
does not mean
“must be done ASYNC”

21. Also important
Can’t do proper ES without identify relevant domain
state changes. (from the business point of view)
Look for Events that affect domain state and activity.

22. Example with
implementation
Shopping cart
Possible to add/remove products from the cart
New request: we need to know what was removed

23. gem 'rails_event_store'

24. http://bit.ly/cart-es

25. module Events
ItemRemovedFromBasket = Class.new(RailsEventStore::Event)
end
lib/events.rb

26. module EventStoreSetup
def event_store
@event_store ||= RailsEventStore::Client.new.tap do |es|
es.subscribe(Denormalizers::ItemRemovedFromBasket.new,
[Events::ItemRemovedFromBasket])
end
end
end
lib/event_store_setup.rb

27. app/controllers/orders_controller.rb
def remove_item
cmd = Command::RemoveItemFromBasket.new(product_params)
execute(cmd)
head :ok
end

28. lib/command/remove_item_to_basket.rb
module Command
class RemoveItemFromBasket < Base
attr_accessor :order_id
attr_accessor :product_id
validates :order_id, presence: true
validates :product_id, presence: true
alias :aggregate_id :order_id
end
end

29. lib/command/execute.rb
def handler_for(command)
{
Command::RemoveItemFromBasket =>
CommandHandlers::RemoveItemFromBasket,
}.fetch(command.class)
end

30. lib/command_handlers/remove_item_from_basket.rb
module CommandHandlers
class RemoveItemFromBasket < Command::Handler
def call(command)
with_aggregate(command.aggregate_id) do |order|
order.remove_item(command.product_id)
end
end
private
def aggregate_class
Domain::Order
end
end
end

31. lib/domain/order.rb
def remove_item(product_id)
apply Events::ItemRemovedFromBasket.new
(data: {order_id: id, product_id: product_id})
end
def apply_events_item_removed_from_basket(event)
order_line.decrease_quantity
remove_order_line(order_line) if order_line.empty?
end

32. lib/denormalizers/item_removed_from_basket.rb
module Denormalizers
class ItemRemovedFromBasket
def call(event)
item = find(event.data.order_id, event.data.product_id)
item.quantity -= 1
item.quantity > 0 ? item.save! : item.destroy!
end
...
end
end

33. On Deck
Add Projection usage examples

34. Approaches

35. Approaches
New projects: Event Sourcing

36. Approaches
New projects: Event Sourcing
Legacy: Event Publishing

37. Conclusions
Help you not to loose data
Clean, testable structure with domain events
Shines with CQRS
May get handy when working on Legacy apps

38. Materials
Robert Pankowecki about Saga pattern:
http://bit.ly/robert-saga

39. Materials
Great article about ES from business perspective:
http://bit.ly/abstract-es
Greg Young’s talk recording: http://bit.ly/greg-es
Martin Fowler has a lot of content on the topic
http://docs.geteventstore.com

40. Arkency blogposts on
ES and DDD in Ruby
http://blog.arkency.com/tags/eventstore/
http://blog.arkency.com/tags/ddd/

41. http://bit.ly/feedback-anton
Feedback appreciated!