15 years with Rails and DDD

The problem with big Rails apps is that it's one big model all ActiveRecords together with service objects.
There are no clear boundaries.

This talk presents a solution based on DDD - create many models - domain models and read models.

Transcript

1. Andrzej Krzywda, Chicago Ruby User Group, 2025 15 years with Rails and DDD Lessons learnt [ahn-jay]
3. 2004 Wrocław, Poland
4. Young, idealistic programmer
5. Plain Old Java Objects No getters
6. Hi, I'm Andrzej and I'm a Java Ruby developer
8. Helping Rails apps since 2007 a boutique consultancy
9. ❤
10. 2012 2015 1B domain events
11. 2025 Lemonade
12. Current Rails landscape Main bulding blocks ActiveRecord Service Objects
13. Bene fi ts • AR for UI, queries, forms, json • Service objects - well-named • Service objects - one place • Simplicity of CRUD • Business-e ff ective The Rails Way architecture
14. • Big classes (User, Order, Product) - over 50 columns • No modularization (microservices don't help either) • Hard to unit test • Service objects rules • Coupling • Cohesion • Tracking Callbacks • Deadlocks in transactions • Slow reports (multiple joins) • Not junior friendly • Not AI friendly Problems in bigger Rails apps
15. What is DDD?
16. • Maps business into code • Model per domain • Shared language • Modelling process • Strategical DDD • Tactical patterns - repository, value object, entity and many others DDD
17. problems • overkill for small apps • risk of over-engineering • boilerplate code • devs focus on tactical patterns DDD
18. What could be the dream architecture?
19. • No new inventions • Keep what's great in Rails • Keep what's great in DDD • Allow CRUD domains • Allow rich domains • Junior-friendly • AI-friendly The Dream Rails Architecture What
20. Domain Driven Rails architecture
21. App layer vs domain layer
22. App Domain
23. App layer
24. App Domain Admin panel Public app Mobile API Shopify GraphQL Service Objects Controllers AllProducts Ledger AllOrders Month Report Read models webhooks ImportOrderService SupplyProductService SubmitOrderService O ff er Cart Public::OrdersController Admin::ProductsController Writes Reads Public::CartController commands
25. Service object or controller
26. Service object or controller
27. Service object or controller
28. Domain layer
29. App Domain Domains Pricing Inventory Catalog Shipping Invoicing Ordering Events Commands events events
30. Domain events as communication
31. 3 building blocks
32. Aggregate events, cmd -> event
33. Read model events -> data
34. Process manager events -> commands
35. Domains
36. App Domain Admin panel Public app Mobile API Shopify GraphQL Domains Pricing Inventory Catalog Shipping Invoicing Ordering Events Commands
39. App Domain Admin panel Public app Mobile API Shopify GraphQL Service Objects Controllers Domains Pricing Inventory Catalog Shipping Invoicing Ordering AllProducts Ledger AllOrders Month Report Read models Events Commands SupplyProductService Admin::ProductsController Writes Reads Process Managers
40. App Domain Admin panel Public app Mobile API Shopify GraphQL Service Objects Controllers Domains Pricing Inventory Catalog Shipping Invoicing Ordering AllProducts Ledger AllOrders Month Report Read models webhooks ImportOrderService Events Commands SupplyProductService SubmitOrderService O ff er Cart Public::OrdersController Admin::ProductsController Writes Reads Public::CartController Process Managers
41. Commands and Events as the input/output
42. Events published from aggregates
44. Process managers instead of callbacks
45. Read models increase performance decrease deadlocks denormalize view data unit testable
48. Event Storming
49. Event Modelling
50. https://www.youtube.com/watch?v=0KjmEK2TAxc
51. App Domain Admin panel Public app Mobile API Shopify GraphQL Service Objects Controllers Domains Pricing Inventory Catalog Shipping Invoicing Ordering AllProducts Ledger AllOrders Month Report Read models webhooks ImportOrderService Form objects Events Commands SupplyProductService SubmitOrderService O ff er Cart Public::OrdersController Admin::ProductsController Writes Reads Public::CartController Process Managers
52. From Rails to Domain-Driven Rails in safe steps
53. 1. Publish events from service objects
54. 2. Extract read models (and eliminate columns from AR)
55. 3. Turn Callbacks into Process Managers
56. Domain-Driven Rails and AI
57. Small context for AI
58. How to start?
59. Ecommerce repository gh: RailsEventStore/ecommerce
60. Prepare a PR, I will review it
61. THANK YOU! Andrzej Krzywda
62. THANK YOU! Andrzej Krzywda email: andrzej@arkency.com X: andrzejkrzywda BlueSky: andrzejkrzywda LinkedIn: Andrzej Krzywda blog.arkency.com