Rage against the state machine

From LRUG March 2014 - A story about the problems we faced modelling state and recording state changes at GoCardless and how we generalised our solution to those problems into a new gem, Statesman.

Transcript

1. RAGE AGAINST THE STATE MACHINE
2. ANDY APPLETON @appltn
4. BACKGROUND
5. Payment failure submitted DAY 5 DAY 4 Merchant receives failure notification DAY 3 Payment to merchant reversed DAY 0 DAY 1 Payer's bank receives request DAY 2 Merchant credited with payment Payment request submitted
6. LOOKS LIKE A STATE MACHINE
7. class Payment < ActiveRecord::Base
 has_many :payment_actions
 
 def mark_as_submitted
 self.payment_actions.create(..)
 end
 end

8. DAY 0 Mandate created DAY 1 Payer's bank receives mandate DAY 2 Mandate approved (DAY 3) Merchant notified of failure
9. class Mandate < ActiveRecord::Base
 has_many :mandate_histories
 
 def mark_as_submitted
 self.mandate_histories.create(..) end
 end

10. DUPLICATION INCONSISTENCY BUT IT WORKS (AND WE HAVE A PRODUCT TO BUILD) DUPLICATION INCONSISTENCY
11. € NEXT ON THE ROADMAP
12. EURO PAYMENTS FOLLOW A DIFFERENT TIMELINE
13. class Payment < ActiveRecord::Base
 has_many :payment_actions
 
 def gbp_mark_as_submitted
 self.payment_actions.create(..)
 end
 
 def eur_mark_as_submitted
 self.payment_actions.create(..)
 end
 end

14. TIME FOR SOME THINKING
15. TIME FOR A STATE MACHINE ABSTRACTION
16. AUDIT TRAIL DATA INTEGRITY COMPOSABLE AUDIT TRAIL DATA INTEGRITY
17. NOTHING REALLY HIT THE MARK
19. class PaymentStateMachine
 include Statesman::Machine
 
 state :pending, initial: true
 state :submitted
 
 transition from: :pending,
 to: :submitted
 end
20. class Payment < ActiveRecord::Base
 has_many :payment_actions
 # ...
 def state_machine
 @state_machine ||= PaymentStateMachine.new(self,
 transition_class: PaymentAction)
 end
 end
21. class PaymentStateMachine
 # ...
 guard_transition to: :paid do |pmnt|
 pmnt.has_active_mandate?
 end
 end

22. class PaymentStateMachine
 # ...
 after_transition to: :paid do |pmnt|
 PaymentNotifier.send_mail(pmnt)
 end
 end

23. BACKEND AGNOSTIC EASY TO ADD AN ADAPTER BACKEND AGNOSTIC
24. /gocardless/statesman
25. ?