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
   

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2. ANDY APPLETON
   @appltn
   

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4. BACKGROUND
   

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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
   

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6. LOOKS LIKE
   A STATE MACHINE
   

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7. class Payment < ActiveRecord::Base

   has_many :payment_actions

   

   def mark_as_submitted

   self.payment_actions.create(..)

   end

   end

   

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8. DAY 0
   Mandate
   created
   DAY 1
   Payer's bank
   receives mandate
   DAY 2
   Mandate
   approved
   (DAY 3)
   Merchant notified
   of failure
   

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9. class Mandate < ActiveRecord::Base

   has_many :mandate_histories

   

   def mark_as_submitted

   self.mandate_histories.create(..)
   end

   end

   

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10. DUPLICATION
    INCONSISTENCY
    BUT IT WORKS
    (AND WE HAVE A
    PRODUCT TO BUILD)
    DUPLICATION
    INCONSISTENCY
    

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11. €
    NEXT ON THE
    ROADMAP
    

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12. EURO PAYMENTS
    FOLLOW A DIFFERENT
    TIMELINE
    

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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

    

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14. TIME FOR
    SOME THINKING
    

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15. TIME FOR
    A STATE MACHINE
    ABSTRACTION
    

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16. AUDIT TRAIL
    DATA INTEGRITY
    COMPOSABLE
    AUDIT TRAIL
    DATA INTEGRITY
    

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17. NOTHING REALLY
    HIT THE MARK
    

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19. class PaymentStateMachine

    include Statesman::Machine

    

    state :pending, initial: true

    state :submitted

    

    transition from: :pending,

    to: :submitted

    end
    

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20. class Payment < ActiveRecord::Base

    has_many :payment_actions

    # ...

    def state_machine

    @state_machine ||=
    PaymentStateMachine.new(self,

    transition_class: PaymentAction)

    end

    end
    

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21. class PaymentStateMachine

    # ...

    guard_transition to: :paid do |pmnt|

    pmnt.has_active_mandate?

    end

    end

    

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22. class PaymentStateMachine

    # ...

    after_transition to: :paid do |pmnt|

    PaymentNotifier.send_mail(pmnt)

    end

    end

    

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23. BACKEND AGNOSTIC
    EASY TO ADD AN
    ADAPTER
    BACKEND AGNOSTIC
    

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24. /gocardless/statesman
    

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25. ?
    

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