DDD Boundaries Review — Examples

# app/services/billing/invoice_service.rb
class Billing::InvoiceService
  def call(reservation_id)
    reservation = Fleet::Reservation.find(reservation_id)
    reservation.update!(status: :invoiced)  # Billing mutating Fleet state
    create_invoice(reservation)
  end
end

# Fleet publishes an outcome; Billing subscribes via a job or hook
class Fleet::Reservation < ApplicationRecord
  def complete!
    update!(status: :completed)
    ReservationCompletedJob.perform_later(id)  # Fire-and-forget event
  end
end

# app/services/billing/invoice_service.rb — no Fleet constants
class Billing::InvoiceService
  def call(reservation_id:, amount_cents:)
    create_invoice(reservation_id:, amount_cents:)
  end
end

Severity: High
Contexts: Billing → Fleet
Leaked term: reservation.update!(status: :invoiced)
Risk: Billing owns Fleet state transitions. Changes to Fleet lifecycle break Billing silently.
Smallest credible fix: Fleet emits ReservationCompleted event; Billing reacts without touching Fleet models.