ruby 43 lines · 9 steps

Atomic money transfers with Rails transactions

A service object moves funds between two accounts atomically using row locks and a database transaction.

Explained by highlit

Rails Advanced service object database transactions row locking error handling concurrency

1class FundsTransfer
class InsufficientFundsError < StandardError; end
3 
def initialize(source:, destination:, amount:)
  @source = source
  @destination = destination
  @amount = amount
end
9 
def call
  ActiveRecord::Base.transaction do
    @source.lock!
    @destination.lock!
14 
    raise InsufficientFundsError if @source.balance < @amount
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    @source.update!(balance: @source.balance - @amount)
    @destination.update!(balance: @destination.balance + @amount)
19 
    transfer = Transfer.create!(
      source_account: @source,
      destination_account: @destination,
      amount: @amount,
      status: :completed
    )
26 
    LedgerEntry.create!(account: @source, transfer: transfer, change: -@amount)
    LedgerEntry.create!(account: @destination, transfer: transfer, change: @amount)
29 
    transfer
  end
rescue InsufficientFundsError
  errors << "Source account has insufficient funds"
  false
rescue ActiveRecord::RecordInvalid => e
  errors << e.record.errors.full_messages.to_sentence
  false
end
39 
def errors
  @errors ||= []
end
43end

01 / 01

STEP 01

Walkthrough

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Three takeaways

1Wrapping mutations in a transaction guarantees that a partial failure rolls everything back.
2Pessimistic row locks serialize concurrent transfers so two requests can't double-spend the same balance.
3Rescuing specific exceptions lets a service return a clean false-or-result instead of leaking raw errors.

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