Idempotent Job with Advisory Lock

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                  module AdvisoryLock
  module_function

  def with_lock(key)
    lock_id = Zlib.crc32(key.to_s)
    got_lock = ApplicationRecord.connection.select_value("SELECT pg_try_advisory_lock(#{lock_id})")
    return false unless got_lock

    yield
  ensure
    ApplicationRecord.connection.execute("SELECT pg_advisory_unlock(#{lock_id})")
  end
end

                  class Payments::CaptureJob < ApplicationJob
  queue_as :payments

  def perform(payment_id)
    payment = Payment.find(payment_id)
    key = "payment:capture:#{payment.id}"

    AdvisoryLock.with_lock(key) do
      payment.with_lock do
        return if payment.captured_at?

        gateway.capture!(payment.gateway_reference)
        payment.update!(captured_at: Time.current)
      end
    end
  end

  private

  def gateway
    @gateway ||= PaymentGateway.new
  end
end

I reached for idempotency the moment retries started duplicating side effects. In Advisory lock helper, I generate a deterministic lock ID and use pg_try_advisory_lock to ensure only one worker owns the critical section; the ensure block always calls pg_advisory_unlock so I don’t leak locks. Then, in Idempotent job, I build an execution key like payment:capture:<id> and wrap the entire capture flow in AdvisoryLock.with_lock. Inside payment.with_lock, I do a second layer of safety by checking captured_at? before calling the gateway, then I persist captured_at as the one true “done” marker. The result is “exactly-once-ish” behavior without pretending the world is perfect.

Sarah Chen

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Idempotent Job with Advisory Lock

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