Implement Background Job

EVERY job MUST have its test written and validated BEFORE implementation.
  1. Write the job spec (idempotency, retry, error handling)
  2. Run the spec — verify it fails because the job does not exist yet
  3. ONLY THEN write the job class

EVERY job that performs a side effect (charge, email, API call) MUST have
an idempotency check BEFORE the side effect.

EVERY perform method should do only three things:
  1. Load the record from the passed ID
  2. Guard for idempotency / permanent no-op conditions
  3. Delegate the side effect or orchestration to a service object

If perform needs more than that, extract a service.

After implementation: run full suite, confirm job appears in queue dashboard,
verify idempotency by enqueueing twice and checking the second run is a no-op.

# Bad — object may be stale or deleted by perform time
SomeJob.perform_later(@order)

# Good — reload fresh inside perform
SomeJob.perform_later(@order.id)

class SendInvoiceReminderJob < ApplicationJob
  queue_as :default
  retry_on Net::OpenTimeout, wait: :polynomially_longer, attempts: 5
  discard_on ActiveRecord::RecordNotFound

  def perform(invoice_id)
    invoice = Invoice.find(invoice_id)
    return if invoice.reminder_sent_at?

    InvoiceReminders::Send.call(invoice:)
  end
end

module InvoiceReminders
  class Send
    def self.call(invoice:)
      InvoiceMailer.overdue(invoice).deliver_now
      invoice.update!(reminder_sent_at: Time.current)
    end
  end
end

# config/recurring.yml
production:
  nightly_cleanup:
    class: "NightlyCleanupJob"
    schedule: "0 2 * * *"
  hourly_sync:
    class: "HourlySyncJob"
    schedule: "every 1 hour"
    queue: low