1_before.rb

class PaymentGatewayCallbackService
  # New custom exception
  TransactionFailed = Class.new(StandardError)

  def callback(order_id, gateway_transaction_attributes)
    order = Order.find(order_id)
    transaction = order.order_transactions.create(callback: gateway_transaction_attributes)
    # raise the exception when things went wrong
    transaction.successful? or raise TransactionFailed
    order.paid!
    OrderMailer.order_paid(order.id).deliver
  rescue ActiveRecord::RecordNotFound, TransactionFailed => e
    raise
  rescue => e
    Honeybadger.notify(e)
    AdminOrderMailer.order_problem(order.id).deliver
    raise
  end
end

class PaymentGatewayController < ApplicationController
  ALLOWED_IPS = ["127.0.0.1"]
  before_filter :whitelist_ip

  def callback
    PaymentGatewayCallbackService.new.callback(params[:order_id], gateway_transaction_attributes)
    redirect_to successful_order_path(params[:order_id]) 
  # Rescue and redirect
  rescue PaymentGatewayCallbackService::TransactionFailed => f
    redirect_to retry_order_path(params[:order_id])
  rescue ActiveRecord::RecordNotFound => e
    redirect_to missing_order_path(params[:order_id])
  rescue
    redirect_to failed_order_path(params[:order_id]), alert: t("order.problems")
  end

  # ...
end

2_after.rb

class PaymentGatewayService < Struct.new(:listener)
  def callback(order_id, gateway_transaction_attributes)
    order = Order.find_by_id(order_id)
    return listener.order_missing unless order
    transaction = order.order_transactions.create(callback: gateway_transaction_attributes)
    if transaction.successful?
      order.paid!
      OrderMailer.order_paid(order.id).deliver
      listener.order_successful(order)
    else
      listener.transaction_unsuccessful(order)
    end
  rescue => e
    listener.order_error(order, e)
  end
end

class PaymentGatewayController < ApplicationController
  ALLOWED_IPS = ["127.0.0.1"]

  before_filter :whitelist_ip

  def callback
   PaymentGatewayService.new(self).callback(params[:order_id], gateway_transaction_attributes)
  end

  # listener callbacks
  def order_successful(order)
    redirect_to successful_order_path(order.id)
  end

  def transaction_unsuccessful(order)
    redirect_to retry_order_path(order.id)
  end

  def order_missing
    redirect_to missing_order_path(params[:order_id])
  end

  def order_error(order, exception)
    Honeybadger.notify(exception)
    AdminOrderMailer.order_problem(order.id).deliver
    redirect_to failed_order_path(order.id), alert: t("order.problems")
  end

  private

  def whitelist_ip
    raise UnauthorizedIpAccess unless ALLOWED_IPS.include?(request.remote_ip)
  end
end