ActiveMQ in Action–Asynchronous Processes in Amazon

• The  order  placement  process  that’s  employed  by  Amazon  is  a  good
  example of the first stage in a much larger set of asynchronous processes.
• Each stage  of  the  order  is  handled  discretely by  a  separate  service. 
• When  the  user places the order, there’s a synchronous call to submit the order,
• but the entire order process doesn’t take place behind a synchronous call via the web browser.
• Instead, the order is accepted and acknowledged immediately.
• The rest of the steps  in  the  process  are  handled  asynchronously.
• If  a  problem  occurs  that prevents the process from proceeding, the user is notified via email.

Users would be notified via emails when something bad happens will create some inconvenience. However, that’s the way we trade off for high scaling.