title: Whats the best way to add listener to external resource?
body: <p>I have a django project and external resource. The external resource has XML data which changes time to time but it still static if nothing happened. I need to collect new data if something happened.
  The ways I see:
  1. Grab content one time in some period and check if it is different.
  2. Save content as file and check length (worse way).</p>
  
  <p>Unfortunately, I can't predict when content will change also I can't use WebSockets, Long Polling and WebHooks.
  Please, describe me the ways that in modern WWW today may solve my problem!</p>