title: Can client web browsers subscribe to updates with a RESTful web application? (Observer Pattern)
body: <p>I'm currently developing a website where the front-end client (HTML/JavaScript running in web browsers) uses RESTful Ajax calls to communicate with my back-end server application. In many of the pages, the client absolutely needs to know if a few particular things change. But, some of these changes are very rare, so it doesn't seem very efficient to constantly have the client send requests to the server to get the state to check if it changed and do something if the state did change.</p>
  
  <p>I'm wondering if there are recommended ways to have the client (user's browser) act like an observer and subscribe to specific subjects in the server and have the server notify the client whenever a state change is made to something client subscribed to. I currently know of no realistic way to do this.</p>
  
  <p>Any guidance would be deeply appreciated, thanks!</p>