title: Is there any other way to send notification messages to a user without firebase?
body: <p>I am currently working on an app which will be used to send important messages as fast as possible to multiple people.</p>
  <p>I found out that there is such a thing like firebase with which it is possible to send messages from a custom server to iOS and android devices.</p>
  <p>But I need a solid and secure solution and firebase is therefore only my plan b. I would prefer a custom solution.</p>
  <p>How can I achieve that a message will be sent to a client without firebase? I do not want the client to check for a new message every x seconds. That would be a waste of data. The triggering must come from the server. Can this be achieved using the observer pattern?</p>