title: How to get all of device tokens from Firebase?
body: <p>How to get device tokens (FCM registration tokens) from Firebase? </p>
  
  <p>I'm making mobile app using firebase and its server using node.js. </p>
  
  <p>I want to send <code>push notification</code> message from server, but I don't know how to get devices token. </p>
  
  <ol>
  <li><p>How to get device tokens from external server? I'm using Firebase admin SDK now.</p></li>
  <li><p>Is the device token only generated when the app is connected fcm server?</p></li>
  <li><p>Should I save token to another database when user first run the app and register FCM server?</p></li>
  </ol>