title: Sending notification from one app to another app in another device in Android
body: <p>I am creating <em>two different ANDROID apps</em> (each on two separate devices and different locations), one for the <em>user</em> and another for the <em>merchant</em>. Here, I need to send a notification from the merchant's app to the user's app and vice versa  via a button clicks (both typically being on different devices, but both ANDROIDS), which the notification should be a popup or a dialog. I'm truly embarrassed by this concept. Could some one help me on this. A sample full source code would be really appreciable, since I am a beginner in ANDROID.
  Thanks in advance..!</p>