Another option is to use Firebase Cloud Messaging:
Using FCM, you can notify a client app that new email or other data is
available to sync.
How does it work?
An FCM implementation includes two main components for sending and
receiving:
- A trusted environment such as Cloud Functions for Firebase or an app server on which to build, target and send messages.
- An iOS, Android, or Web (JavaScript) client app that receives messages.
Client registers its Firebase key to a server. When updates are available, server sends push notification to the Firebase key associated with the client. Client may receive data in notification structure or sync it with a server after receiving a notification.
