I want that firebase sends a notification to my app when my database is updated. For example when I write a new entry in this database, the app should receive a notification, even if the app is closed.
Since you are using Firebase I would suggest you to use the feature
Cloud Messaging. You can send a
Data Message and the application knows that the database was updated. Check more info about it here: https://firebase.google.com/docs/cloud-messaging/concept-options