Firebase Database as back end for your Android/iOS app

Take a look at Cloud Firestore.

The author talks a lot about scalability but Google recently posted [0] that Cloud Firestore is similar to Firebase Realtime DB and more scalable.

[0] https://firebase.google.com/docs/database/rtdb-vs-firestore

"Cloud Firestore is Firebase's new flagship database for mobile app development. It improves on the successes of the Realtime Database with a new, more intuitive data model. Cloud Firestore also features richer, faster queries and scales better than the Realtime Database."