Firestore is perfectly usable on the backend without a client app . But if you are shipping an app, I’d strongly recommend integrating Firebase Authentication first, and you should think carefully about structuring your data and rules from there.
Read moreCan I use Firestore without authentication?
To be honest, without Firebase Authentication, it’s not possible to accept writes to a database without Authentication and also avoid abuse, since anyone could write anything from anywhere on the internet. This could also cost you large amounts of money if someone discovers your “open” database.
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