I develop an iOS application and I'm using Node & Postgres for handling user data and authentication. I would like to add a chat feature to my app and I chose Firestore for this.
My question is how should I link the existing user data with a Firebase collection of users without re-implementing the authentication part from the ground up. Right now the authentication is based on JWTs with access and refresh tokens.
In order to link my existing users, I can add the userId already present in my database to the users collection that I will create in Firestore, but I need some kind of authentication to ensure security.
While the token for Firebase is also a JWT, it will probably have to be separate from your existing JWT, as it contains Firebase-specific information (such as the project ID), and they're signed with different keys.
After you authenticate the user with your own backend, mint a custom token for Firebase authentication with the information you want to use in Firebase/Firestore and security rules, pass that to the client, and sign them in to Firebase with it.