How to get Firebase user id from FCM token? (in admin code on server)

Question

My signed-in Android clients register their tokens with my server. The problem is I am not sure whether the token sent by a particular client genuinely belongs to that clien

Answer 1

On a Firebase level there is no connection between a Authentication UID, and a Cloud Messaging Instance ID token.

A Firebase Authentication UID uniquely identifies a user. If that user signs in on a different device, they have the same UID value.

A Firebase Authentication Instance ID token identifies a single app on a single device. If a different user signs in to that app, the app will keep the same Instance ID token.

If you want to associate a UID with an FCM tokens, you have to do that in your application code. For example, many developers clear the FCM Instance ID tokens when the user signs out of their app. This ensures that a new user who signs in on the same device will get a new Instance ID token.

Also see:

• Firebase Cloud Messaging Auth Tokens vs Registration Token
• What is the proper way to capture device registration tokens in order to send notifications via Firebase Cloud Messages?