this is a bit of a composite question, I'll try my best to separate the various parts even if they all have a common intent.
Common intent
Having a clear way of delivering notifications about posts regarding specific topics to Firebase Users (and not simply to application instances).
I have tried various methods, and I can't find a definitive answer about which one is the best one.
Method 1 - Relying only on the database
Each Firebase User has its own document in the Firestore at users/{userId}
This document contains two collections: tokens and interests.
The token collection contains a list of documents which have FCM tokens as one of their fields. Each time an user signs in the application or FirebaseInstanceIDService.onTokenRefresh() is called, the collection is updated to add the new token.
The interest collection contains a list of interests which simply are strings and are used as tags for posts. This collection has a mirror as interests/{interestId}/users/{userId} showing all the users interested in something. (This is kept updated and synchronised via a Cloud Function)
When a new post is created under a specific interest, I can get a list of all the users interested and then get their tokens from their document. Finally, I send a notification to each individual token.
Problems
- This solution is not elegant (this isn't that big of a problem)
- With the new GDPR rules I fear I might not be allowed to save tokens directly on the Database
- If the user signs out when he's offline, the token isn't removed from his document, and the new user receives notifications for the old interests.
- Should I keep track of what the current token is and update it each time an user signs in ignoring
FirebaseInstanceIDService.onTokenRefresh()? Else only the user signed in when the service is called would update the database.
Method 2 - Using FCM topic subscriptions
This should be the best option for me, but I can't understand how to make it work with multiple users on the same phone (always one at a time though)
The way I would handle this is still have the users/{userId}/interests collection, removing users/{userId}/tokens and interests/{interestId}/users, and subscribe/unsubscribe from the various topics as the user signs in and out.
Problems
- What happens if the user signs out when he's offline? There is no way to retrieve the current subscriptions and remove each one, potentially resulting in conflicting topics subscriptions.
Thank you very much for your time
