user specific topics isn't the right path, conventional wisdom would be that you want < 1000s of topics.

It looks like that limit has been changed more recently: https://stackoverflow.com/questions/32950503/can-i-have-100s... but I'm still skeptical it's what you want. You'd need a consumer for each of them, you're tracking offsets for all of them.

Much more normal would be a single topic here, partitioned on user.

You can have 2 consumers then. One that pulls things off and notifies. Another one that pulls things off and stores to the DB, which is indexed by user ID.

Then when they read a notification, you flip a bit in the DB for the notifications.