I Have an app with over 12k active users. Now I want to implement push notifications using Firebase Cloud Messaging, the problem here is I need to store all the users devices tokens somewhere on a server in a Json file to store and retrieve these IDs later to send push notifications to all of the users. Now I am thinking of using Firebase RealTime Database to store these tokens, but I don’t know whether it is the right choice or not, because I don’t want to pay for a server that stores just array of device IDs ( tokens ). Later my app grows and I don’t know if Firebase Realtime Database is free or not for storing just some device IDs.