Comment by abeisgreat

Writing directly to Firebase is rarely done past the MVP stage. Normally it's the reading which is done directly from the client. Generally writes are bounced through Cloud Functions or a traditional server of some form. Some also "fan out" data, where a user has a private area to write to (say a list of tweets) then they get "fanned out" to follower's timelines via an async backend process which does any verification / cleansing as needed.