How to write custom code (logic) when using firebase [duplicate]

Answer 1

Nowadays there isn't.

But they are working on Firebase Triggers that will supply that need and should be released soon. (There is no release date yet).

What you can do, is create a nodejs server running a background task that process a queue. You can use https://github.com/firebase/firebase-queue for that. It is already prepared to work with many "workers" so it is easy to scale if you need. (Creating more nodejs servers).