Ionic 2 and Firebase Cloud Functions

Hello guys,
I am kind of new to web development and already got some usermanagement set up in Ionic 3 (which doesn’t differ too much from Ionic 2 in that regard) using the Firebase Realtime Database.
Now I need some server side logic with a simple request from the client resulting in a response from the server.
Is a simple API call to my Firebase-Website (where the Firebase Cloud Functions are) with HTTP the way to go or are there better ways to achieve server side logic? I suppose doing the logic on the client side by just using the Realtime Database is not the best idea, since the user needs more access-rights on my Database than he is supposed to have.

So what is the best way to call Firebase Cloud Functions ?