[akanet]

I can't tell from the docs - are server functions able to to modify or reject writes to refs in the realtime DB before they happen?

[inlined]

No. All events are asynchronous. This use case is best simulated by having clients post to a queue. Cloud functions then either discards or applies writes in that queue.

[RoboTeddy]

You should add this technique to the docs! It makes you realize that the Cloud Functions feature is a lot more powerful than you might initially think.

[troycarlson]

A lot of people use the Firebase Queue library for this:

https://github.com/firebase/firebase-queue