Cloud Functions for Firebase OnWrite

Question

I'm looking at the New Cloud Functions for Firebase and it says when doing an OnWrite you should be careful not to save data back to the same Child. (which will fire off the trigger again).

So I'm trying to figure out, how do I set a modification date on a record ?

Answer 1

The issue isn't that you can't or shouldn't change the data, but that you need to guard against infinite loops. For instance, setting a timestamp could retrigger the function which would set the timestamp which would retrigger...and so on.

What you can do, however, is guard your code by making sure to mark the state in an idempotent way so the same code doesn't retrigger. For example:

exports.doThing = functions.database.ref('/events/{id}').onWrite(ev => {
  // prevent loops by guarding on data state
  if (ev.data.child('didThingAt').exists()) return;

  // do thing here...

  return ev.data.adminRef.update({
    didThingAt: admin.database.ServerValue.TIMESTAMP
  });
});