Cloud Functions for Firebase OnWrite

18
votes

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 ?

2
firebasefirebase-realtime-databasegoogle-cloud-functions
Have you considered setting the modification date as part of the write you make from the client? Using ServerValue.TIMESTAMP? - cartant
Thats really only an example... I don't want a solution on how to set a modification date. I want a solution on how I can change data serverside. - Mark
Lets say, the client changes data and I need to process a financial transaction and then delete the record. A delete is a change to OnWrite so I'm not sure how to organise that code. - Mark
No worries. However, I'd suggest you edit the question to use your last comment as the example, as I think it's a much more likely use case. - cartant

2 Answers

19
votes

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
  });
});
6
votes

I understand firebase functions provides a better method:

The method is to verify if a previous event exists. If so, do nothing and return else, do you job... Besides you can verify if item is being deleted.

  if (event.data.previous.exists()) {
    return;
  }
  // Exit when the data is deleted.
  if (!event.data.exists()) {
    return;
  }

This is full example from firebase documentation.

exports.makeUppercase = functions.database.ref('/messages/{pushId}/original')
    .onWrite(event => {
      // Only edit data when it is first created.
      if (event.data.previous.exists()) {
        return;
      }
      // Exit when the data is deleted.
      if (!event.data.exists()) {
        return;
      }
// Grab the current value of what was written to the Realtime Database.
const original = event.data.val();
console.log('Uppercasing', event.params.pushId, original);
const uppercase = original.toUpperCase();
// You must return a Promise when performing asynchronous tasks inside a Functions such as
// writing to the Firebase Realtime Database.
// Setting an "uppercase" sibling in the Realtime Database returns a Promise.
return event.data.ref.parent.child('uppercase').set(uppercase);