Is it possible to have a Firebase Function that is triggered by changes to a Firestore that lives in a seperate Firebase project to the Function?

Question

Let's say I have a Firebase project named "A". Within this project, I have a Cloud Firestore triggered Firebase function that needs to run when a document within Firestore changes. By default, the Firebase Function will listen to changes within Firestore on project A.

However, let's say I have a particular use case where there is a second Firebase project named "B". I need the Firebase Function within Project A to be triggered on Firestore changes that happen to Firestore within project B.

Is this possible? Firebase docs do show initializing multiple projects, which would allow me to connect to multiple databases as such:

const admin = require("firebase-admin");
const serviceAccount = require("path/to/serviceAccountKey.json");

const secondaryAppConfig = {
  credential: admin.credential.cert(serviceAccount),
  databaseURL: "https://<DATABASE_NAME>.firebaseio.com"
};

// Initialize another app with a different config
const secondary = firebase.initializeApp(secondaryAppConfig, "secondary");

// Retrieve the database.
const secondaryDatabase = secondary.database();

But this doesn't allow me to trigger a Firestore Triggered Firebase Function on my secondary project. Firebase functions call the firebase-functions methods directly, whereas calling a database calls the initialized project.

const functions = require('firebase-functions');

exports.myFunction = functions.firestore
  .document('...')
  .onWrite((change, context) => { /* ... */ });

Is what I would like to do possible? Or does anyone have a workaround (other than creating this Firebase Function within project B)?

Answer 1

It's not possible. Cloud Functions triggers can only fire in response to changes in the resources of the project where they are deployed. This is true for all types of triggers, including Firestore.

If you want code to run in response to changes in another project, the function will have to be deploy to that project.