This template shows you how to build a presence system (understanding which users are online / offline) in Firestore with some help from the Realtime Database. An explanation of why this is necessary, how it works, and what changes you need to implement in a client app can be found in the docs.

See file functions/index.js for the code.

The dependencies are listed in functions/package.json.

To install:

cd functions
npm install

As an example we'll be using a secure note structure:

/status
    /UID_A
        state: "online"
    /UID_B
        state: "offline"

Whenever a new note is created or modified a Function sends the content to be indexed to the Firestore instance.

The sample client app in public/index.html and public/index.js will anonymously log in the user, create status/UID_A with a last_changed and state in Realtime Database, then mirror that over to a UID_A document in the app's Firestore collection status.

To deploy the sample to your Firebase app,

1. Run npm install to install dependencies for the server-side functions as detailed above.
2. From this top-level sample directory, deploy the Realtime Database trigger defined in functions to Firebase Functions and the public directory app to Firebase Hosting.

Assumimg you've created a Firebase application called firebase-example-123 (make sure it's upgraded to the Spark plan and that Anonymous Authentication are enabled).

firebase use --add firebase-example-123
firebase deploy

Then visit https://firebase-example-123.web.app in your browser and you should see User <UID_A> is online written to the index file, as well as the associated data in Realtime Database and Firestore.