Why does only writing data to Firebase Database cost me 12kB/minute in downloads?

Question

I am using native Firebase Javascript SDK's on IoT device running Node-red environment.

• My code consist only WRITE and DELETE operations on the Firebase RealtimeDatabase data.
• When the IoT device turns on the connection is also established.
• But other then that I never READ any data from Firebase Realtime database.

Even though I don't READ any data, when I looked this morning into the Firebase console I saw a graph that showed me constant 12kB/minute downloads and 1 peak connection. I though some bot connected to my database. So I turned off my IoT device and I saw in the graph that also the connection went from 1 to 0 (so I am certain only my IoT device was connected to the database).

Also the downloads went from 12kB/minute to 0kB/minute:

• I will add my code below - but I am confused what data did I downloaded if I didn't use any on, onChildAdded, onValue etc. methods.
• I am pretty new to Firebase Database so if I am doing some rookie mistake just point me to the right direction.

Why does my IoT device download any data when I am only using READ methods?

---

Code which runs when the IoT device turn's on:

//Load data from Global contexta
const app = global.get('app');
const database = global.get('database');
const firestore = global.get('firestore');
const auth = global.get('auth');


const firebaseConfig = {
  //my credentials 
};

//Set up Firebase
const fb_app = app.initializeApp(firebaseConfig);
const fb_db = database.getDatabase(); //kdybychom měli vice projektu tak app dame do parametru
const fb_ft = firestore.getFirestore();
const fb_auth = auth.getAuth();

//Save the database reference to Global context
global.set('fb_app', fb_app);
global.set('fb_db', fb_db);
global.set('fb_ft', fb_ft);
global.set('fb_auth', fb_auth);

Code that writes data every 1 second:

const fb = global.get('database');
const fb_db = global.get('fb_db');

var timestamp = Math.round(msg.payload / 1000);
var UID = 'uid1';

var a1 = 0  + Math.floor(Math.random() * 100);
var p1 = 50 + Math.floor(Math.random() * 20);
var t1 = 20 + Math.floor(Math.random() * 20);

//Send data to Firebase
const ref = fb.ref(
    fb_db,
    'device_realtime/' + UID + '/' + timestamp.toString()
);

fb.set(ref, {
    a1: a1,
    p1: p1,
    t1: t1
});

Code that deletes data older then 60 seconds:

const fb = global.get('database');
const fb_db = global.get('fb_db');

var timestamp1 = Math.round(msg.payload / 1000) - 60;
var timestamp2 = Math.round(msg.payload / 1000) - 61;
var UID = 'uid1';

//Delete old data from firebase
var reference1 = fb.ref(
    fb_db,
    'device_realtime/' + UID + '/' + timestamp1.toString()
);

var reference2 = fb.ref(
    fb_db,
    'device_realtime/' + UID + '/' + timestamp2.toString()
);

fb.remove(reference1)
fb.remove(reference2)

Code that writes data every 1 minute:

const fb = global.get('database');
const fb_db = global.get('fb_db');

var timestamp = Math.round(msg.payload / 1000);
var UID = 'uid1';

var a1 = 0  + Math.floor(Math.random() * 100);
var p1 = 50 + Math.floor(Math.random() * 20);
var t1 = 20 + Math.floor(Math.random() * 20);

//Send data to Firebase
const ref = fb.ref(
    fb_db,
    'device_trends/' + UID + '/' + timestamp.toString()
);

fb.set(ref, {
    a1: a1,
    p1: p1,
    t1: t1
});

Screenshot of the entire flow:

Answer 1

To expand on the discussion in comments:

That Downloads figure takes into account all outbound traffic from the database, including e.g. any possible OK responses to writes made.

Based on the figures quoted and some napkin math, it seems that every write corresponds to about 200 bytes of data downloaded.