Once you write and deploy a function, Google's servers start listening to those functions immediately, that is listening for events and running the function when it gets triggered. As the load of your app increases or decreases, it responds by rapidly scaling the number of virtual server instances needed to run your function faster. If the function is deleted, idle, or updated by you, then instances are cleaned up and replaced by new instances. In the case of deletion, it also removes the connection between functions and the event provider.

Given here are the events that are supported by Cloud Functions:

• onWrite(): It triggers when data is created, destroyed, or changed in the Realtime Database
• onCreate(): It triggers when new data is created in the Realtime Database
• onUpdate(): It triggers when data is updated in the Realtime Database
• onDelete(): It triggers when data is deleted from the Realtime Database

Here's a code sample of the cloud function makeUppercase:

exports.makeUppercase = functions.database.ref('/messages/{pushId}/original')
 .onWrite(event => {
 // 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);
 });