We\'ve been doing some research for many hours already.
The first solution that appears everywhere (other StackExchange questions too) is using third-party service <
As gracefully stated by @ajostergaard, TODAY Google has announced its much waited Google Cloud Functions, but precisely for Firebase they've implemented their own specific Firebase Functions which substitutes the Triggers shown in conferences since Nov. 2014 (more than 2 years ago!) and mentioned in Firebase groups .
This is an example link that explains exactly how to do what is asked in this question: