Infrastructure for the one we have here in .au, which started out as a copy/paste from the .sg opentrace app's git repo, is minuscule.
The only backend touches that apps do is a few login/register APIs when you first launch it, and if you ever test positive and agree to send your data it sends a big json blob to an AWS Lambda nodejs function that pretty much just uploads the json to S3.
I don't recall how complex the login/register stuff is, but I don't remember thinking "That's gonna need heroics to register 10 or 20 million users quickly" when reviewing the code.
The only backend touches that apps do is a few login/register APIs when you first launch it, and if you ever test positive and agree to send your data it sends a big json blob to an AWS Lambda nodejs function that pretty much just uploads the json to S3.
I don't recall how complex the login/register stuff is, but I don't remember thinking "That's gonna need heroics to register 10 or 20 million users quickly" when reviewing the code.
(If you wanna go look yourself, try here: https://github.com/opentrace-community )