Wow, we came across the same thing while building epimetrics.in back in 2014-2015. We built the analytics engine and tested it in one mega Indian festival calle

Wow, we came across the same thing while building epimetrics.in back in 2014-2015. We built the analytics engine and tested it in one mega Indian festival called "Kumbh Mela" with over 20 Million people gathering at one place. In our case, we had to struggle with the sheer amount of people as well. We learnt a bunch of stuff and right away decided to have offline storage option and send data back to db when connection resumes. We also did the research on how to save battery life and use muscular memory to enable our users to use our system in that setup as efficiently as possible. But at the end, it was very satisfying when we were leading on the front to spot the diseases. We helped attend over 2 million patients efficiently. If interested, I would love to share our insights plus few decisions which were very helpful for our adoption from health workers as well as govt health department officials. Good work guys! Keep plugging! :) Edit: Added festival name.