I built a real time leaderboard that uses SSE (Server Sent Events) as the main access pattern. When I tried to scale it I hit a limit of 28232 connections, with resource usage being nominal.

This was very peculiar, and the post describes my journey figuring out the cause of the issue and how I went past that limit, eventually scaling to 150k connections on a single machine (and my ideas on how to take this further, as I hit memory limits at this point).