My recent project - a replacement for CodeMaster's RaceNet, runs on flat files! https://dirtforever.net/
Just have to use locks to be careful with writes.
I figured I'd migrate it to a database after maybe 10k users or so.
Neat, what happened to the original system? Last I checked multiplayer was working in DR2.
And crashes you can exclude? Good luck!
Neat, what happened to the original system? Last I checked multiplayer was working in DR2.