Nice work and some thoughs

The last time I checked, App Engine offered about 1GB database in the free tier. Which is enough to store all pairings and all uploaded battles, as long as you delete data from retired bots once in a while.

As for the amount of clients the server can handle, it should not really be an issue, since there are usually 3 to 4 simultaneous clients at most.

If you want to use some batch processing, adding a Ranked Pairs ranking would make my day. I has O(n^4) complexity, but I think it can still fit inside the 10 minutes window from cron, so no need for a backend.

The problem isn't so much total storage space, but that I'm limited at 50k writes per day. Each bot counts as 2 writes, so effectively I have 25k updates I can do. I've figured out a caching scheme so that each melee battle comes out to 10 updates (1 per bot) instead of 45 updates (1 per pairing). I also need to update the total rumble battles count and the user upload count, so that leaves a bit of overhead, meaning I can have ~2000 melee battles per day uploaded.

I'l see what ratings systems are feasible..

I've actually figured out a sort of temporary caching between requests where I wait for bots to accumulate a certain number of pairings before pushing them to disk. I don't think it's necessary to re-work the rumble upload protocol yet. One thing I would like the rumble to tell me is how many bots are in a melee battle though. Right now I just have it hardcoded at 10. It would help with my caching if I knew how many they were uploading per battle.

Flush them to the database on a period basis (constant number of commits regardless of number of uploads) instead of being per number of uploads?