The forum runs on 6 dedicated servers with mongo replication, redis cluster and balancer at the nginx level.
Seems like you've ticked most of the boxes. I wonder, however, if you have explored mongodb sharding?
I prefer PostgreSQL. Hence, my latest putzing about is with postgres. And MongoDB's licensing games are a concern. But I don't think it sucks as bad as being portrayed here. Especially when you've not implemented sharding, which is what I was referencing here:
Mongodb scales horizontally very well.
@volanar Uh, yeah, I now recall we discussed this previously. Not consistent with my last round of testing, which has admittedly been some while.
YMMV. Have fun! 🐕