One of authors is here. Redis saves data to the disk by default but this can be turned off. That's what we did. GUIDs of connected users are stored in memory w

One of authors is here. Redis saves data to the disk by default but this can be turned off. That's what we did. GUIDs of connected users are stored in memory with expiration by the 10 min timeout. I think scalability does not hurt privacy. It's nice to have 10 servers with 40GB memory instead of one with 4GB. Connections between servers can be encrypted with TLS.