All entities and groups are stored in a database for persistence. You must choose what kind of database you wish to use with NetAuth. A discussion of available databases can be found below.
ProtoDB is the default storage engine for NetAuth. This option will simply use directories and files on disk to store the protocol buffers that represent NetAuth's internal state. This option should be reasonably performant for many users and should scale well across small to medium installations.
ProtoDB is safe to synchronize across multiple servers for high availability and the disk state is consistent as much as possible, but a crash during a write may corrupt the entity or group that was being written, so it is recommended to backup the data directory regularly.