Courtney Couch describes in much detail the solution used to scale Redis. Including application-level sharding, replication, and persistency.
The web is abound with warnings and cautionary tales about going this route.
There are horror stories about lost data, hitting memory limits, or people
unable to effectively manage the data within Redis, so you might be
wondering “What on earth were you thinking?!” So here is our story, why we
decided to use Redis anyway, and how we overcame those issues.
Original title and link: Redis as the Primary Data Store ( ©myNoSQL)