At some point you realize that the data has grown too large for a single server, so you need to shard the data across multiple servers. You bring up another RavenDB server with the sharding bundle installed. You wait for the data to re-shard (during which time you can still read / write to the servers). You are done.
At least, that is the goal. In practice, there is one step that you would have to do, you would have to tell us how to shard your data.
In theory, auto sharding would be available in every database. In practice, there are just a few where auto sharding actually works.
Original title and link: RavenDB to Add Auto Sharding (NoSQL databases © myNoSQL)