Riak, REE-ahk, combines the power of Erlang with the ubiquity of JSON to deliver a highly scalable document store over REST. Hold on CouchDB fans, before you pounce, you haven’t heard this one before.
Riak is architected for master-less replication from the GET-go. All nodes participate equally in the replication so scaling horizontally means just adding new servers to join the party.
Links for powerful object graphs
Riak supports link-walking which is far more powerful than traditional relational DB joins. As Sean Cribbs explains:
Every datum stored in Riak can have one-way relationships to other data via the Link HTTP header. In the canonical example, you know the key of a band that you have stored in the “artists” bucket (Riak buckets are like database tables or S3 buckets). If that artist is linked to its albums, which are in turn linked to the tracks on the albums, you can find all of the tracks produced in a single request. As I’ll describe in the next section, this is much less painful than a JOIN in SQL because each item is operated on independently, rather than a table at a time. Here’s what that query would look like:
More powerful Map/Reduce
Map/Reduce works a bit differently in Riak. Riak’s Map function expects a list of keys on which it should run. You can of course pass every key in your bucket, but doing Map in this way allows Map to be run on the node where the data is actually stored.