When you should use Riak
- You need a reliable, extremely fast non-relational database.
- You need private cloud storage for non-huge files which can be called directly through a browser.
- You need to play with your database via a simple REST interface.
- AP + Eventual consistency from CAP is OK with you.
Link: An Overview of Riak: an Open Source NoSQL Database at Uptime & Performance Tips via blog.monitis.com