As Netflix moved into the cloud, we needed to find the appropriate mechanisms to persist and query data within our highly distributed infrastructure. Our goal is to build fast, fault tolerant systems at Internet scale. We realized that in order to achieve this goal, we needed to move beyond the constraints of the traditional relational model. In the distributed world governed by Eric Brewer’s CAP theorem , high availability (a.k.a. better customer experience) usually trumps strong consistency. There is little room for vertical scalability or single points of failure. And while it is not easy to re-architect your systems to not run join queries, or not rely on read-after-write consistency (hey, just cache the value in your app!), we have found ourselves braving the new frontier of NoSQL distributed databases.
Source: Netflix Tech Blog