So what do we mean by always on? Five 9s, ten 9s? This talk is about the hardest part of the stack to make available: the database.;Do you remember CAP theorem from University? How many AP systems have you built or used? Trading off consistency for being always on is a decision some companies have had to make (we can’t live without Netflix or Amazon can we?). ;Most of the talk will be about Apache Cassandra. However we'll first go through the general requirements of any database system when you want to remain online 24/7, this will help you to evaluate any database technology. ;What you’ll learn is if you want to build an AP systems with a database like Cassandra, we as developers need to get out of a relational mindset (no free lunch). We need to model our data differently and develop with fault tolerance in mind to handle things like: concurrent updates from multiple datacenters, hardware and network failure and throughputs in the millions of transactions a second.