There are two parts to this that are really challenging -- designing a robust system (since there are a million different edge cases that can happen in a distri

There are two parts to this that are really challenging -- designing a robust system (since there are a million different edge cases that can happen in a distributed system), and designing an intuitive interface so that the database does the right thing out of the box, is configurable if users want that, and is actually easy to configure in practice. Doing each thing independently is hard; doing both of them together in one system is really really hard. I'm particularly proud of how nicely everything clicks together, and how elegant the administration API is. It seems very simple and is extremely powerful; getting that right took many iterations over the course of a year.