Living Without Atomic Clocks

It’s a fact that the design of CockroachDB is based on Google’s Spanner data storage system. One of the most surprising and inspired facets of Spanner is its use of atomic clocks and GPS clocks to give participating nodes really accurate wall time synchronization. The designers of Spanner call this ‘TrueTime’, and it provides a tight bound on clock offset between any two nodes in the system. TrueTime enables high levels of external consistency. As an open source database based on Spanner, our challenge was in providing similar guarantees of external consistency without atomic clocks.

If someone knows even a little about Spanner, one of the first questions they have is: “You can’t be using atomic clocks if you’re building an open source database; so how the heck does CockroachDB work?”

It’s a very good question.

CockroachDB was designed to work without atomic clocks or GPS clocks. It’s an open source database intended to be run on arbitrary collections of nodes: from physical servers in a corp development cluster to public cloud infrastructure using the flavor-of-the-month virtualization layer. It’d be a showstopper to require an external dependency on specialized hardware for clock synchronization.

So what does CockroachDB do instead? Well, before answering that question, it’ll be helpful to dig a little deeper into why TrueTime was conceived for Spanner.