Simon Willison’s Weblog


Before we even started writing the database, we first wrote a fully-deterministic event-based network simulation that our database could plug into. This system let us simulate an entire cluster of interacting database processes, all within a single-threaded, single-process application, and all driven by the same random number generator. We could run this virtual cluster, inject network faults, kill machines, simulate whatever crazy behavior we wanted, and see how it reacted. Best of all, if one particular simulation run found a bug in our application logic, we could run it over and over again with the same random seed, and the exact same series of events would happen in the exact same order. That meant that even for the weirdest and rarest bugs, we got infinity “tries” at figuring it out, and could add logging, or do whatever else we needed to do to track it down.

[...] At FoundationDB, once we hit the point of having ~zero bugs and confidence that any new ones would be found immediately, we entered into this blessed condition and we flew.

[...] We had built this sophisticated testing system to make our database more solid, but to our shock that wasn’t the biggest effect it had. The biggest effect was that it gave our tiny engineering team the productivity of a team 50x its size.

Will Wilson, on FoundationDB