In this paper, we present a novel, robust, scalable, and open-source online code execution system called Judge0. It features a modern modular architecture that can be deployed over an arbitrary number of computers and operating systems. We study its design, comment on the various challenges that arise in building such systems, compare it with other available online code execution systems and online judge systems, and finally comment on several scenarios how it can be used to build a wide range of applications varying from competitive programming platforms, educational and recruitment platforms, to online code editors. Though first presented now, Judge0 is in active use since October 2017 and has become a crucial part of several production systems.