Giant monolithic source-code repositories are one of the fundamental pillars of the back end infrastructure in large and fast-paced software companies. The sheer volume of everyday code changes demands a reliable and efficient change management system with three uncompromisable key requirements-always green master, high throughput, and low commit turnaround time. Green refers to a master branch that always successfully compiles and passes all build steps, the opposite being red. A broken master (red) leads to delayed feature rollouts because a faulty code commit needs to be detected and rolled backed. Additionally, a red master has a cascading effect that hampers developer productivitydevelopers might face local test/build failures, or might end up working on a codebase that will eventually be rolled back. This paper presents the design and implementation of SubmitQueue. It guarantees an always green master branch at scale: all build steps (e.g., compilation, unit tests, UI tests) successfully execute for every commit point. SubmitQueue has been in production for over a year, and can scale to thousands of daily commits to giant monolithic repositories.