Evergreen Continuous Integration is a custom-built CI system designed by MongoDB to meet their specific needs. Unlike most companies that rely on off-the-shelf solutions, MongoDB reinvented the wheel and created an efficient infrastructure for testing changes quickly, keeping engineers happy. The journey to creating Evergreen was born of necessity and uncertainty but yielded a powerful tool with dynamic parallelization, elastic machines allocation, and easy commit-based testing. This approach allowed MongoDB to test their codebase on many different operating system and architecture combinations, validate the correctness of builds, and iterate aggressively with confidence. By building an in-house CI tool, MongoDB was able to evaluate Go as a language, leveraging its static compilation, concurrency primitives, and cross-platform support. The creation of Evergreen has been a successful experiment that not only improved their development practices but also brought joy and excitement to the engineering team, making it a valuable experience for the company.