How we migrated application servers from Unicorn to Puma | GitLab

Both Unicorn and Puma are web servers for Ruby on Rails. The big difference is that Unicorn is a single-threaded process model and Puma uses a multithreaded model.Unicorn has a multi-process, single-threaded architecture to make better use of available CPU cores (processes can run on different cores) and to have stronger fault tolerance (most failures stay isolated in only one process and cannot take down GitLab entirely). On startup, the Unicorn ‘main’ process loads a clean Ruby environment with the GitLab application code, and then spawns ‘workers’ which inherit this clean initial environment. The ‘main’ never handles any requests.