Puma

Uses multi-threaded and multi-process architecture to handle concurrent requests efficiently, with cluster mode reducing memory via copy-on-write for better throughput.

Inherits a robust HTTP parser from Mongrel with over 15 years of production use, making it the default and most popular server for Ruby on Rails.

Includes built-in SSL support, zero-downtime rolling restarts, and a request bufferer, allowing deployment without a reverse proxy and reducing infrastructure overhead.

Offers extensive setup via CLI, environment variables, or config files with lifecycle hooks, plus a control server for runtime management and monitoring.