Rainbows! 是一个为 Rack 应用而开发的 HTTP 服务器，基于 Unicorn 。主要为一些长时间请求和慢速的客户端而设计。
1. Two new concurrency models were added: WriterThreadSpawn and WriterThreadPool.
2. Both are designed for serving large static files and work best under Ruby 1.9.
3. They may also be... used to dynamically generate long-running, streaming responses after headers are sent (use "proxy_buffering off" with nginx).
4. Unlike other concurrency options, these are designed to run behind nginx (or haproxy if you don't support POST/PUT requests) and are vulnerable to slow client denial of service attacks