Unicorn is an HTTP server for Rack applications designed to only serve fast clients on low-latency, high-bandwidth connections and take advantage of features in Unix/Unix-like kernels. Slow clients should only be served by placing a reverse proxy capable of fully buffering both the the request and response in between Unicorn and slow clients


  • Designed for Rack, Unix, fast clients, and ease-of-debugging. We cut out everything that is better supported by the operating system, nginx or Rack.
  • Compatible with both Ruby 1.8 and 1.9. Rubinius support is in-progress.
  • Process management: Unicorn will reap and restart workers that die from broken apps. There is no need to manage multiple processes or ports yourself. Unicorn can spawn and manage any number of worker processes you choose to scale to your backend.
  • Load balancing is done entirely by the operating system kernel. Requests never pile up behind a busy worker process.
  • Does not care if your application is thread-safe or not, workers all run within their own isolated address space and only serve one client at a time for maximum robustness.
  • Supports all Rack applications, along with pre-Rack versions of Ruby on Rails via a Rack wrapper.
  • Builtin reopening of all log files in your application via USR1 signal. This allows logrotate to rotate files atomically and quickly via rename instead of the racy and slow copytruncate method. Unicorn also takes steps to ensure multi-line log entries from one request all stay within the same file.
  • nginx-style binary upgrades without losing connections. You can upgrade Unicorn, your entire application, libraries and even your Ruby interpreter without dropping clients. before_fork and after_fork hooks in case your application has special needs when dealing with forked processes. These should not be needed when the “preload_app” directive is false (the default).
  • Can be used with copy-on-write-friendly memory management to save memory (by setting “preload_app” to true).
  • Able to listen on multiple interfaces including UNIX sockets, each worker process can also bind to a private port via the after_fork hook for easy debugging.
  • Simple and easy Ruby DSL for configuration.
  • Decodes chunked transfers on-the-fly, thus allowing upload progress notification to be implemented as well as being able to tunnel arbitrary stream-based protocols over HTTP.


  • Unicorn is copyright 2009 by all contributors (see logs in git). It is based on Mongrel 1.1.5 and carries the same license.

  • Mongrel is copyright 2007 Zed A. Shaw and contributors. It is tri-licensed under (your choice) of the GPLv3, GPLv2 or Ruby-specific terms. See the included LICENSE file for details.

  • Unicorn is 100% Free Software.

Used by...

GitHub uses unicorn and has a great tech document about it. They also use Thin and Passenger, not only unicorn!


Configuration, installation, deployment, performance, optimization.... But please first read unicorn documentation!

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