unicorn: Rack HTTP server for fast clients and Unix

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 Ruby 2.0.0 and later.

  • 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.

  • 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.

  • transparent upgrades using systemd socket activation is supported since unicorn 5.0

  • 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 GC in Ruby 2.0+ 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 requests on-the-fly.


unicorn is copyright 2009-2018 by all contributors (see logs in git). It is based on Mongrel 1.1.5. Mongrel is copyright 2007 Zed A. Shaw and contributors.

unicorn is licensed under (your choice) of the GPLv2 or later (GPLv3+ preferred), or Ruby (1.8)-specific terms. See the included LICENSE file for details.

unicorn is 100% Free Software (including all development tools used).


The library consists of a C extension so you'll need a C compiler and Ruby development libraries/headers.

You may install it via RubyGems on RubyGems.org:

gem install unicorn

You can get the latest source via git from the following locations (these versions may not be stable):

https://repo.or.cz/unicorn.git (mirror)

You may browse the code from the web:

See the HACKING guide on how to contribute and build prerelease gems from git.


Rack (including Rails 3+) applications

In APP_ROOT, run:


unicorn will bind to all interfaces on TCP port 8080 by default. You may use the --listen/-l switch to bind to a different address:port or a UNIX socket.

Configuration File(s)

unicorn will look for the config.ru file used by rackup in APP_ROOT.

For deployments, it can use a config file for unicorn-specific options specified by the --config-file/-c command-line switch. See Unicorn::Configurator for the syntax of the unicorn-specific options. The default settings are designed for maximum out-of-the-box compatibility with existing applications.

Most command-line options for other Rack applications (above) are also supported. Run `unicorn -h` to see command-line options.


There is NO WARRANTY whatsoever if anything goes wrong, but let us know and we'll try our best to fix it.

unicorn is designed to only serve fast clients either on the local host or a fast LAN. See the PHILOSOPHY and DESIGN documents for more details regarding this.

Due to its ability to tolerate crashes and isolate clients, unicorn is unfortunately known to prolong the existence of bugs in applications and libraries which run on top of it.


All feedback (bug reports, user/development dicussion, patches, pull requests) go to the mailing list/newsgroup. See the ISSUES document for information on the mailing list.

The mailing list is archived at yhbt.net/unicorn-public/

Read-only NNTP access is available at: nntps://news.public-inbox.org/inbox.comp.lang.ruby.unicorn and nntp://news.gmane.io/gmane.comp.lang.ruby.unicorn.general

Read-only IMAP access is also avaialble at: imaps://yhbt.net/inbox.comp.lang.ruby.unicorn.0 and imap://7fh6tueqddpjyxjmgtdiueylzoqt6pt7hec3pukyptlmohoowvhde4yd.onion/inbox.comp.lang.ruby.unicorn.0 The AUTH=ANONYMOUS mechanism is supported, as is any username+password combination.

For the latest on unicorn releases, you may also finger us at [email protected] or check our NEWS page (and subscribe to our Atom feed).