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Ubuntu upstart is an event-based replacement for the init daemon, it handles starting and supervising Linux services. While originally developed for Ubuntu, it can be used on any other Linux distribution.

Ruby unicorn is a popular HTTP server for Rack applications that has its own system of process management.

Unfortunately upstart is incompatible with unicorn's master "phoenix" way of respawning to a new process upon receiving the SIGUSR2 signal.

This little demo is a proof of concept utilizing upstart's little known utility called upstart-socket-bridge. The U-S-B is similar to how xinetd works, it listens to a socket and allows a process to take over the file descriptor once a connection has been made. U-S-B is passing the file descriptor information via the environment variable UPSTART_FDS.

Ruby unicorn has its own little known feature, one that is actually used to respawn into a new master. Unicorn is passing the previous file descriptors to a new master process via the UPSTART_FD environment variable.

The idea in this project, especially the unicorn_wrapper.rb file, is forgoing the Unicorn launch of its first master, and allowing Upstart to become that master via U-S-B unix sockets.

While this is an interesting experiment, there are a lot of bugs involved and this process is not recommended for actual production use. YMMV.

To test this demo, you will need an Ubuntu system (tested on Ubuntu 12.04 Precise), and a Rails project configured to be served via unicorn using the unicorn_wrapper.rb file.

Setup:

• place unicorn_wrapper.rb in your rails project
• edit project location and place unicorn_upstart.conf as /etc/init/unicorn.conf
• copy nginx configuration unicorn_nginx.conf to /etc/nginx/sites-enabled/default
• restart nginx

Troubleshooting:

• upstart log file shows as /var/log/uptart/unicorn.log and has useful information
• install socat with sudo apt-get install socat and test connectivity using sudo socat stdin unix-client:/var/run/unicorn.sock
• nginx is using www-data as its user by default, configured in /etc/nginx/nginx.conf which might not have permissions writing to the unicorn socket owned by root. change nginx user to root helps resolve this.