deploy.rb

# config valid only for current version of Capistrano
lock '3.4.0'

set :application, 'application'
set :repo_url, 'repository'

# Default branch is :master
# ask :branch, `git rev-parse --abbrev-ref HEAD`.chomp

# Default deploy_to directory is /var/www/my_app_name
set :deploy_to, '/var/application'

# Default value for :scm is :git
# set :scm, :git

# Default value for :format is :pretty
# set :format, :pretty

# Default value for :log_level is :debug
# set :log_level, :debug

# Default value for :pty is false
set :pty, true

# Default value for :linked_files is []
set :linked_files, fetch(:linked_files, []).push('config/database.yml', 'config/secrets.yml', 'config/unicorn.rb')

# Default value for linked_dirs is []
set :linked_dirs, fetch(:linked_dirs, []).push('log', 'tmp/pids', 'tmp/cache', 'tmp/sockets', 'vendor/bundle', 'public/system')

# Default value for keep_releases is 5
# set :keep_releases, 5

namespace :deploy do

  task :restart do
    invoke "unicorn:restart"
  end
  after 'deploy:publishing', 'deploy:restart'

  task :push_config do
    next unless any? :linked_files
    on release_roles :all do
      fetch(:linked_files).each do |file|
        unless test "[ -f #{shared_path.join file} ]"
          upload! file, "#{shared_path.join file}"
        end
      end
    end
  end
  before 'deploy:check:linked_files', 'deploy:push_config'

end

namespace :unicorn do
  task :stop do
    on roles(:web) do
      execute :sudo, :systemctl, :stop, :unicorn
    end
  end

  task :restart do
    on roles(:web) do
      invoke 'unicorn:stop'
      invoke 'unicorn:start'
    end
  end

  task :start do
    on roles(:web) do
      execute :sudo, :systemctl, :start, :unicorn
    end
  end
end

unicorn.rb

# Copyed from http://benjaminknofe.com/blog/2014/03/08/zero-downtime-deployment-with-unicorn-and-capistrano/
# Note: http://stackoverflow.com/questions/15111902/rails-mongoid-unicorn-config-for-heroku

root = "/var/application/current"

pid           "#{root}/tmp/pids/unicorn.pid"
stderr_path   "#{root}/log/unicorn_error.log"
stdout_path   "#{root}/log/unicorn.log"

listen "#{root}/tmp/unicorn.socket", :backlog => 2048

preload_app true
working_directory root
worker_processes 4
timeout 30

before_fork do |server, worker|
  # the following is highly recomended for Rails + "preload_app true"
  # as there's no need for the master process to hold a connection
  defined?(ActiveRecord::Base) and ActiveRecord::Base.connection.disconnect!

  ##
  # When sent a USR2, Unicorn will suffix its pidfile with .oldbin and
  # immediately start loading up a new version of itself (loaded with a new
  # version of our app). When this new Unicorn is completely loaded
  # it will begin spawning workers. The first worker spawned will check to
  # see if an .oldbin pidfile exists. If so, this means we've just booted up
  # a new Unicorn and need to tell the old one that it can now die. To do so
  # we send it a QUIT.
  #
  # Using this method we get 0 downtime deploys.

  old_pid = "#{root}/tmp/pids/unicorn.pid.oldbin"
  if File.exists?(old_pid) && server.pid != old_pid
    begin
      Process.kill("QUIT", File.read(old_pid).to_i)
    rescue Errno::ENOENT, Errno::ESRCH
      # someone else did our job for us
    end
  end
end

after_fork do |server, worker|
  ##
  # Unicorn master loads the app then forks off workers - because of the way
  # Unix forking works, we need to make sure we aren't using any of the parent's
  # sockets, e.g. db connection

  defined?(ActiveRecord::Base) and ActiveRecord::Base.establish_connection
  # Redis and Memcached would go here but their connections are established
  # on demand, so the master never opens a socket

  ##
  # Unicorn master is started as root, which is fine, but let's
  # drop the workers to deployer:deployer

  begin
    uid, gid = Process.euid, Process.egid
    user, group = 'nia', 'nia'
    target_uid = Etc.getpwnam(user).uid
    target_gid = Etc.getgrnam(group).gid
    worker.tmp.chown(target_uid, target_gid)
    if uid != target_uid || gid != target_gid
      Process.initgroups(user, target_gid)
      Process::GID.change_privilege(target_gid)
      Process::UID.change_privilege(target_uid)
    end
  rescue => e
    if RAILS_ENV == 'development'
      STDERR.puts "couldn't change user, oh well"
    else
      raise e
    end
  end
end