#373 Zero-Downtime Deployment pro

Aug 09, 2012 | 12 minutes | Performance, Deployment, Production

Here I show how to accomplish zero-downtime deployment using Unicorn. I also cover gotchas when working with migrations and how to put up a maintenance page when you do need to take down the site.

Click to Play Video ▶

Download:
source codeProject Files in Zip (74.5 KB)
mp4Full Size H.264 Video (28.2 MB)
m4vSmaller H.264 Video (16.8 MB)
webmFull Size VP8 Video (17.5 MB)
ogvFull Size Theora Video (42.6 MB)

Browse Source Code

Resources

Update February 16, 2013: The cap deploy:web:disable tasks have been removed from Capistrano. Use the capistrano-maintenance gem instead.

          terminal
        
cap unicorn:setup unicorn:stop unicorn:start
rails d migration rename_content_to_body_in_articles
rails g migration add_content_to_articles content:text
rails g migration copy_article_content
rails g migration remove_body_from_articles body:text
cap nginx:setup
cap deploy:web:disable
cap -e deploy:web:disable
cap deploy:web:enable

          config/application.rb
        
sleep 10 # to simulate slow loading app

          config/recipes/templates/unicorn.rb.erb
        
working_directory "<%= current_path %>"
pid "<%= unicorn_pid %>"
stderr_path "<%= unicorn_log %>"
stdout_path "<%= unicorn_log %>"

listen "/tmp/unicorn.<%= application %>.sock"
worker_processes <%= unicorn_workers %>
timeout 30

preload_app true

before_fork do |server, worker|
  # Disconnect since the database connection will not carry over
  if defined? ActiveRecord::Base
    ActiveRecord::Base.connection.disconnect!
  end

  # Quit the old unicorn process
  old_pid = "#{server.config[: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|
  # Start up the database connection again in the worker
  if defined?(ActiveRecord::Base)
    ActiveRecord::Base.establish_connection
  end
end

          config/recipes/templates/unicorn_init.erb
        
restart|reload)
  sig USR2 && echo reloaded OK && exit 0

          db/migrate/copy_article_content.rb
        
class CopyArticleContent < ActiveRecord::Migration
  def up
    execute "update articles set content=body"
  end

  def down
    execute "update articles set body=content"
  end
end

          config/recipes/templates/nginx_unicorn.erb
        
if (-f $document_root/system/maintenance.html) {
  return 503;
}
error_page 503 @maintenance;
location @maintenance {
  rewrite  ^(.*)$  /system/maintenance.html last;
  break;
}

          config/deploy.rb
        
set :maintenance_template_path, File.expand_path("../recipes/templates/maintenance.html.erb", __FILE__)

          config/recipes/templates/maintenance.html.erb
        
<!DOCTYPE html>
<html>
  <head>
    <title>Blog</title>
    <style type="text/css">
    html, body {
      background-color: #4B7399;
      font-family: Verdana, Helvetica, Arial;
      font-size: 14px;
    }

    a {
      color: #0000FF;
      img { border: none; }
    }

    #container {
      width: 80%;
      margin: 0 auto;
      background-color: #FFF;
      padding: 20px 40px;
      border: solid 1px black;
      margin-top: 20px;
    }
    </style>
  </head>
  <body>
    <div id="container">
      <h1>The system is down for <%= reason ? reason : "maintenance" %></h1>
      <p>It will be back <%= deadline ? deadline : "soon" %></p>
    </div>
  </body>
</html>

          models/article.rb
        
attr_accessible :body # ...

def body=(body)
  self.content = body
end