Rails 6 + webpack assets + capistrano + unicorn + nginx : Zero downtime deploy and code reload

00.Capistrano-Rails-Unicorn-Nginx-Webpack

cap staging deploy:check
cap staging deploy

APP.conf.example

# /etc/nginx/sites-available/APP.conf

upstream APP {
# Path to Unicorn socket file
server unix:/home/USER/deployed_apps/APP/current/tmp/sockets/unicorn.sock fail_timeout=0;
}

server {
listen 80;
server_name XXXX.XXXXX.XXXXX;

root /home/USER/deployed_apps/APP/current;

try_files $uri/index.html $uri @APP;

location @APP {
   proxy_pass http://APP;
   proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header Host $http_host;
   proxy_redirect off;
}

# serve a static "maintenance" html page in case unicorn is not started yet
error_page 500 502 503 504 /error/5xx.html;
location ^~ /error/ {
    internal;
    root /var/www;
}
client_max_body_size 4G;
keepalive_timeout 10;
}

Capfile

# Load DSL and set up stages
require "capistrano/setup"

# Include default deployment tasks
require "capistrano/deploy"

# Load the SCM plugin appropriate to your project:
#
# require "capistrano/scm/hg"
# install_plugin Capistrano::SCM::Hg
# or
# require "capistrano/scm/svn"
# install_plugin Capistrano::SCM::Svn
# or
require "capistrano/scm/git"
install_plugin Capistrano::SCM::Git

# Include tasks from other gems included in your Gemfile
#
# For documentation on these, see for example:
#
#   https://github.com/capistrano/rvm
#   https://github.com/capistrano/rbenv
#   https://github.com/capistrano/chruby
#   https://github.com/capistrano/bundler
#   https://github.com/capistrano/rails
#   https://github.com/capistrano/passenger
#
require "capistrano/rvm"
# require "capistrano/rbenv"
# require "capistrano/chruby"
require "capistrano/bundler"
require "capistrano/rails/assets"
require "capistrano/rails/migrations"
require 'capistrano3/unicorn'
# require "capistrano/passenger"

# Load custom tasks from `lib/capistrano/tasks` if you have any defined
Dir.glob("lib/capistrano/tasks/*.rake").each { |r| import r }

deploy.rb

# config/deploy.rb
lock "~> 3.11.2"

set :application, "APP"
set :user, "USER"
set :repo_url, "git@XXXX.com:XXXX/XXXXX.git"

set :migration_role, :app
set :conditionally_migrate, true

set :assets_roles, [:USER]

# clean assets every 2 deploys
set :keep_assets, 2

set :deploy_to, "/home/#{fetch(:user)}/deployed_apps/#{fetch(:application)}"

append :linked_files, "config/database.yml"
append :linked_files, "config/secrets.yml"
append :linked_files, "config/master.key"
append :linked_dirs, "log", "tmp/pids", "tmp/cache", "tmp/sockets", "tmp/logs"

namespace :deploy do
  namespace :check do
    # upload your local master.key in case it's missing on the deploy target
    before :linked_files, :set_master_key do
      on roles(:app), in: :sequence, wait: 10 do
        unless test("[ -f #{shared_path}/config/master.key ]")
          upload! 'config/master.key', "#{shared_path}/config/master.key"
        end
      end
    end
  end
  task :assets do
    on roles(:app) do
      within release_path do
        # not using rails_env will probably result in this: https://github.com/capistrano/rails/issues/237
        with rails_env: fetch(:rails_env) do
          execute :bundle, 'exec rails assets:precompile'
        end
      end
    end
  end
  task :restart do
    on roles(:app) do
      invoke 'unicorn:restart'
    end
  end
  before "deploy:assets:precompile", "deploy:assets"
  after 'deploy:publishing', 'deploy:restart'
end

Gemfile

# ...

gem 'rails', '~> 6.0.0'
gem 'webpacker'

# to complicate things we could have different web server 
# DO NOT USE if and block logic, otherwise the Gemfile.lock will be incorrect on each deployment
# Use Puma as the app server on Windows
gem 'puma', '~> 3.11', platforms: [:mingw, :mswin, :x64_mingw, :jruby]
# Use unicorn on debian
gem 'unicorn', platform: :ruby

group :development do
  gem "capistrano", "~> 3.10", require: false
  gem "capistrano-rails", "~> 1.4", require: false
  gem 'capistrano-rvm', require: false
  gem 'capistrano3-unicorn', require: false
end

production.rb

# config/deploy/production.rb
# default branch is master
set :rails_env, 'production'
set :unicorn_env, :production
set :unicorn_rack_env, 'production'
server "YYYY.YYY.YYYYY",
  user: "USER",
  roles: %w{web app db}

staging.rb

# config/deploy/staging.rb
set :branch, ENV['BRANCH'] || 'develop'
set :rails_env, 'production'
set :unicorn_env, :production
set :unicorn_rack_env, 'production'
server "XXX.XXX.XXXX",
  user: "USER",
  roles: %w{web app db}

unicorn.rb

# config/unicorn/production.rb

# set path to the application
root = "/home/USER/deployed_apps/APP/current"
working_directory root

# Set unicorn options
worker_processes 2
preload_app true
timeout 30

# Path for the Unicorn socket
listen "#{root}/tmp/sockets/unicorn.sock", :backlog => 64

# Set path for logging
stderr_path "#{root}/log/unicorn.stderr.log"
stdout_path "#{root}/log/unicorn.stdout.log"

# Set proccess id path
pid "#{root}/tmp/pids/unicorn.pid"

before_fork do |server, worker|
  if defined?(ActiveRecord::Base)
    ActiveRecord::Base.connection.disconnect!
  end

  # Before forking, kill the master process that belongs to the .oldbin PID.
  # This enables 0 downtime deploys and cleans master(old) hanging around
  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|
  if defined?(ActiveRecord::Base)
    ActiveRecord::Base.establish_connection
  end
end