deploy.rb

#!/usr/bin/env ruby

def system(arg)
  puts "--> #{arg}"
end

# no need to wait for backup to finish
# see http://dba.stackexchange.com/questions/13742/does-running-pg-dump-on-live-db-produce-consistent-backups
# the only question: does backup block DDL operations?
def launch_pg_backup
  puts "Initiating PG Backup in a thread"
  puts "And returning as soon as we start seeing it's logs"
end

system "git push heroku HEAD:master --force"
migration_status = `heroku run bin/probe_migrations.rb`

case migration_status
  when /NEED MIGRATIONS/
    launch_pg_backup
    system 'heroku run rake db:migrate'
  when /UP TO DATE/
    puts 'OK Migrations not needed'
  else
    puts '!!! Deploy failed'
    system 'heroku releases:rollback'
end

probe_migrations.rb

#!/usr/bin/env ruby
require 'rubygems'
require 'active_record'
require 'yaml'

dbconfig = YAML::load(File.open('config/database.yml'))
ActiveRecord::Base.establish_connection(ENV["DATABASE_URL"] || dbconfig[ENV['RAILS_ENV'] || 'development'])

puts(ActiveRecord::Migrator.needs_migration? ? "NEED MIGRATIONS" : "UP TO DATE")

Procfile

web: bin/watchdog.rb && bundle exec rails server thin -p $PORT
resque: bin/watchdog.rb && bundle exec resque 

watchdog.rb

#!/usr/bin/env ruby
require 'rubygems'
require 'active_record'
require 'yaml'

dbconfig = YAML::load(File.open('config/database.yml'))
ActiveRecord::Base.establish_connection(ENV["DATABASE_URL"] || dbconfig[ENV['RAILS_ENV'] || 'development'])

if ActiveRecord::Migrator.needs_migration?
  Thread.new do
    while ActiveRecord::Migrator.needs_migration?
      puts '--> Migrations still pending...'
      sleep 3
    end
    exit 0
  end

  if ENV['PORT']
    require 'sinatra'
    get '*' do
      "I'm under maintenance right now. BRB"
    end
  else
    sleep 3600
  end
end