puma config file

Procfile

# ./Procfile
web: bundle exec puma -p $PORT -C ./config/puma.rb

puma.rb

# ./config/puma.rb
environment ENV['RACK_ENV']
threads 0,5

workers 3

preload_app!

on_worker_boot do
  ActiveSupport.on_load(:active_record) do
    ActiveRecord::Base.establish_connection
  end
end