active_record.rb

module MyApp
  module Database
    def connect(pool_size = nil, reap_time = nil)
      return unless defined? ActiveRecord::Base
      config = Rails.application.config.database_configuration[Rails.env]
      config['reaping_frequency'] = reap_time || ENV['AR_DB_REAP_FREQ'] || 10  # seconds
      config['pool']              = pool_size || ENV['AR_DB_POOL']      || 5
      ActiveRecord::Base.establish_connection(config)
    end

    def disconnect
      return unless defined? ActiveRecord::Base
      ActiveRecord::Base.connection_pool.disconnect! 
    end

    def reconnect(pool_size = nil, reap_time = nil)
      disconnect
      connect(pool_size, reap_time)
    end

    module_function :disconnect, :connect, :reconnect
  end
end

puma.rb

# doc
# https://github.com/puma/puma/blob/master/examples/config.rb
max_threads = Integer(ENV['PUMA_MAX_THREADS'] || '16')
threads Integer(ENV['PUMA_MIN_THREADS'] || '0'), max_threads
workers Integer(ENV['PUMA_WORKERS'] || '1')

require './lib/config/active_record'
on_worker_boot do
  ActiveSupport.on_load(:active_record) do
    MyApp::Database.reconnect(max_threads)
  end
end

if ENV['PUMA_CONTROL_ENDPOINT']
  if ENV['PUMA_CONTROL_ENDPOINT_AUTH_TOKEN']
    activate_control_app ENV['PUMA_CONTROL_ENDPOINT'], { auth_token: ENV['PUMA_CONTROL_ENDPOINT_AUTH_TOKEN'] }
  elsif ENV['PUMA_CONTROL_ENDPOINT_NO_AUTH_TOKEN']
    activate_control_app ENV['PUMA_CONTROL_ENDPOINT'], { no_token: true }
  else
    activate_control_app ENV['PUMA_CONTROL_ENDPOINT']
  end
end

preload_app!

ENV['PUMA_LOADED'] = 'y'