database_connections.rb

#config/initializers/database_connection.rb
Rails.application.config.after_initialize do
  ActiveRecord::Base.connection_pool.disconnect!

  ActiveSupport.on_load(:active_record) do
    if Rails.application.config.database_configuration
      config = Rails.application.config.database_configuration[Rails.env]
      config['reaping_frequency'] = ENV['DB_REAP_FREQ'] || 10 # seconds
      config['pool']              = ENV['DB_POOL']      || 5
      ActiveRecord::Base.establish_connection(config)
    end
  end
end

Gemfile

gem 'foreman'
gem 'puma'

Procfile

web: bundle exec puma -t ${PUMA_MIN_THREADS:-8}:${PUMA_MAX_THREADS:-12} -w ${PUMA_WORKERS:-2} -p $PORT -e ${RACK_ENV:-development}