configuration.rb

class Configuration
  attr_accessor :tail_logs, :max_connections, :admin_password
  attr_accessor :app_server

  def app_server
    @app_server_config ||= AppServer.new
    yield @app_server_config if block_given?
    @app_server_config
  end
end

class AppServer
  attr_accessor :port, :admin_password
end

def configure
  c = Configuration.new
  yield c if block_given?
  c
end

configuration = configure do |config|
  config.tail_logs = true
  config.max_connections = 55
  config.admin_password = 'secret'
  config.app_server do |app_server_config|
    app_server_config.port = 8808
    app_server_config.admin_password = config.admin_password
  end
end

puts configuration.class # => Configuration
puts configuration.tail_logs # => true
puts configuration.app_server.admin_password # => 'secret'