Mixlib::Config

Mixlib::Config provides a class-based configuration object, as used in Chef. To use in your project:

  require 'mixlib/config'

  module MyConfig
    extend Mixlib::Config
    config_strict_mode true
    default :first_value, 'something'
    default :other_value, 'something_else'
  end

You can use this to provide a configuration file for a user. For example, if you do this:

  MyConfig.from_file('~/.myconfig.rb')

A user could write a Ruby config file that looked like this:

  first_value 'hi'
  second_value "#{first_value}!  10 times 10 is #{10*10}!"

Inside your app, you can check configuration values with this syntax:

  MyConfig.first_value   # returns 'something'
  MyConfig[:first_value] # returns 'something'

And you can modify configuration values with this syntax:

  MyConfig.first_value('foobar')    # sets first_value to 'foobar'
  MyConfig.first_value = 'foobar'   # sets first_value to 'foobar'
  MyConfig[:first_value] = 'foobar' # sets first_value to 'foobar'

Nested Configuration

Often you want to be able to group configuration options to provide a common context. Mixlib::Config supports this thus:

  require 'mixlib/config'

  module MyConfig
    extend Mixlib::Config
    config_context :logging do
      default :base_filename, 'mylog'
      default :max_log_files, 10
    end
  end

The user can write their config file like this:

  logging.base_filename 'superlog'
  logging.max_log_files 2

You can access these variables thus:

  MyConfig.logging.base_filename
  MyConfig[:logging][:max_log_files]

Default Values

Mixlib::Config has a powerful default value facility. In addition to being able to specify explicit default values, you can even specify Ruby code blocks that will run if the config value is not set. This can allow you to build options whose values are based on other options.

  require 'mixlib/config'

  module MyConfig
    extend Mixlib::Config
    config_strict_mode true
    default :verbosity, 1
    default(:print_network_requests) { verbosity >= 2 }
    default(:print_ridiculously_unimportant_stuff) { verbosity >= 10 }
  end

This allows the user to quickly specify a number of values with one default, while still allowing them to override anything:

  verbosity 5
  print_network_requests false

Strict Mode

Misspellings are a common configuration problem, and Mixlib::Config has an answer: config_strict_mode. Setting config_strict_mode to true will cause any misspelled or incorrect configuration option references to throw Mixlib::Config::UnknownConfigOptionError.

  require 'mixlib/config'

  module MyConfig
    extend Mixlib::Config
    config_strict_mode true
    default :filename, '~/output.txt'
    configurable :server_url # configurable declares an option with no default value
    config_context :logging do
      default :base_name, 'log'
      default :max_files, 20
    end
  end

Now if a user types fielname "~/output-mine.txt" in their configuration file, it will toss an exception telling them that the option "fielname" is unknown. If you do not set config_strict_mode, the fielname option will be merrily set and the application just won't know about it.

Different config_contexts can have different strict modes; but they inherit the strict mode of their parent if you don't explicitly set it. So setting it once at the top level is sufficient. In the above example, logging.base_naem 'mylog' will raise an error.

In conclusion: always set config_strict_mode to true. You know you want to.

Testing and Reset

Testing your application with different sets of arguments can by simplified with reset. Call MyConfig.reset before each test and all configuration will be reset to its default value. There's no need to explicitly unset all your options between each run.

NOTE: if you have arrays of arrays, or other deep nesting, we suggest you use code blocks to set up your default values (default(:option) { [ [ 1, 2 ], [ 3, 4 ] ] }). Deep children will not always be reset to their default values.

Enjoy!