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DiceBag is a library of rake tasks for configuring web apps in the style of The Twelve-Factor App. Configuration values are picked up from the environment and used to populate configuration files from templates. Pre-packaged templates for common configuration files are provided.

Although Rails already supports ERB syntax for its YML configuration files, DiceBag will generate a final static file that will work without keeping your deployment environment variables in sync with your production environment variables. For security reasons, these environments may sometimes differ.

Also DiceBag will work with any kind of text files, not only YML files. It can be very useful for ruby initializer files for instance.


Add the following to your Gemfile:

gem 'dice_bag'

If you are using these tasks outside of a Rails project, add the following to your Rakefile or wherever your local rake tasks are defined:

require 'dice_bag/tasks'

Run the following command to see the new tasks:

[bundle exec] rake -T | grep "rake config"

Create configuration files from templates

When the rake "config" task is run, configuration files are populated for all ERB template files in the project that have a ".dice" extension. Configuration values from the environment are made available to the templates through the configured object.

For example, take a "database.yml.dice" file containing this template:

  database: development
  username: <%= configured.database_username || 'root' %>
  password: <%= configured.database_password %>

Then running the following command:

DATABASE_USERNAME=alice DATABASE_PASSWORD=xyzzy [bundle exec] rake config

will generate a "database.yml" file with the following contents:

  database: development
  username: alice
  password: xyzzy

See the feature documentation for further examples and functionality.

As discussed in The Twelve-Factor App section on configuration, do not commit your generated configuration files to source control. Instead, commit the templates to source control and then regenerate the configuration files at deployment time by running the rake config:deploy task.

Ensuring variables are set in production

It is a common pattern to use default information for development but to not allow defaults in production, instead we want to always set up the environment variables in production.

It is very easy to discover what variables have not been set in production using a bang after the variable name, for instance:

secret_key: <%= configured.secret_key_base! || 'any text is ok' %>

Will raise an explanatory error if we are using Rails, we are in production and the variable SECRET_KEY_BASE is not set. In other environments will not care about it not being set and will use the default.

Generating the templates of given gems only

config:generate_all will generate all the templates it can find. Since sometimes this behavior is not desirable you can use the config:generate_from_gems task to specify gem names:

[bundle exec] rake config:generate_from_gems gem1 gem2 gemN

will generate only the templates provided by gem1, gem2 and gemN.

To force-generate set the DICEBAG_FORCE environment variable to any value when running the task.

Generating the pre-packaged templates

If the corresponding gems are installed, the following pre-packaged templates are provided:

  • mysql2 or pg: database.yml.dice for Rails
  • aws-sdk: aws.yml.dice
  • dalli: dalli.yml.dice

Run the following command to generate them:

[bundle exec] rake config:generate_all

This command provides options to compare changes between source and local templates, so new additions to the source templates can be safely added while preserving any project specific customization to local templates.

Alternatively, to force generate templates (replacing existing local templates with the source), run the following:

[bundle exec] rake config:generate_all:force

As with your own templates, you should commit these pre-packaged templates to source control.

You can customize these pre-packaged template to your needs but if the change is a generic fix or extension, please consider contributing it back to this project so that everyone benefits.

Defining your own pre-packaged templates

If you want DiceBag to generate your own pre-packaged templates when you run the rake "config:generate_all" task, you can create a plug-in. Read the templates.md file to learn how to do this.


rake config fails in Rails project with file not found

Due to rake running config/application.rb before kicking off a task, if config/application.rb loads any configuration files that dice_bag must generate a 'file not found' error may occur. Makes sense that a file rake config:generate_all needs to create, does not exist before it has ran.

Solution: Move any config loading that depends on files generated by dice_bag out of application.rb and into config/initializers/*. Since the commands rails server or rails console etc. always run the initializers, moving the logic here should be a safe bet.