Plugin Development: Configuration

This page documents how to add new configuration options to Vagrant, settable with config.YOURKEY in Vagrantfiles. Prior to reading this, you should be familiar with the plugin development basics.

Definition Component

Within the context of a plugin definition, new configuration keys can be defined like so:

config "foo" do
  require_relative "config"
  Config
end

Configuration keys are defined with the config method, which takes as an argument the name of the configuration variable as the argument. This means that the configuration object will be accessible via config.foo in Vagrantfiles. Then, the block argument returns a class that implements the Vagrant.plugin(2, :config) interface.

Implementation

Implementations of configuration keys should subclass Vagrant.plugin(2, :config), which is a Vagrant method that will return the proper subclass for a version 2 configuration section. The implementation is very simple, and acts mostly as a plain Ruby object. Here is an example:

class Config < Vagrant.plugin(2, :config)
  attr_accessor :widgets

  def initialize
    @widgets = UNSET_VALUE
  end

  def finalize!
    @widgets = 0 if @widgets == UNSET_VALUE
  end
end

When using this configuration class, it looks like the following:

Vagrant.configure("2") do |config|
  # ...

  config.foo.widgets = 12
end

Easy. The only odd thing is the UNSET_VALUE bits above. This is actually so that Vagrant can properly automatically merge multiple configurations. Merging is covered in the next section, and UNSET_VALUE will be explained there.

Merging

Vagrant works by loading multiple Vagrantfiles and merging them. This merge logic is built-in to configuration classes. When merging two configuration objects, we will call them "old" and "new", it'll by default take all the instance variables defined on "new" that are not UNSET_VALUE and set them onto the merged result.

The reason UNSET_VALUE is used instead of Ruby's nil is because it is possible that you want the default to be some value, and the user actually wants to set the value to nil, and it is impossible for Vagrant to automatically determine whether the user set the instance variable, or if it was defaulted as nil.

This merge logic is what you want almost every time. Hence, in the example above, @widgets is set to UNSET_VALUE. If we had two Vagrant configuration objects in the same file, then Vagrant would properly merge the follows. The example below shows this:

Vagrant.configure("2") do |config|
  config.widgets = 1
end

Vagrant.configure("2") do |config|
  # ... other stuff
end

Vagrant.configure("2") do |config|
  config.widgets = 2
end

If this were placed in a Vagrantfile, after merging, the value of widgets would be "2".

The finalize! method is called only once ever on the final configuration object in order to set defaults. If finalize! is called, that configuration will never be merged again, it is final. This lets you detect any UNSET_VALUE and set the proper default, as we do in the above example.

Of course, sometimes you want custom merge logic. Let us say we wanted our widgets to be additive. We can override the merge method to do this:

class Config < Vagrant.config("2", :config)
  attr_accessor :widgets

  def initialize
    @widgets = 0
  end

  def merge(other)
    super.tap do |result|
      result.widgets = @widgets + other.widgets
    end
  end
end

In this case, we did not use UNSET_VALUE for widgets because we did not need that behavior. We default to 0 and always merge by summing the two widgets. Now, if we ran the example above that had the 3 configuration blocks, the final value of widgets would be "3".

Validation

Configuration classes are also responsible for validating their own values. Vagrant will call the validate method to do this. An example validation method is shown below:

class Config < Vagrant.plugin("2", :config)
  # ...

  def validate(machine)
    errors = _detected_errors
    if @widgets <= 5
      errors << "widgets must be greater than 5"
    end

    { "foo" => errors }
  end
end

The validation method is given a machine object, since validation is done for each machine that Vagrant is managing. This allows you to conditionally validate some keys based on the state of the machine and so on.

The _detected_errors method returns any errors already detected by Vagrant, such as unknown configuration keys. This returns an array of error messages, so be sure to turn it into the proper Hash object to return later.

The return value is a Ruby Hash object, where the key is a section name, and the value is a list of error messages. These will be displayed by Vagrant. The hash must not contain any values if there are no errors.

Accessing

After all the configuration options are merged and finalized, you will likely want to access the finalized value in your plugin. The initializer function varies with each type of plugin, but most plugins expose an initializer like this:

def initialize(machine, config)
  @machine = machine
  @config  = config
end

When authoring a plugin, simply call super in your initialize function to setup these instance variables:

def initialize(*)
  super

  @config.is_now_available
  # ...existing code
end

def my_helper
  @config.is_here_too
end

For examples, take a look at Vagrant's own internal plugins in the plugins folder in Vagrant's source on GitHub.