FeatureFlipper

FeatureFlipper is a simple library that allows you to restrict certain blocks of code to certain environments. This is mainly useful in projects where you deploy your application from HEAD and don't use branches.

Read more about using feature flips here: Feature Toggles.

Install

FeatureFlipper is packaged as a gem:

$ gem install feature_flipper

In your project you have to configure the path to the app specific configuration file after requiring FeatureFlipper:

require 'feature_flipper'
FeatureFlipper::Config.path_to_file = "#{Rails.root}/config/features.rb"

Example config file

FeatureFlipper.features do
  in_state :development do
    feature :rating_game, :description => 'play a game to get recommendations'
  end

  in_state :live do
    feature :city_feed, :description => 'stream of content for each city'
  end
end

FeatureFlipper.states do
  state :development, ['development', 'test'].include?(Rails.env)
  state :live, true
end

This is your complete features.rb config file. In the example there are two states: :development is active on development servers and :live is always active (this is the last state a feature goes through).

The feature :rating_game is still in development and not shown on the production site. The feature :city_feed is done and already enabled everywhere. You transition features between states by just moving the line to the new state block and deploying your code.

You can take a look at the static_states.rb in the examples folder to see this in detail.

Configuration

You need to create a configuration file which defines the two entities FeatureFlipper cares about:

states
features

You first define multiple 'states' which normally depend on the environment (for example: the state 'development' is only active on development servers). After that you add 'features' which correspond to logical chunks of work in your project. These features then move through the different states as they get developed (for example: :development -> :staging -> :live).

Defining features

A feature needs to have a name and you can add additional information like a more detailed description, a ticket number, a date when it was started, etc. Features are always defined in a state, you cannot define a feature which doesn't belong to a state.

in_state :development do
  feature :rating_game, :description => 'play a game to get recommendations'
end

Defining states

A state is just a name and a boolean check. The check needs to evaluate to true when it is active. For a Rails app you can just use environments:

FeatureFlipper.states do
  state :development, ['development', 'test'].include?(Rails.env)
  state :staging, ['staging', development', 'test'].include?(Rails.env)
end

Usage

In your code you then use the show_feature? method to branch depending on wether a feature is active or not:

if show_feature?(:rating_game)
  # new code
else
  # old code
end

The show_feature? method is defined on Object, so you can use it everywhere.

Dynamic feature groups

As soon as we have the feature_flipper infrastructure in place, we can start doing more interesting things with it. For example, dynamic features which are enabled on a per user basis. This allows you to release features to employees only or to a private beta group, etc.

Defining dynamic states

A dynamic state is defined using a Proc:

FeatureFlipper.states do
  state :development, ['development', 'test'].include?(Rails.env)
  state :employees, Proc.new { |feature_name| respond_to?(:current_user, true) && current_user.employee? }
end

The Proc get's evaluated in the context of where you call the show_feature? method from, so it depends on your app what you can do there. In a typical Rails app you could do checks on the current user, as shown above. This way the condition if someone should see a feature or not can be anything: You can store it in the database, in Redis, look at request parameters, based on the current time, etc.

Take a look at dynamic_states.rb in the examples folder to see this in detail.

theflow/feature_flipper