Flipflop your features

Flipflop provides a declarative, layered way of enabling and disabling application functionality at run-time. It is originally based on Flip. Compared to the original gem Flipflop has:

• an improved dashboard
• thread safety
• better database performance due to per-request caching, enabled by default
• more strategies (Redis, query strings, sessions, custom code)
• more strategy options (cookie options, strategy names and descriptions, custom database models)
• the ability to use the same strategy twice, with different options
• configuration in a fixed location (config/features.rb) that is usable even if you don't use the database strategy
• dashboard is inaccessible in production by default, for safety in case of misconfiguration
• removes controller filters and view helpers, to promote uniform semantics to check for features (facilitates project-wide searching)

You can configure strategy layers that will evaluate if a feature is currently enabled or disabled. Available strategies are:

• a per-feature default setting
• database (with Active Record or Redis), to flipflop features site-wide for all users
• cookie or session, to flipflop features for single users
• query string parameters, to flipflop features occasionally (in development mode for example)
• custom strategy code

Flipflop has a dashboard interface that's easy to understand and use.

Installation

Add the gem to your Gemfile:

gem "flipflop"

Generate routes, feature settings and database migration:

rails g flipflop:install

Run the migration to store feature settings in your database:

rake db:migrate

Declaring features

Features and strategies are declared in config/features.rb:

Flipflop.configure do
  # Strategies will be used in the order listed here.
  strategy :cookie
  strategy :active_record
  strategy :default

  # Basic feature declaration:
  feature :shiny_things

  # Enable features by default:
  feature :world_domination, default: true
end

This file is automatically reloaded in development mode. No need to restart your server after making changes.

Strategies

The following strategies are provided:

• :active_record – Save feature settings in the database.
  • :class – Provide the feature model. Flipflop::Feature by default (which uses the table flipflop_features). Honors default_scope when features are resolved or switched on/off.
• :cookie – Save feature settings in browser cookies for the current user.
  • :prefix – String prefix for all cookie names. Defaults to no prefix.
  • :path – The path for which the cookies apply. Defaults to the root of the application.
  • :domain – Cookie domain. Is nil by default (no specific domain). Can be :all to use the topmost domain. Can be an array of domains.
  • :secure – Only set cookies if the connection is secured with TLS. Default is false.
  • :httponly – Whether the cookies are accessible via scripting or only HTTP. Default is false.
• :query_string – Interpret query string parameters as features. This strategy is only used for resolving. It does not allow switching features on/off.
  • :prefix – String prefix for all query string parameters. Defaults to no prefix.
• :redis – Save feature settings in Redis.
  • :client – Use the specified Redis client instead of Redis.new.
  • :prefix – String prefix for all Redis keys. Defaults to no prefix.
• :session – Save feature settings in the current user's application session.
  • :prefix – String prefix for all session variables. Defaults to no prefix.
• :default – Not strictly needed, all feature defaults will be applied if no strategies match a feature. Include this strategy to determine the order of using the default value, and to make it appear in the dashboard.
• :test – Simple strategy that stores features in memory. Useful for testing. If you call Flipflop::FeatureSet.current.test! this strategy is enabled and replaces all configured strategies.

All strategies support these options, to change the appearance of the dashboard:

• :name – The name of the strategy. Defaults to the name of the selected strategy.
• :description – The description of the strategy. Every strategy has a default description.
• :hidden – Optionally hides the strategy from the dashboard. Default is false.

The same strategy type can be used multiple times, as long as the options are different. To prevent subtle bugs, an error is raised if two identical strategies are configured.

Checking if a feature is enabled

Flipflop.enabled? or the dynamic predicate methods can be used to check feature state:

Flipflop.enabled?(:world_domination)  # true
Flipflop.world_domination?            # true

Flipflop.enabled?(:shiny_things)      # false
Flipflop.shiny_things?                # false

This works everywhere. In your views:

<div>
  <% if Flipflop.world_domination? %>
    <%= link_to "Dominate World", world_dominations_path %>
  <% end %>
</div>

In your controllers:

class ShinyThingsController < ApplicationController
  def index
    return head :forbidden unless Flipflop.shiny_things?
    # Proceed with shiny things...
  end
end

In your models:

class ShinyThing < ActiveRecord::Base
  after_initialize do
    if !Flipflop.shiny_things?
      raise ActiveRecord::RecordNotFound
    end
  end
end

Custom strategies

Custom light-weight strategies can be defined with a block:

Flipflop.configure do
  strategy :random do |feature|
    rand(2).zero?
  end
  # ...
end

You can define your own custom strategies by inheriting from Flipflop::Strategies::AbstractStrategy:

class UserPreferenceStrategy < Flipflop::Strategies::AbstractStrategy
  class << self
    def default_description
      "Allows configuration of features per user."
    end
  end

  def switchable?
    # Can only switch features on/off if we have the user's session.
    # The `request` method is provided by AbstractStrategy.
    request?
  end

  def enabled?(feature)
    # Can only check features if we have the user's session.
    return unless request?
    find_current_user.enabled_features[feature]
  end

  def switch!(feature, enabled)
    user = find_current_user
    user.enabled_features[feature] = enabled
    user.save!
  end

  def clear!(feature)
    user = find_current_user
    user.enabled_features.delete(feature)
    user.save!
  end

  private

  def find_current_user
    # The `request` method is provided by AbstractStrategy.
    User.find_by_id(request.session[:user_id])
  end
end

Use it in config/features.rb:

Flipflop.configure do
  strategy UserPreferenceStrategy # name: "my strategy", description: "..."
end

If you define your class inside Flipflop::Strategies, you can use the shorthand name to refer to your strategy:

module Flipflop::Strategies
  class UserPreferenceStrategy < AbstractStrategy
    # ...
  end
end

Flipflop.configure do
  strategy :user_preference
end

Dashboard access control

The dashboard provides visibility and control over the features.

You don't want the dashboard to be public. For that reason it is only available in the development and test environments by default. Here's one way of implementing access control.

In app/config/application.rb:

config.flipflop.dashboard_access_filter = :require_authenticated_user

In app/controllers/application_controller.rb:

class ApplicationController < ActionController::Base
  def require_authenticated_user
    head :forbidden unless User.logged_in?
  end
end

Or directly in app/config/application.rb:

config.flipflop.dashboard_access_filter = -> {
  head :forbidden unless User.logged_in?
}

Testing

In your test environment, you typically want to keep your features. But to make testing easier, you may not want to use any of the strategies you use in development and production. You can replace all strategies with a single :test strategy by calling Flipflop::FeatureSet.current.test!. The test strategy will be returned. You can use this strategy to enable and disable features.

describe WorldDomination do
  before do
    test_strategy = Flipflop::FeatureSet.current.test!
    test_strategy.switch!(:world_domination, true)
  end

  it "should dominate the world" do
     # ...
  end
end

If you are not happy with the default test strategy (which is essentially a simple thread-safe hash object), you can provide your own implementation as argument to the test! method.

License

This software is licensed under the MIT License. View the license.