Flip — flip your features

Learnable uses feature flippers (so does Flickr) as a tool to help achieve continuous deployment.

Flip gives us a declarative, layered mechanism to enable and disable features. There's a configurable system-wide default (default: !Rails.env.production? works nicely), plus three layers of strategies to determine status per-feature:

• The declared default, e.g. feature :world_domination, default: true,
• A database-backed strategy, for flipping features site-wide for all users.
• A cookie-backed strategy, for privately previewing features in your own browser only.

(Hint: that last one is a a killer feature..)

Install

Note: the alpha version number indicates Flip is currently being extracted from its host application. The process described here is currently fictional. But it does have a happy ending.

Rails 3.0 and 3.1+

# Gemfile
gem "flip"

# Generate the model and migration
> rails g flip:install

# Run the migration
> rake db:migrate

# They lived happily ever after.

Declaring Features

# This is the model class generated by rails g flip:install
class Feature < ActiveRecord::Base
  include Flip::Declarable

  # The recommended Flip strategy stack.
  strategy Flip::CookieStrategy
  strategy Flip::DatabaseStrategy
  strategy Flip::DefaultStrategy
  default false

  # A basic feature declaration.
  feature :shiny_things

  # Override the system-wide default.
  feature :world_domination, default: true

  # Enabled half the time..? Sure, we can do that.
  feature :flakey,
    default: proc { rand(2).zero? }

  # Provide a description, normally derived from the feature name.
  feature :something,
    default: true,
    description: "Ability to purchase enrollments in courses",

end

Checking Features

Feature status can be checked by any code using on? or using the dynamic predicate methods:

Flip.on? :world_domination   # true
Flip.world_domination?       # true

Flip.on? :shiny_things       # false
Flip.shiny_things?           # false

Within view and controller methods, the FlipHelper module provides a feature?(key) method:

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

Feature Flipping Controllers

The Flip::ControllerFilters module is mixed into the base ApplicationController class. The following controller will respond with 404 Page Not Found to all but the index action unless the :new_stuff feature is enabled:

class SampleController < ApplicationController

  require_feature :something, :except => :index

  def show
  end

  def index
  end

end

Note that conditionally declared routes require a server restart to notice changes to feature flags, so they're not a good idea; database/cookie feature flipping will be ignored.

Command Center

A dashboard allows you to view the current state of the feature set, and flip any switchable strategies (database, cookie). Screenshot coming…

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Created by Paul Annesley
Copyright © 2011 Learnable Pty Ltd, MIT Licence.