Feature Flags on Rails

During the previous post I looked at what feature flags are and why you'd want to use them. In this post I will explore how I've implemented them in the current web-application I'm working on. It's a very simple process which means there are no excuses not to use a feature flag in your application.

everything is easier on Rails

The Rollout gem

I use the Rollout gem as a way of implementing Feature flags. It's pretty easy to use although there are a few things I don't like about it. The important things are:

1. It relies on redis which I already have in my stack.
2. It doesn't have a user-interface so you must enable features with the production console.

Here's how you'd use rollout:

if $rollout.active? :my_cool_feature
  my_cool_feature
end

Then you'd be able to activate that feature on the console by typing:

$rollout.activate :my_cool_feature

Activating a single user or a group

Often you'll want to activate a single user or a group of users. To do this we need to modify our feature flag invocation as follows:

if $rollout.active? :my_cool_feature, current_user
  my_cool_feature
end

Then you could activate it for a specific user as follows:

$rollout.activate_user :my_cool_feature, User.find(1)

Alternatively you can define a group of users:

# config/initializers/rollout.rb

$rollout.define_group(:staff) do |user|
  user.staff?
end

Then you can specify a group to activate in the console instead:

$rollout.activate_group :my_cool_feature, :staff

Global variables are yucky

Personally I don't like litering my codebase with $global $variables. Also it's normally useful that feature flags are always enabled in development and test environments. What I do is wrap Rollout in a very small class:

class Feature
  def self.active?(*args)
    (!feature_flags_enabled?) || $rollout.active?(*args) rescue true
  end

  def self.feature_flags_enabled?
    Rails.env.production? || Rails.env.staging?
  end
end

This lets me use Feature.active? in my code which I believe is more intention-revealing than $rollout.active? and more aesthetically pleasing.

Ready to Rollout?

Rollout has other features (like enabling flags based on percentages) and the README has links detailing how to automatically disable features based on errors. The reason I like Rollout is that it's very simple to get going, and integrates easily into your own stack (assuming you have Redis).

There are alternative libraries you can use such as Flip which have different features such as ActiveRecord support, or a web-dashboard for enabling/disabling flags.