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Feature flippers.
Latest commit 897100b @EricR EricR Merge pull request #71 from DarbySmart/randomize-percentages
Randomize user percentage groups



Feature flippers.


MAKE SURE TO READ THIS: 2.X Changes and Migration Path

As of rollout-2.x, only one key is used per feature for performance reasons. The data format is percentage|user_id,user_id,...|group,_group.... This has the effect of making concurrent feature modifications unsafe, but in practice, I doubt this will actually be a problem.

This also has the effect of rollout no longer being dependent on redis. Just give it something that responds to set(key,value) and get(key).

If you have been using the 1.x format, you can initialize Rollout with :migrate => true and it'll do its best to automatically migrate your old features to the new format. There will be some performance impact, but it should be limited and short-lived since each feature only needs to get migrated once.


If you'd prefer to continue to use the old layout in redis, Rollout::Legacy is a copy and paste of the old code :-).

Install it

gem install rollout

How it works

Initialize a rollout object. I assign it to a global var.

require 'redis'

$redis   =
$rollout =$redis)

Check whether a feature is active for a particular user:

$, User.first) # => true/false

Check whether a feature is active globally:


You can activate features using a number of different mechanisms.


Rollout ships with one group by default: “all”, which does exactly what it sounds like.

You can activate the all group for the chat feature like this:

$rollout.activate_group(:chat, :all)

You might also want to define your own groups. We have one for our caretakers:

$rollout.define_group(:caretakers) do |user|

You can activate multiple groups per feature.

Deactivate groups like this:

$rollout.deactivate_group(:chat, :all)

Specific Users

You might want to let a specific user into a beta test or something. If that user isn't part of an existing group, you can let them in specifically:

$rollout.activate_user(:chat, @user)

Deactivate them like this:

$rollout.deactivate_user(:chat, @user)

User Percentages

If you're rolling out a new feature, you might want to test the waters by slowly enabling it for a percentage of your users.

$rollout.activate_percentage(:chat, 20)

The algorithm for determining which users get let in is this:

CRC32( % 100 < percentage

So, for 20%, users 0, 1, 10, 11, 20, 21, etc would be allowed in. Those users would remain in as the percentage increases.

Deactivate all percentages like this:


_Note that activating a feature for 100% of users will also make it active “globally”. That is when calling Rollout#active? without a user object._

In some cases you might want to have a feature activated for a random set of users. It can come specially handy when using Rollout for split tests.

$rollout =$redis, randomize_percentage: true)

When on `randomize_percentage` will make sure that 50% of users for feature A are selected independently from users for feature B.

Feature is broken

Deactivate everybody at once:


For many of our features, we keep track of error rates using redis, and deactivate them automatically when a threshold is reached to prevent service failures from cascading. See for the failure detection code.


Rollout separates its keys from other keys in the data store using the “feature” keyspace.

If you're using redis, you can namespace keys further to support multiple environments by using the gem.

$ns =, :redis => $redis)
$rollout =$ns)
$rollout.activate_group(:chat, :all)

This example would use the “development:feature:chat:groups” key.


In our infrastructure, rollout obviously allows us to progressively enable new features but we also use it to automatically disable features and services that break or fail to prevent them from causing cascading failures and wiping out our entire system.

When a feature reaches “maturity” - in other words, expected to be at 100% rollout all the time - we use check_rollout.rb to setup nagios alerts on the rollouts so that we get paged if one of them gets disabled.

Implementations in other languages



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