Feature Switches

Status: Alpha (Don't use in production)

Summary

A gem that manages features switches in an application. Feature switches are dynamically configured. They can be turned on, on to a percentage of actors, on to a specific group of actors, or off without a code deploy. There are some excellent and mature projects that provide this functionality already: rollout and flipper. This project is an experiment aiming for a specific set of design goals:

1. Reduce the chatter in the protocol between a node checking to see if a feature is enabled for an actor and the backend storage system. We're going to be reading this data far more often than we're writing it so we want to aggressively cache, but...

2. Ensure all nodes get the latest configuration data as soon as possible. A cache that expires after a certain TTL can't work as a client isn't guaranteed to talk to the same instance of our application on each request. A feature disappearing and reappearing depending on which application server instance a user hits is a bug.

3. Allow for extension with new backends that support change notification; specifically distributed system synchronization backends like Zookeeper.

4. Ensure that any kind of actor identifier can be used; not just an object that responds to id We want to gate our switches on things that aren't ActiveRecord objects (e.g., incoming phone numbers, username, etc).

5. Expose an easy-to-remember CLI since irb is how we'll configure the feature switches.

Supported Backends

• Redis
• Postgres
• In-memory (for testing)

Design

Switches uses a backend for both storage of feature configuration data and for notifying sibling nodes that a change has been made. We'll look at how this works against the Redis backend.

On startup, switches will connect to Redis twice: once for querying and setting configuration data and one for subscribing to a pub/sub channel of change notifications. When a change is made to configuration data, an extra call is made to Redis to publish a change notification. Once this change notification is received by other listening nodes they will refetch the configuration data and update their local stores.

This allows a node to validate if a user can pass through a feature switch using in-memory data without a querying a backend but ensures that each node is using the same data to make the decision.

Installation

In your Gemfile:

  gem "feature_switches"

Postgres Backend

Note that switches will connect to Postgres twice for each node. This is important as Postgres will fork a new process for each connection so ensure you have the overhead before using this backend.

To use Postgres a table called switches must be created in your database. Two rake tasks have been included to create and drop this table:

1. Add this to your Rakefile:

  require "switches"
  require "switches/tasks"

2. To create the table:

  DATABASE_URL=postgres://root:sekret@localhost/my_application rake switches:postgres:setup

3. To drop the table

  DATABASE_URL=postgres://root:sekret@localhost/my_application rake switches:postgres:remove

Usage

# Initialize
$switches = Switches do |config|
  config.backend = "redis://localhost:6379/0"
end
# => #<Switches redis://localhost:6379/0>

# Check to see if a feature is active for an identifier
$switches.feature(:redesign).on?(current_user.id)
# => true

$switches.feature(:redesign).on?(current_user.phone_number)
# => true

# Turn a feature on globally
$switches.feature(:redesign).on
# => #<Feature redesign; 100%>

# Turn a feature on for a given percentage of identifiers
$switches.feature(:redesign).on(25)
# => #<Feature redesign; 25%>

# Turn a feature off globally
$switches.feature(:redesign).off
# => #<Feature redesign; 0%>

# Add or remove an identifier from a cohort group
$switches.cohort(:power_users).add(424)
# => #<Cohort power_users; 1 member>

$switches.cohort(:power_users).remove(424)
# => #<Cohort power_users; 0 members>

# Add a cohort group to a feature
$switches.feature(:redesign).add(:power_users)
# => #<Feature redesign; 0%; power_users>

# Remove a cohort group from a feature
$switches.feature(:redesign).remove(:power_users)
# => #<Feature redesign; 0%>

License

Please see LICENSE.