Pennant

Feature flags for your application, with pluggable backends.

Installation

1. Add the dependency to your shard.yml:

   dependencies:
     pennant:
       github: jgaskins/pennant
   

2. Run shards install

Usage

Load and configure Pennant:

require "pennant"
require "redis"

redis = Redis::Client.new

Pennant.config do |c|
  c.adapter = Pennant::RedisAdapter.new(redis)
end

Now, anywhere in your application, you can simply call Pennant.enabled?("my-feature") to see if a particular feature flag is enabled.

To enable or disable a feature flag, you can simply call Pennant.enable("my-feature") or Pennant.disable("my-feature"), respectively. To run this in production, you can run the following command at a production Bash prompt (assuming your Pennant configuration exists in ./config/pennant.cr):

crystal eval 'require "./config/pennant"; Pennant.enable "my-feature"'

For example, if your Crystal application is running on Heroku, you can prefix the command with heroku run .... If it's running on Kubernetes, you could prefix it with kubectl exec -it $POD -- ....

Enabling for a given actor

Pennant allows enabling a feature for specific actors, such as specific users, groups. Any object in your application can be used for this purpose as long as it includes the Pennant::Actor mixin, which requires defining the pennant_id method:

struct User
  include Pennant::Actor

  getter id : UUID

  def pennant_id : String
    "User:#{id}"
  end
end

The format of pennant_id is up to you, as long as it uniquely identifies that person, group, or concept and resolves to the same value for it every time. For database-backed objects, this is often a class (or table) name and a primary key.

Once you have this in place, you can check whether a feature is enabled by passing for: actor to the enable, disable, and enabled? method:

if Pennant.enabled?("my-feature", for: current_user)
  # new hotness
else
  # old and busted
end

Enabling for a percentage of requests

Sometimes you don't necessarily want to enable a feature for the same people, but instead for a percentage of your application's traffic. For this, you can pass percentage_of_time: 0.1 to enable:

Pennant.enable("my-feature", percentage_of_time: 0.05)

All calls to Pennant.enabled?("my-feature") will now return true 5% of the time.

Web UI

There is a web UI in progress to manage feature flags so that you don't need to enable/disable them via production shells. The intent is to be able to insert it into your HTTP::Server middleware:

http = HTTP::Server.new([
  HTTP::LogHandler.new,
  Pennant::Web.new(mount_at: "/feature_flags"),
  # the rest of your HTTP handler entries ...
])

With the Lucky framework, you'd insert this into your middleware array.

With this in place, if you visit your application at /feature_flags you will see a list of your feature flags and their current states.

Contributing

1. Fork it (https://github.com/jgaskins/pennant/fork)
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create a new Pull Request

Contributors

• Jamie Gaskins - creator and maintainer