Trebuchet

Trebuchet launches features at people. Wisely choose a strategy, aim, and launch!

Installation

Trebuchet can be used with Rails or standalone.

To use with Rails: gem 'trebuchet', :require => 'trebuchet_rails'

Setup

Trebuchet defaults to storing data in memory, or can be used with Redis or Memcache as a data store:

Trebuchet.set_backend :memcached
Trebuchet.set_backend :redis, :client => Redis.new(:host => 'example.com')
Trebuchet.set_backend :redis_cached, :client => Redis.new(:host => 'example.com')

A Rails initializer is a great spot for this. You may want to use a few other settings, either hardcoded values or procs (eval'd in the context of the controller):

Trebuchet.admin_view = proc { current_user.try(:admin?) } # /trebuchet admin interface access
Trebuchet.time_zone = proc { current_user.time_zone } # or just "Mountain Time (US & Canada)"

Aim

Trebuchet can be aimed while your application is running. The syntax is:

Trebuchet.aim('awesome_feature', :percent, 1)

Which will launch 'awesome_feature' to 1% of users.

Another builtin strategy allows launching to particular user IDs:

Trebuchet.aim('awesome_feature', :users, [23, 42])

You can also combine multiple strategies, in which case the feature is launched if any of them is true:

Trebuchet.feature('awesome_feature').aim(:percent, 1).aim(:users, [23, 42])

If you don't aim Trebuchet for a feature, the default action is not to launch it to anyone.

Launch

In a view, do this:

<% trebuchet.launch('time_machine') do %>
    <p>Welcome to the future!</p>
<% end %>

The code between do .. end will only run if the strategy for 'time_machine' allows launching to current_user.

You can also use it in a controller:

def index
    trebuchet.launch('time_machine') do
        @time_machine = TimeMachine.new
    end
end

Custom Strategies

Trebuchet ships with a number of default strategies but you can also define your own custom strategies like so:

Trebuchet.define_strategy(:admins) do |user|
    !!(user && user.has_role?(:admin))
end

controller.current_user is yielded to the block and it should return true for users you want to launch to. You can use parameters with custom strategies too:

Trebuchet.define_strategy(:markets) do |user, markets|
    markets.include?(user.market)
end

Like parameters for builtin strategies, these can be changed while the application is running. For example:

Trebuchet.aim('time_machine', :markets, ['San Francisco', 'New York City'])

When using Trebuchet together with Rails, a good place to define custom strategies is in an initializer.

Visitor Strategy

Trebuchet can be used to launch to visitors (no user object present). First, set the visitor id either directly (in a before filter) or as a proc:

Trebuchet.visitor_id = 123

Trebuchet.visitor_id = proc { |request| request && request.cookies[:visitor] && request.cookies[:visitor].hash }

If you're using a proc, Trebuchet passes in the request object. It expects that the proc returns an integer. If it returns anything else, Trebuchet will not launch.

Fiber and Thread Safety

Trebuchet stores global state such as Trebuchet.current which is thread and fiber unsafe behavior. In order to use these features in a fiber or threaded environment, Trebuchet.threadsafe_state = true will cause Trebuchet to store these values in a thread-local state object instead. This is not the default for backward compatability reasons.

2legit/trebuchet