car_experiment.rb

class CarExperiment < ApplicationExperiment
  class CappedRollout < Gitlab::Experiment::Rollout
    def execute_assignment
      assignments = Users::ExperimentAssignment.where(experiment_name: @experiment.name).group(:variant_name).count
      variants    = %i[polo golf touran].select { |variant_name| assignments[variant_name.to_s].to_i < 5000 }
      return if variants.blank?

      variants[Digest::MD5.hexdigest(id).to_i(16) % variants.size]
    end
  end

  default_rollout CappedRollout

  control          { nil }
  variant(:polo)   { update_car_size "XS" }
  variant(:golf)   { update_car_size "S" }
  variant(:touran) { update_car_size "M" }

  def enabled?
    return false unless Feature.enabled?("#{name}_experiment", user)

    !user.experiment_assignments.where(experiment_name: name).exists?
  end

  def should_track?
    super && enabled? && "control" != assigned.name
  end

  private

  def update_car_size(size)
    # do something
  end
end