gistfile1.rb

client = Client.new RedisWhere.new
client.track event

class RedisWhere
  def name
    'redis'
  end

  def track message
    $redis.rpush *message
  end
end

class Client
  attr_accessor :where
  attr_writer :formatter

  def initialize where
    self.where = where
  end

  def formatter
    @formatter ||= begin
      format = {'redis' => 'Redis'}.fetch(where.name, 'Plain')
      "Formatter::#{format}".constantize.new
    end
  end

  def track event
    where.track formatter.format(event)
  end
end

module Formatter
  class Redis
    def format event
      [event.payload[:user_id], event.to_json]
    end
  end

  class Plain
    def format event
      "#{event.payload[:user_id]}: #{event.name}"
    end
  end
end