Define hookable API in module that’s included, this way other clients can include a module that overrides those hooks. Because including modules inserts the methods in the ancestor lookup chain (i.e. the power of Ruby), this allows the use of super etc.

gistfile1.rb

module APIExposable
  def expose_api(mod = nil, &block)
    if mod.nil?
      if block.nil?
        raise "Either a module or a block that's used to create a module is required."
      else
        mod = Module.new(&block)
      end
    elsif mod && block
      raise "Only a module *or* is required, not both."
    end
    include mod
  end

  alias_method :override_api, :expose_api
end

module API
  def execute(command)
    puts command
  end
end

class Downloader
  extend APIExposable
  expose_api API

  def download!
    execute('git clone URL')
  end
end

d = Downloader.new
d.download!

class Downloader
  override_api do
    def execute(command)
      puts "\nExecuting:"
      super
    end
  end
end

d.download!