api_steps.rb

When /^I send a GET request for "([^"]*)"$/ do |path|
    get path
end

When /^I send more than one GET request in a second to "([^"]*)"$/ do |path|
    # We'll assume this happens in < 1 second
    get path
    get path
end

Then /^the response code should be "([^"]*)"$/ do |code|
    last_response.status.should == code.to_i
end

my_dumb_api.feature

Feature: My Dumb API
  In order to retrieve an API response
  As a web API developer
  I want an API to respond to my requests

  Scenario: API is available
    When I send a GET request for "http://example.com/api/"
    Then the response code should be "200"

  Scenario: Exceeding API Query Rate
    When I send more than one GET request in a second to "http://example.com/api"
    Then the response code should be "403"

routes.rb

RailsApp::Application.routes.draw do
  mount Rack::Builder.new {
    use Rack::Throttle::Interval, :min => 1.0
    run proc { |env|
      [200, {}, ["OK"]]
    }
  } => "/api"
end