I will show you a technique to impose a rate limit (aka API Throttling) on a Ruby Web Service. I will be using Rack middleware so you can use this no matter what Ruby Web Framework you are using, as long as it is Rack-compliant.
Awesome. I can think of so many uses for this basic pattern to replace stuff I usually have to do at the app level (slow) or at the Apache level (difficult to port, prone to decay).
Comments note that Memcache might be a better pick than Redis, which works even better with the setup I have in mind, though Redis sounded fun too.