Interfaces are Dumb (and that's a Very Good Thing) - RailsConf
Both Webhooks and Rack enable creative uses that exceed the vision of the system implementor.
By following their example, you can allow extensibility and unforeseen advantages. Webhooks have enabled Github to offload development, and gives people an easy way do what they want with their data. Rack has taken the Ruby world by storm, and with good reason. You’ll see the simplest Rack server possible (just one line of code).