How to stop accepting requests when you are upgrading your rails site?

Whenever I want to deploy my new version of rails app. I want to stop stop accepting requests from client side.

Example: User is filling form. In the back I am upgrading my rails web (not the rails or ruby version). When user submits the form while upgrade is happening or has happened, I want to show something like 'Web site has upgraded, please try again'. Is it actually possible, or some way to achieve it.