Often times we don’t want an HTTP client to blindly cache content that it has been given. We may not be able to rely on setting specific expiration headers either. Instead we need a way for the client to ask the server whether or not a resource has been updated.

HTTP provides us with the ability to do this with conditional caching. A client can make a request to the server and find out of the server has a new version of the resource available. Rails provides us with tools to take advantage of this.