Software Architect's Handbook

Using a validation token

A common scenario with HTTP caching occurs when a response has expired from the cache but has not changed in any way. The client would be required to download the response again, which is wasteful since the resource has not changed.

A validation token in the ETag header of a response can be used to check whether an expired resource has changed. Clients can send the validation token along with a request. If a response has expired from the cache but the resource has not changed, there is no reason to download it again. The server will return a 304 Not Modified response and the browser will then know that it can renew the response in the cache and use it.