Google caches the results of URL Fetch requests so subsequent requests can be supplied from the cache, thereby speeding up the request and your application in general.
This can be troublesome though, especially if an application is accessing a web page that changes quickly; URL fetch may be returning stale results without the application understanding this. Fortunately, there’s a way to detect whether or not the page was retrieved from a Google cache server.
For all URL Fetch requests from the production App Engine servers, the X-Google-Cache-Control header is added to URL Fetch responses. If the header has a value of remote-fetch, then the fetch retrieved a fresh copy of the page. If the value is remote-cache-hit, then the page was retrieved from Google’s cache and may have stale data.
Here’s how the header will look like if it’s a cache hit:
While a freshly retrieved page will have this header: