Web Caching

Forcing a Cache to Refresh

One of the tradeoffs of caching is that you may occasionally receive stale data. What can you do if you believe (or know) that a cache has given you stale data? You need some way to refresh or validate the data received from the cache. HTTP provides a couple of mechanisms for doing just that. Clients can generate requests with Cache-control directives, the two most common of which are no-cache and max-age. We’ll discuss no-cache first because it has been around the longest.

The no-cache Directive

The no-cache directive notifies a cache that it cannot return a cached copy. Even if a fresh copy of the response—with a specific expiration time—is in the cache, the client’s request must be forwarded to the origin server. RFC 2616 calls such a request an “end-to-end validation” (Section 14.9.4). The no-cache directive is sent when you click on the Reload button on your browser. In an HTTP request, it looks like this:

GET /index.html HTTP/1.1
Cache-control: no-cache

Recall that the Cache-control header does not exist in the HTTP/1.0 standard. Instead, HTTP/1.0 clients use a Pragma header for the no-cache directive:

Pragma: no-cache

no-cache is the only directive defined for the Pragma header in RFC 1945. For backwards compatibility, RFC 2616 also defines the Pragma header. In fact, many of the recent HTTP/1.1 browsers still use Pragma for the no-cache directive instead of the newer Cache-control.

Note that the no-cache directive does not necessarily require the ...