Prevent an HTTP client from hitting a server with cache (iphone)

Question

Ok, I'm confused. I'm trying to send back the magic headers from my server that will prevent a client from hitting the server again until a resource is stale.

I understand how ETag or Last-Modified works (Validation) - the client will ALWAYS still hit the server, and the server needs to validate the date or etag against the current value to know whether to bother serving up a new one.

Cache-Control and Expires, however, I don't think I understand. I've set the following:

Cache-Control: max-age=86400, must-revalidate

No matter what I do, my client (my browser, curl, NSURLConnection) always hits the server again on the second request. Is this a client thing? What headers should I send back to get the client to use it's private cache for a certain length of time?

Answer 1

The server needs to check the If-Modified-Since header and return a 304 not modified header if it wants the browser to keep caching.

Answer 2

As Nathan hints at in his answer, clients can issue a subsequent request with an If-Modified-Since header to determine whether or not their cache is stale. If the client receives a 304 Not Modified response, it will serve the content out of the local cache.

According to RFC 2616 (the HTTP 1.1 specification), the presence of must-revalidate within the Cache-control header forces clients to re-check their cache's status with the originating server prior to serving out of the cache.

Answer 3

For future reference - Mark Nottingham has written a great guide to HTTP caching:

http://www.mnot.net/cache_docs/#CACHE-CONTROL