I’ve already discussed cache validation in the context of cache hits versus misses. Upon receiving a request for a cached object, the cache may want to validate the object with the origin server. If the cached object is still valid, the server replies with a short HTTP 304 (Not Modified) message. Otherwise, the entire new object is sent. HTTP/1.1 provides two validation mechanisms: last-modified timestamps and entity tags.
Under HTTP/1.0, timestamps are the only type of validator.
Even though HTTP/1.1 provides a new technique, last-modified
timestamps remain in widespread use. Most HTTP responses
Last-modified header that specifies the time when the
resource was last changed on the origin server. The
Last-modified timestamp is given in Greenwich Mean Time (GMT)
with one-second resolution, for example:
HTTP/1.1 200 OK Date: Sun, 04 Mar 2001 03:57:45 GMT Last-Modified: Fri, 02 Mar 2001 04:09:20 GMT
For objects that correspond to regular files on the origin server, this timestamp is the filesystem modification time.
When a cache validates an object, this same timestamp is
sent in the
If-modified-since header of
a conditional GET request.
GET http://www.ircache.net/ HTTP/1.1 If-Modified-Since: Wed, 14 Feb 2001 15:35:26 GMT
If the server’s response is 304 (Not Modified), the cache’s
object is still valid. In this case, the cache must
update the object to reflect any new HTTP response header values,
Expires. If the server’s ...