June 2001
Intermediate to advanced
320 pages
9h 18m
English
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Validation
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.
Last-modified Timestamps
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
include a 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,
such as Date and Expires. If the server’s ...
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O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.Julian F.
I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.