June 2000
Beginner to intermediate
80 pages
2h 49m
English
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If-Modified-Since
To accommodate client-side caching of documents, the client can use
the If-Modified-Since header with the GET method.
When using this option, the client requests the server to send the
requested information associated with the URL only if it has been
modified since a client-specified time.
If the document was modified, the server will give a status code of 200 and returns the document in the entity-body of its reply. If the document was not modified, the server will give a response code of 304 (Not Modified).
An example If-Modified-Since header might read:
If-Modified-Since: Fri, 02-Jun-95 02:42:43 GMT
If the server returns a code of 304, the document has not been
modified since the specified time. The client can use the cached
version of the document. If the document is newer, the server will
send it along with a 200 (OK) code. Servers may also include a
Last-Modified header
with the document, to let the user know when the last change was made
to the document.
Another related client header is
If-Unmodified-Since,
which says to only send the document if it hasn’t been changed
since the specified date. This is useful for ensuring that the data
is exactly the way you wanted it to be. For example, if you GET a
document from a server, make changes in a publishing tool, and PUT it
back to the server, you can use the
If-Unmodified-Since header to verify that the
changes you made are accepted by the server only if the previous one
you were looking at is still there.
If the ...
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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.