Last-Modified header with genereted content

How do I use the Last-Modified header when generating pages from a database

[Egil]

When you don't use URL rewriting the webpage’s will be served with the different headers that help the browsers cache pages and save bandwidth when re-requesting them.

However, when I use URL rewriting and generated a page from content in a database, that may or may not have been updated since the visitor was last on my page, I loose the ability to tell the user that they already have a current version of a given page in their cache.

How do I regain that option? I assume I need to read the HTTP header from the client and see what version the client has on her computer, and then either do a short http header response saying “nothing changed” or resent the entire page. Am I going in the right direction here?

Best regards, Egil.

[eelixduppy]

Welcome to WebmasterWorld!

Many proxies and clients can be forced to disable caching with:

<?php
header [us3.php.net]("Cache-Control: no-cache, must-revalidate"); // HTTP/1.1
header("Expires: Mon, 26 Jul 1997 05:00:00 GMT"); // Date in the past
?>

--Excerpted from [php.net...]

[Egil]

Hi eelixduppy
I don't want to disable caching on the client, I want to utilize it to save bandwidth.
Let me try to explain myself differently:

I want to set the Last Modified header when I send a page in response to a normal HTTP GET (thanks for showing me how).

If the clients send a “conditional HTTP GET”, e.g. a HTTP GET that looks something like this:

GET /somepath/ HTTP/1.1
Host: www.exampel.com
If-modified-since: Wed, 2 Jul 2006 09:23:24

Then I want to check the “If-modified-since” parameter against the “last updated” field in the database for the given page, and if the page has not been modified, I want to respond with a HTTP/1.1 304 Not Modified …, otherwise I’ll return the page of course.

Has anybody done this before, if so, how did you do it?

[jatar_k]

you could store the date that generated page was last modified and just output a Last-Modified header every time. The when you edit any given row in your db just change the modified date.

[Egil]

hi jatar_k
That was indeed my plan. The thing I'm trying to figure out is how to read the "If-modified-since" in the conditional get and how to return a HTTP 304 instead of HTTP 200 from php.

Also, more generally, if anybody else had considered what I'm trying to do, and what their thoughts is about it - is it worth the hassle, extra cpu time etc.

Thanks for replying, Egil.

[jatar_k]

I don't know if it is worth the hassle, as long as you are setting the proper Last-Modified header the browser/spider should react accordingly.

[eeek]

Then I want to check the &#8220;If-modified-since&#8221; parameter against the &#8220;last updated&#8221; field in the database

Why not just use the last updated field in the database to set the http Last-Modified: header?

[Egil]

My plan IS to set the Last-Modified header field when ever I return a page.

But if a client does a "Conditional GET" (look a few posts back for a definition of a Conditional GET) I want to responde with HTTP 304 if the client already has the most up to date version of the page.