HTTP status code 200 (cache) vs status code 304?

Question

I'm using the Google "Page Speed" plug-in for Firefox to access my web site.

Some of the components on my page is indicated as HTTP status:

200 200 (cache) 304

By Google's "Page Speed".

What I'm confused about is the difference between 200 (cache) and 304.

I've refreshed the page multiple times (but have not cleared my cache) and it always seems that my favicon.ico and a few images are status=200 (cache) while some other images are http status 304.

I don't understand why the difference.

UPDATE:

Using Google "Page Speed", I receive a "200 (cache)" for http://example.com/favicon.ico as well as http://cdn.example.com/js/ga.js

But, I receive a http status "304" for http://cdn.example.com/js/combined.min.js

I don't understand why I have two JavaScript files located in the same directory /js/, one returning a http status 304 and the other returning a 200 (cache) status code.

Answer 1

304 is unmodified. i get this code a lot in my media files like css and js.

http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.5

Answer 2

The items likely have long-lasting Expires: or similar headers that denote that they will be valid for awhile without refresh, or are a special case like favicons (most browsers cache them awhile without checking for changes unless you force-refresh or clear cache), So the browser is not even checking whether these items were modified, just pulling them straight from cache.

304s, on the other hand, are the response of the server after the browser has checked if the file was modified since the last version it had cached (the answer being "no").

When Firebug shows the 200 responses, there was not in fact a 200 response from the server (there wasn't even a request sent to the server), that's just the status code used to indicate that there was a successful retrieval from local cache of the item.

Answer 3

HTTP 304 is "not modified". Your web server is basically telling the browser "this file hasn't changed since the last time you requested it." Whereas an HTTP 200 is telling the browser "here is a successful response" - which should be returned when it's either the first time your browser is accessing the file or the first time a modified copy is being accessed.

For more info on status codes check out http://en.wikipedia.org/wiki/List%5Fof%5FHTTP%5Fstatus%5Fcodes.

Answer 4

This new pagespeed craze is driving me nuts! There is 0 documentation on some of the data outputted, including this "200(cache)" issue... Almost more frustrating though are the explanations that are given. As a 'noob' to this caching thing, another thing that bothered me is lack of real guidance after advice. The page does a good job of explaining what things like 'serving static content from a cookieless domain' and/or 'Parallelize downloads across hostnames' are, but do not do a good job of explaining exactly how to go about doing these things...

That's my rant and now I also have a question... i am frequently calling different 'static' elements (ie: header.php, sidebar.php, porn-widget.php, etc) throughout my site with statements. For page load times should these elements use require_once as they are not doing anything dynamic?

Thanks and you are a hero if you read all this.

Answer 5

200 (cache) means Firefox is simply using the locally cached version. This is the fastest because no request to the Web server is made.

304 means Firefox is sending a "If-Modified-Since" conditional request to the Web server. If the file has not been updated since the date sent by the browser, the Web server returns a 304 response which essentially tells Firefox to use its cached version. It is not as fast as 200 (cache) because the request is still sent to the Web server, but the server doesn't have to send the contents of the file.

To your last question, I don't know why the two JavaScript files in the same directory are returning different results.