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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.
238
The HTTP 304 Not Modified client redirection response code indicates that there is no need to retransmit the requested resources. It is an implicit redirection to a cached resource.
Status Code: 200 OK (from disk cache) shows that the response was indeed served from cache. Browser will serve this response from cache until the response expires i.e. in this case, if it has been in cache for more than 30 seconds.
The 200 status code is by far the most common returned. It means, simply, that the request was received and understood and is being processed. A 201 status code indicates that a request was successful and as a result, a resource has been created (for example a new page).
The items with code "200 (cache)" were fulfilled directly from your browser cache, meaning that the original requests for the items were returned with headers indicating that the browser could cache them (e.g. future-dated Expires or Cache-Control: max-age headers), and that at the time you triggered the new request, those cached objects were still stored in local cache and had not yet expired.
304s, on the other hand, are the response of the server after the browser has checked if a file was modified since the last version it had cached (the answer being "no").
For most optimal web performance, you're best off setting a far-future Expires: or Cache-Control: max-age header for all assets, and then when an asset needs to be changed, changing the actual filename of the asset or appending a version string to requests for that asset. This eliminates the need for any request to be made unless the asset has definitely changed from the version in cache (no need for that 304 response). Google has more details on correct use of long-term caching.
233
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.
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