What is the difference between HTTP status code 200 (cache) vs status code 304?

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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.

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