304 Not Modified with 200 (from cache)

Question

I'm trying to understand what exactly is the difference between "status 304 not modified" and "200 (from cache)"

I'm getting 304 on javascript files that I changed last. Why does this happen? Thanks for the assistance.

Answer 1

https://sookocheff.com/post/api/effective-caching/ is an excellent source to form the required understanding around these 2 HTTP status codes.

After reading this thoroughly, I had this understanding

In typical usage, when a URL is retrieved, the web server will return the resource's current representation along with its corresponding ETag value, which is placed in an HTTP response header "ETag" field. The client may then decide to cache the representation, along with its ETag. Later, if the client wants to retrieve the same URL resource again, it will first determine whether the local cached version of the URL has expired (through the Cache-Control and the Expire headers). If the URL has not expired, it will retrieve the local cached resource. If it determined that the URL has expired (is stale), then the client will contact the server and send its previously saved copy of the ETag along with the request in a "If-None-Match" field. (Source: https://en.wikipedia.org/wiki/HTTP_ETag)

But even when expires time for an asset is set in future, browser can still reach the server for a conditional GET using ETag as per the 'Vary' header. Details on 'vary' header: https://www.fastly.com/blog/best-practices-using-vary-header/

Answer 2

"200 (from cache)" means the browser found a cached response for the request made. So instead of making a network call to fetch that resource from the remote server, it simply made use of the cached response.

Now these cached responses have a life time associated with them. With time, the responses can become stale. And if these are not allowed to be served stale (see section 4.2.4 - RFC7234), the browser needs to contact the remote server and verify whether these cached responses are still valid or not. The 304 response status code is the servers' way of letting the browser know that the response hasn't changed and is still valid.

If the browser receives a 304 response, it can "freshen" the relevant stale response(s). And subsequent requests to the resource can again be served from the browser cache without checking with the remote server (until the response becomes stale again).

Answer 3

304 Modified
A 304 Not Modified means that the files have not been modified since the specified version by the "If-Modified-Since", or "If-None-Match".

200 OK
This is the response you'll get if an HTTP request works. GET requests will have something relating to the files. A POST request will have something holding the result of the action.

Happy coding!
Lyfe