What is the difference between 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

User · Answer

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 of HTTP status codes

User · Answer

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

User · Answer

For your last question  why   I ll try to explain with what I know  A brief explanation of those three status codes in layman s terms    200 - success  browser requests and get file from server    If caching is enabled in the server   200  from memory cache  - file found in browser  so browser is not going request from server 304 - browser request a file but it is rejected by server   For some files browser is deciding to request from server and for some it s deciding to read from stored  cached  files  Why is this   Every files has an expiry date  so  If a file is not expired then the browser will use from cache  200 cache     If file is expired  browser requests server for a file  Server check file in both places  browser and server   If same file found  server refuses the request  As per protocol browser uses existing file   look at this nginx configuration  location         add header Cache-Control must-revalidate      expires     60      etag on               Here the expiry is set to 60 seconds  so all static files are cached for 60 seconds  So if u request a file again within 60 seconds browser will read from memory  200 memory   If u request after 60 seconds browser will request server  304    I assumed that the file is not changed after 60 seconds  in that case you would get 200  ie  updated file will be fetched from server    So  if the servers are configured with different expiring and caching headers  policies   the status may differ   In your case you are using cdn  the main purpose of cdn is high availability and fast delivery  Therefore they use multiple servers  Even though it seems like files are in same directory  cdn might use multiple servers to provide u content  if those servers have different configurations  Then these status can change  Hope it helps

User · Answer

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

User · Answer

This threw me for a long time too  The first thing I d verify is that you re not reloading the page by clicking the refresh button  that will always issue a conditional request for resources and will return 304s for many of the page elements  Instead go up to the url bar select the page and hit enter as if you had just typed in the same URL again  that will give you a better indicator of what s being cached properly  This article does a great job explaining the difference between conditional and unconditional requests and how the refresh button affects them  http   blogs msdn com b ieinternals archive 2010 07 08 technical-information-about-conditional-http-requests-and-the-refresh-button aspx

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

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