Is Chrome ignoring Cache-Control: max-age?
I got it. Google Chrome ignores the Cache-Control or Expires header if you make a request immediately after another request to the same URI in the same tab (by clicking the refresh button, pressing the F5 key or pressing Command + R). It probably has an algorithm to guess what does the user really want to do.
A way to test the Cache-Control header is to return an HTML document with a link to itself. When clicking the link, Chrome serves the document from the cache. E.g., name the following document self.html:
<!doctype html><html lang="en"><head> <meta charset="utf-8"> <title>Test Page</title></head><body> <p> <a href="self.html">Link to the same page.</a> If correctly cached, a request should not be made when clicking the link. </p></body></html>Another option is to copy the URL and paste it in the same tab or another tab.
UPDATE: On a Chrome post published on January 26, 2017, it is described what was the previous behavior and how it is changing by doing only revalidation of the main resource, but not of the sub-resources:
Users typically reload either because a page is broken or the content seems stale. The existing reload behavior usually solves broken pages, but stale content is inefficiently addressed by a regular reload, especially on mobile. This feature was originally designed in times when broken pages were quite common, so it was reasonable to address both use cases at once. However, this original concern has now become far less relevant as the quality of web pages has increased. To improve the stale content use case, Chrome now has a simplified reload behavior to only validate the main resource and continue with a regular page load. This new behavior maximizes the reuse of cached resources and results in lower latency, power consumption, and data usage.
In a Facebook post also published on January 26, 2017, it is mentioned that they found a piece of code were Chrome invalidates all cached resources after a POST request:
we found that Chrome would revalidate all resources on pages that were loaded from making a POST request. The Chrome team told us the rationale for this was that POST requests tend to be pages that make a change — like making a purchase or sending an email — and that the user would want to have the most up-to-date page.
It seems this is not the case anymore.
Finally, it is described that Firefox is introducing Cache-Control: immutable to completely stop revalidation of resources:
Firefox implemented a proposal from one of our engineers to add a new cache-control header for some resources in order to tell the browser that this resource should never be revalidated. The idea behind this header is that it's an extra promise from the developer to the browser that this resource will never change during its max-age lifetime. Firefox chose to implement this directive in the form of a cache-control: immutable header.
I hope this helps to untangle the reload mysteries.
Chrome appears to be ignoring your Cache-Control settings if you're reloading in the same tab. If you copy the URL to a new tab and load it there, Chrome will respect the cache control tags and reuse the contents from the cache.
As an example I had this Ruby Sinatra app:
#!/usr/bin/env rubyrequire 'sinatra'before do content_type :txtendget '/' do headers "Cache-Control" => "public, must-revalidate, max-age=3600", "Expires" => Time.at(Time.now.to_i + (60 * 60)).to_s "This page rendered at #{Time.now}."endWhen I continuously reloaded it in the same Chrome tab it would display the new time.
This page rendered at 2014-10-08 13:36:46 -0400.This page rendered at 2014-10-08 13:36:48 -0400.The headers looked like this:
< HTTP/1.1 200 OK< Content-Type: text/plain;charset=utf-8< Cache-Control: public, must-revalidate, max-age=3600< Expires: 2014-10-08 13:36:46 -0400< Content-Length: 48< X-Content-Type-Options: nosniff< Connection: keep-alive* Server thin is not blacklisted< Server: thinHowever accessing the same URL, http://localhost:4567/ from multiple new tabs would recycle the previous result from the cache.
After doing some tests with Cache-Control:max-age=xxx:
- Pressing reload button: header ignored
- Entering same url any tab (current or not): honored
- Using JS (
window.location.reload()): ignored - Using Developer Tools (with Disable cache unselected) or incognito doesn't affect
So, the best option while developing is put the cursor in the omnibox and press enter instead of refresh button.
Note: a right button click on refresh icon will show refresh options (Normal, Hard, Empty Cache). Incredibly, no one of these affect on these headers.