Browser Caching: Assets not revalidated when server sends a 304 ‘Not Modified’ for html page
I’ve been working on some web server middleware, and hit a weird issue that I couldn’t find documented anywhere. First, let’s look at an overview of how browser caching works:
If your web server sends an
ETag header in
a HTTP response, the web browser may choose to cache the response. Next time the
same object is requested, the browser may add an
header to let the server know that the browser might have the object cached. At this point, the server should respond with the
304 Not Modified
code and skip sending the response. This can also happen with the
ETag is not supported as well.
After implementing this in my middleware, I made a quick test website to try it
out. That’s when I ran into a weird behavior: the browser would revalidate the
HTML page itself with the
If-None-Match header, but when the server responded
304 it would not attempt to revalidate the linked stylesheets, scripts,
and images. The browser would not request them at all and immediately use the
cached version. It looks like if the server responds with
304 on the HTML
page, the browser assumes that all the linked assets are not modified as well.
That means that if the asset does change (you weren’t using something like
fingerprinting or versioning on your assets), then the browser will use outdated
Luckily it looks like there’s an easy solution: add
header to your responses.
no-cache doesn’t actually mean “don’t cache at all”,
but rather means that the browser needs to revalidate objects before using
the cached version.