Understanding caching

Hi there,

i need to understand the caching options better. So, i have serval
Questions :wink:

I added some Cacherules like these:

    location  /js {
            add_header        Cache-Control public;
            add_header        Cache-Control must-revalidate;
            expires           7d;
}


    location  /css {
            add_header        Cache-Control public;
            add_header        Cache-Control must-revalidate;
            expires           7d;
}

    location  /img {
            add_header        Cache-Control public;
            add_header        Cache-Control must-revalidate;
            expires           7d;
}

    location  /bundles {
            add_header        Cache-Control public;
            add_header        Cache-Control must-revalidate;
            expires           7d;
}

    location  /alloyeditor {
            add_header        Cache-Control public;
            add_header        Cache-Control must-revalidate;
            expires           7d;
}

No when i browser arround Chrome said „From cache“ that works fine for
me but when i reload the Website it seems that only *.JPGs comming from
the cache.
Is that a Browser issue or a nginx issue? Need to know where i have to
search :wink:

Closing the browser and start again it dont came from cache - but as i
understand i say it will expire in 7 Days - so why nit not comming from
local cache?

Cheers

Daniel

On Thu, Jun 30, 2016 at 10:45:31AM +0200, Daniel Eschner wrote:

Hi there,

i need to understand the caching options better. So, i have serval Questions :wink:

I think in this mail, you refer to the caching done by your browser.

That is entirely controlled by your browser; but it will probably
respect
the suggestions that the server makes.

“A web page” is typically one request for some html, plus multiple
requests for css, js, and image files.

Each of those requests is independent, and each of the responses can be
cached (or not) by your browser, as it sees fit.

            add_header        Cache-Control public;
            add_header        Cache-Control must-revalidate;
            expires           7d;

These lead to http response headers sent to the browser; how the browser
handles them it entirely its business. Essentially the server is
suggesting “do cache this, cache it for 7 days, and then check again”.

You can look at the actual http response headers received by your
browser,
and you can check its documentation for how it is expected to handle
them.

(It is possible that other response headers are sent which say “do not
cache this”.)

No when i browser arround Chrome said „From cache“ that works fine for me but
when i reload the Website it seems that only *.JPGs comming from the cache.

“reload” is probably an instruction to your browser to fetch the (first,
html) page again from outside its local cache.

If you follow a link and then use your browser’s “back” button, that
can be an instruction to your browser to show the page again, from local
cache if possible.

Usually there is also an instruction available to your browser to invite
it to fetch the page again, from the origin web server (bypassing or
breaking through any intermediate caches).

In any case, the multiple subsequent css, js, image requests may be
handled from the local cache, or may be made to “the web”, or may be
made to the origin web server. Your browser decides what requests,
if any, it will make.

Is that a Browser issue or a nginx issue? Need to know where i have to search :wink:

It seems like expected behaviour to me.

It is initially a browser issue.

If you can see a request that you think should not be made; and if you
can see the previous response that you think should have been used
instead;
and if you can see a “please do not cache this” in that response; then
you
can check where in your nginx that “please do not cache this” came from.

Closing the browser and start again it dont came from cache - but as i
understand i say it will expire in 7 Days - so why nit not comming from local
cache?

The server said “please cache this for 7 days”.

The browser may or may not have done that. Perhaps it never writes to
disk; or clears the disk cache on startup, or something.

Cheers,

f

Francis D. [email protected]

To add to Francis’ answer, browsers might not respect server
specification
for expiration.

must-revalidate however forces the browser to check the expiration of
the
resource before attempting of really load it again from the server.
Read: https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9.4
When the browser contacts server with the right version the resource in
its
local cache already, the request is replied with a 304, which you might
see
in you requests log in you browser console.

The probable scenario you will notice is: the base resource (webpage)
will
be downloaded again (200) because there is a user action requesting it.
However, you should see the other included resources, typically small
ones
like pictures, stylesheets, are only revalidated and are replied 304.
That is the standard behavior of nginx, though, you should not need
special
rules for it.

B. R.

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