Gonzalo Arana wrote:
again, I am in the dark: why do cache request headers may need to be replaced or edited in the same entity?
It's a requirement of the HTTP/1.1 spec.HTTP requests can be conditional, in other words a browser (or a proxy) can ask a server "give me this URL, but only if it has changed from my cached copy".
If the server thinks that the file has changed (or Cache-Control: no-cache was specified), then the server will send a full response back headers + body, and the browser/proxy replaces it's cached copy with the new headers+body.
If the server thinks that the file is the same, ie it didn't change, the server sends back the magic code "304 Not Modified", and just the headers - without any body. These new headers must replace the existing headers in the browser/proxy's cached entry, making the cached entry "fresh" again. And here lies the problem.
Doing the request this way means you don't have to ask the backend "is my cached copy still fresh?", get an answer back "No", and then send a second request saying "ok then, give me the new data" - you can implement caching in one request.
The catch is when the server sent "304 Not Modified" - you need to update your cache to say "yep, my cached entry is still fresh", ie update the headers, without touching the body, which hasn't changed.
That's why I suggested a dedicated process/thread for cache administration, which is not a good idea if too many lookups are issued to this process on each request received.
mod_cache definitely needs cache admin, currently it's implemented as an external program that is called via cron, which doesn't help if you're on a box without cron. Cache cleaning can be done either when a connection is complete in the existing process (which may be simpler to implement, but it runs after every connection), or it can be done as you suggest, where a dedicated thread/process handles this independently.
I think in the long run, a dedicated process is the way to go. Regards, Graham --
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