Question about proxy cache when it expires

[Igor Sysoev]

On Tue, May 12, 2009 at 10:38:04AM +0200, J?r?me Loyet wrote:

> Hello igor,
> 
> I have a question about the cache behaviour in proxy mode.
> 
> I have nginx in front head which redirect to an apache back end. Nginx
> caches eveything for M minutes.
> 
> If I have a large number of requests for the same page and this page
> is cached : nginx returns the cached page ... no problems
> After M minutes, the cached page expires
> The first request coming after the expiration makes nginx to ask the
> backend for refresh
> When nginx receives the backend fresh response, it's saved to cache
> and then nginx serves the fresh cached page
> 
> But what happen between the start of the request to the backend and
> the end of the response from the backend ? (let's assume that the
> backend serves the page in 5s ... and in 5s I can have a lot of
> request to this page).:
> - Are the request queued waiting for the backend response ?
> - Every request makes try to refresh the cache from the backend ? (in
> this case, I have multiple request for the same page to the backend
> ... I can have a burst of request and my apache can be overflowed by
> request -- that's why I'm using nignx with cache).
> - Do the requests serve the cached page even if it's expired until the
> backend response has been received ?
> - Maybe something else :)

Currently all requests which found that a cached response expired
are proxied to backend. I plan to implement busy locks to pass the single
request and leave others to wait the response up to specified time.


-- 
Igor Sysoev
http://sysoev.ru/en/