Implementing proxy_cache_lock when updating items

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Hello,

I am currently implementing a caching proxy with many short-lived items,
expiring at a specific date (Expires header set  at an absolute time between
10 and 30 seconds in the future by the origin). For various reasons, my
cache is multi-level (edge, intermediate 2, intermediate 1, origin) and
needs to make the items expire at the edge at 2exactly the time set in the
Expires header. When the item expires, i want an updated version of the item
to be available at the same URL.

I have been able to make it work, and I'm using proxy_cache_lock at every
cache level to ensure i'm not hammering the origin servers nor the proxy
server. As documented, this works perfectly for items not present in the
cache.

I am also using proxy_cache_use_stale updating to avoid this hammering also
in the case of already in-cache items.

My problems begin when an in-cache item expires. 2 top-level caches (let's
name them E1,E2 for example) request an updated fragment from the below
level (INT). The fragment is requested by INT from below for the first
request, but for the second request, a stale fragment is sent (according to
proxy_cache_use_stale setting, with the UPDATING status). So far, all is
working according to the docs. The problem is that fragments in the UPDATING
status are stale, and cannot be cached at all by E1,E2.., and this can be
very impacting for INT, because all the requests made on E1/E2 are now
proxied to INT directly, until INT has a fresh version of the item installed
in cache (this is quite a short duration, but in testing this generate
bursts of 15 to 50 requests in the mean time).

Is there a way to implement the proxy_cache_lock to make it work also for
expired in-cache items in the configuration ? If not, can you suggest a way
to implement this (i'm not familiar to nginx's source, but i'm willing to
dig into it) ?

Thanks,
Best regards,
Aurélien

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,261333,261333#msg-261333