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