proxy_cache_path 'inactive' vs http cache-control / expires headers?

[Maxim Dounin]

Hello!

On Fri, Apr 03, 2020 at 08:33:43AM -0700, Roger Fischer wrote:

> > You can just set the inactive time longer than your possible maximum expire time for the objects  then the cache manager won't purge the cache files even the object is still valid but not accessed.
> 
> That may only have a small impact.
> 
> As far as I understand:
> NGINX will remove an item only when the cache is full (ie. it needs space for a new item).
> Items are removed based on the least-recently used (LRU) queue.
> The least-recently-used (last) item in the LRU queue is unconditionally removed.
> The second and third last items are removed if they are past the invalid time.

Your understanding is wrong.  Cache manager always removes all 
items which were not access for the "inactive=" period of time.  
Quoting docs (http://nginx.org/r/proxy_cache_path):

: Cached data that are not accessed during the time specified by the 
: inactive parameter get removed from the cache regardless of their 
: freshness. By default, inactive is set to 10 minutes.

See ngx_http_file_cache_expire() function for details.

Additionally, cache items can be removed based on the "max_size=" 
parameter of the "proxy_cache_path" directive, or if there isn't 
enough room in the "keys_zone=" shared memory zone.

> The expiry of an item has no influence on the removal of items. 
> It only affects if the item is delivered from the cache, or 
> revalidated with an upstream request.

That's correct, as long as "expire" is meant to be the time from the 
Expires / X-Accel-Expires / Cache-Control / proxy_cache_valid.  In 
nginx documentation this is called "caching time".

-- 
Maxim Dounin
http://mdounin.ru/