Memory usage in nginx proxy setup and use of min_uses

[Maxim Dounin]

Hello!

On Sun, May 16, 2021 at 04:46:17PM +0000, Lucas Rolff wrote:

> Hi everyone,
> 
> I have a few questions regarding proxy_cache and the use of 
> proxy_cache_min_uses in nginx:
> 
> Let’s assume you have an nginx server with proxy_cache enabled, 
> and you’ve set proxy_cache_min_uses to 5;
> 
> Q1: How does nginx internally keep track of the count for 
> min_uses? Is it using SHM to do it (and counts towards the 
> key_zone limit?), or something else?
> 
> Q2: How long time does nginx keep this information for the 
> number of accesses. Let’s say the file gets visited once in a 24 
> hour period; Would nginx keep the counter at 1 for that whole 
> period, or are there some set timeout where it’s “flushed”.
> 
> Q3: If you have a user who decides to access files with a random 
> query string on it; We want to prevent caching this to fill up 
> the storage (The main reason for setting the 
> proxy_cache_min_uses in the first place) – but are we gonna fill 
> up the memory (and keys_zone limit) regardless; If yes – is 
> there a way to prevent this?
> 
> Basically the goal is to understand even just broadly how 
> min_uses are counted, and possibly how to prevent memory from 
> being eaten up in case someone decides to access the same URL 
> once with millions of requests – if there’s any way to flush out 
> the memory for example, for anything that haven’t yet reached 
> the proxy_cache_min_uses if it indeed uses up memory.

The proxy_cache_min_uses basically means that if nginx sees a 
request whose uses count not reached the specified limit yet, it 
won't try to store the response to disk.  It will, however, keep 
the key in the keys_zone with the relevant information, notably 
the number of uses seen so far.  Quoting the proxy_cache_path 
directive description (http://nginx.org/r/proxy_cache_path):

"In addition, all active keys and information about data are stored 
in a shared memory zone, whose name and size are configured by the 
keys_zone parameter. One megabyte zone can store about 8 thousand 
keys."

Much like with any cache item, such keys are removed from the 
keys_zone if no matching requests are seen during the "inactive" 
time.  Similarly, least recently used keys are removed if there is 
not enough room in the keys_zone.

Much like with normal caching, you can control the cache key nginx 
uses.  If you don't want to take query string into account, you 
may want to configure proxy_cache_key without the query string 
(see http://nginx.org/r/proxy_cache_key).

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