[PATCH] proxy_cache_max_range_offset could affect the background updating

[Maxim Dounin]

Hello!

On Mon, Dec 12, 2022 at 07:10:43PM +0400, Sergey Kandaurov wrote:

> > On 7 Dec 2022, at 09:26, Sangdeuk Kwon <sangdeuk.kwon at quantil.com> wrote:
> > 
> > # HG changeset patch
> > # User Sangdeuk Kwon <sangdeuk.kwon at quantil.com>
> > # Date 1670390583 -32400
> > #      Wed Dec 07 14:23:03 2022 +0900
> > # Node ID a1069fbf10ffd806b7c8d6deb3f6546edc7b0427
> > # Parent  0b360747c74e3fa7e439e0684a8cf1da2d14d8f6
> > proxy_cache_max_range_offset could affect the background updating
> > 
> > proxy_cache_max_range_offset doesn't care about the upstream of background updating.
> > So, nginx drops the new cache file after background updating.
> > This behavior is strange because background updating is just to fetch
> > new content after serving a stale cache, not to serve it.
> > 
> > I think the background updating should be not affected by proxy_cache_max_range_offset.
> > 
> 
> Indeed, such configuration with far ranges results in useless background
> (range) subrequests, the response is discarded.
> 
> While the behaviour matches the description of proxy_cache_max_range_offset,
> I believe the original intention was to avoid latencies in sending output
> to the client, imposed by obtaining a new full resource from upstream.
> As long as background subrequests don't delay sending output to the client,
> it should be ok to keep caching of (full) response enabled, regardless of
> whether the proxy_cache_max_range_offset limit is hit.

What is expected to happen if background requests are used for 
something other than cache updates?  For example, such requests 
can be created by the mirror module.  And I believe njs tried to 
use background requests as well.

Also note that the "background subrequests don't delay sending 
output to the client" claim is not really correct.  They do delay 
main request finalization, and therefore can delay further 
requests on the same connection.

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