Use of upstream keepalive_time

[Maxim Dounin]

Hello!

On Fri, Jan 20, 2023 at 11:37:15AM +0530, Vishwas Bm wrote:

> I see that from 1.19.10 keepalive_time has been added.
> http://nginx.org/en/docs/http/ngx_http_upstream_module.html#keepalive_time
> 
> Also keepalive_timeout is present for idle connection
> http://nginx.org/en/docs/http/ngx_http_upstream_module.html#keepalive_timeout
> 
> On checking the behaviour between these two, I see that keepalive_time is
> having higher precedence over keepalive_timeout.
> Even if connection is not idle based on keepqlive_timeout, connection is
> still  getting closed because of keepalive_time.
> 
> Is this expected behaviour?
> Also can I set keepalive_time to higher value say 24hours ? Any drawbacks
> with this ?
> Can this keepalive_time be disabled and priority given only to
> keepalive_timeout ?

The "keepalive_time" is a directive to limit total lifetime of the 
connection, making it possible to free any resources associated 
with the connection, notably allocated memory.  Further, in some 
setups it might be important to periodically redo connection 
authentication, notably re-validate peer certificates.

The "keepalive_time" directive is mostly equivalent to 
keepalive_requests, which is documented as follows:

: Closing connections periodically is necessary to free 
: per-connection memory allocations. Therefore, using too high 
: maximum number of requests could result in excessive memory usage 
: and not recommended.

Note though that keepalive_time is 1 hour by default, and reopening 
connections once per hour is not expected to have any performance 
impact.  Rather, it is expected to be a hard limit on the total 
connection lifetime on connections which are mostly idle and 
therefore do not reach the "keepalive_requests" limit in a 
reasonable time.

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