Re: Improve handling of parameter differences in physical replication

On Sat, 29 Feb 2020 at 06:39, Alvaro Herrera <alvherre@2ndquadrant.com> wrote:
>
> On 2020-Feb-27, Peter Eisentraut wrote:
>
> > So this patch relaxes this a bit.  Upon receipt of
> > XLOG_PARAMETER_CHANGE, we still check the settings but only issue a
> > warning and set a global flag if there is a problem.  Then when we
> > actually hit the resource issue and the flag was set, we issue another
> > warning message with relevant information.  Additionally, at that
> > point we pause recovery, so a hot standby remains usable.
>
> Hmm, so what is the actual end-user behavior?  As I read the code, we
> first send the WARNING, then pause recovery until the user resumes
> replication; at that point we raise the original error.

I think after recovery is paused users will be better to restart the
server rather than resume the recovery. I agree with this idea but I'm
slightly concerned that users might not realize that recovery is
paused until they look at that line in server log or at
pg_stat_replication because the standby server is still functional. So
I think we can periodically send WARNING to inform user that we're
still waiting for parameter change and restart.

Regards,

-- 
Masahiko Sawada            http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services