Re: Synchronizing slots from primary to standby

On Mon, Dec 11, 2023 at 1:22 PM Drouvot, Bertrand
<bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
>
> Hi,
>
> On 12/8/23 10:06 AM, Amit Kapila wrote:
> > On Wed, Dec 6, 2023 at 4:53 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> >>
> >> PFA v43, changes are:
> >>
> >
> > I wanted to discuss 0003 patch about cascading standby's. It is not
> > clear to me whether we want to allow physical standbys to further wait
> > for cascading standby to sync their slots. If we allow such a feature
> > one may expect even primary to wait for all the cascading standby's
> > because otherwise still logical subscriber can be ahead of one of the
> > cascading standby.
>
> I've the same feeling here. I think it would probably be expected that
> the primary also wait for all the cascading standby.
>
> > I feel even if we want to allow such a behaviour we
> > can do it later once the main feature is committed.
>
> Agree.
>
> > I think it would
> > be good to just allow logical walsenders on primary to wait for
> > physical standbys represented by GUC 'standby_slot_names'.
>
> That makes sense for me for v1.
>
> > If we agree
> > on that then it would be good to prohibit setting this GUC on standby
> > or at least it should be a no-op even if this GUC should be set on
> > physical standby.
>
> I'd prefer to completely prohibit it on standby (to make it very clear it's not
> working at all) as long as one can enable it without downtime once the standby
> is promoted (which is the case currently).

And I think slot-sync worker should exit as well on cascading standby. Thoughts?

If we agree on the above, then we need to look for a way to
distinguish between first and cascading standby. I could not find any
existing way to do so. One possible approach is to connect to the
remote using PrimaryConninfo and run 'pg_is_in_recovery()' there, if
it returns true, then it means we are cascading standby. Any simpler
way to achieve this?

thanks
Shveta