RE: Synchronizing slots from primary to standby

On Thursday, November 23, 2023 11:45 PM Drouvot, Bertrand <bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:

Hi,

>
> On 11/23/23 6:13 AM, Amit Kapila wrote:
> > On Tue, Nov 21, 2023 at 4:35 PM Drouvot, Bertrand
> > <bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
> >>
> >> On 11/21/23 10:32 AM, shveta malik wrote:
> >>> On Tue, Nov 21, 2023 at 2:02 PM shveta malik <shveta(dot)malik(at)gmail(dot)com>
> wrote:
> >>>>
> >>
> >>> v37 fails to apply to HEAD due to a recent commit e83aa9f92fdd,
> >>> rebased the patches. PFA v37_2 patches.
> >>
> >> Thanks!
> >>
> >> Regarding the promotion flow: If the primary is available and
> >> reachable I don't think we currently try to ensure that slots are in
> >> sync. I think we'd miss the activity since the last sync and the promotion
> request or am I missing something?
> >>
> >> If the primary is available and reachable shouldn't we launch a last
> >> round of synchronization (skipping all the slots that are not in 'r' state)?
> >>
> >
> > We may miss the last round but there is no guarantee that we can
> > ensure to sync of everything if the primary is available. Because
> > after our last sync, there could probably be some more activity.
>
> I don't think so thanks to the fact that we ensure that logical walsenders on the
> primary wait for the physical standby.
>
> Indeed that should prevent any decoding activity on the primary while the
> promotion is in progress on the standby (at least as soon as the walreceiver is
> shutdown).
>
> So that I think that a promotion flow like:
>
> - walreceiver shutdown
> - last round of sync
> - sync-worker shutdown
>
> Should ensure that slots are in sync (as logical slots on the primary should not
> be able to advance as soon as the walreceiver is shutdown during the
> promotion).
>

I think it could not ensure the slots are in sync, because there is no
guarantee that the logical slot has caught up to the physical standby on
promotion and logical publisher and subscriber both could still be active
during promotion. IOW, the logical slot's LSN can still be advanced after the
walreceiver shutdown if it was far bebind the physical slot's LSN.

Best Regards,
Hou zj