Re: Improve pg_sync_replication_slots() to wait for primary to advance

On Fri, Oct 31, 2025 at 11:04 AM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
>
> On Thu, Oct 30, 2025 at 3:48 PM Ajin Cherian <itsajin(at)gmail(dot)com> wrote:
> >
> >
> > Thanks for your review, Japin. Here's patch v20 addressing the comments.
> >
>
> Thank You for the patch. Please find a few comment son test:
>
>
> 1)
> +# until the slot becomes sync-ready (when the standby catches up to the
> +# slot's restart_lsn).
>
> I think it should be 'when the primary server catches up' or 'when the
> remote slot catches up with the locally reserved position.'
>
> 2)
> +# Attempt to synchronize slots using API. This will initially fail because
> +# the slot is not yet sync-ready (standby hasn't caught up to slot's
> restart_lsn),
> +# but the API will wait and retry. Call the API in a background process.
>
> a)
> 'This will initially fail ' seems like the API will give an error,
> which is not the case
>
> b) 'standby hasn't caught up to slot's restart_lsn' is not correct.
>
> We can rephrase to:
> # Attempt to synchronize slots using the API. The API will continue
> retrying synchronization until the remote slot catches up with the
> locally reserved position.
>
> 3)
> +# Enable the Subscription, so that the slot catches up
>
> slot --> remote slot
>
> 4)
> +# Create xl_running_xacts records on the primary for which the
> standby is waiting
>
> Shall we rephrase to below or anything better if you have?:
> Create xl_running_xacts on the primary to speed up restart_lsn advancement.
>
> 5)
> +# Confirm that the logical failover slot is created on the standby and is
> +# flagged as 'synced'
>
> Suggestion:
> Verify that the logical failover slot is created on the standby,
> marked as 'synced', and persisted.
>
> (It is important to mention persisted because even temporary slot is
> marked as synced)
>

Shall we remove this change as it does not belong to the current patch
directly? I think it was a suggestion earlier, but we shall remove it.

6)
-# Confirm the synced slot 'lsub1_slot' is retained on the new primary
+# Confirm that the synced slots 'lsub1_slot' and 'snap_test_slot' are
retained on the new primary
is( $standby1->safe_psql(
'postgres',
q{SELECT count(*) = 2 FROM pg_replication_slots WHERE slot_name IN
('lsub1_slot', 'snap_test_slot') AND synced AND NOT temporary;}
+

thanks
Shveta