Re: Synchronizing slots from primary to standby

Hi,

On Thu, Apr 04, 2024 at 05:31:45PM +0530, shveta malik wrote:
> On Thu, Apr 4, 2024 at 2:59 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> >
> >
> > Prior to commit 2ec005b, this check was okay, as we did not expect
> > restart_lsn of the synced slot to be ahead of remote since we were
> > directly copying the lsns. But now when we use 'advance' to do logical
> > decoding on standby, there is a possibility that restart lsn of the
> > synced slot is ahead of remote slot, if there are running txns records
> > found after reaching consistent-point while consuming WALs from
> > restart_lsn till confirmed_lsn. In such a case, slot-sync's advance
> > may end up serializing snapshots and setting restart_lsn to the
> > serialized snapshot point, ahead of remote one.
> >
> > Fix:
> > The sanity check needs to be corrected. Attached a patch to address the issue.
>

Thanks for reporting, explaining the issue and providing a patch.

Regarding the patch:

1 ===

+ * Attempt to sync lsns and xmins only if remote slot is ahead of local

s/lsns/LSNs/?

2 ===

+ if (slot->data.confirmed_flush != remote_slot->confirmed_lsn)
+ elog(LOG,
+ "could not synchronize local slot \"%s\" LSN(%X/%X)"
+ " to remote slot's LSN(%X/%X) ",
+ remote_slot->name,
+ LSN_FORMAT_ARGS(slot->data.confirmed_flush),
+ LSN_FORMAT_ARGS(remote_slot->confirmed_lsn));

I don't think that the message is correct here. Unless I am missing something
there is nothing in the following code path that would prevent the slot to be
sync during this cycle.

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com