Re: Synchronizing slots from primary to standby

On Thu, Jan 25, 2024 at 10:39 AM Peter Smith <smithpb2250@gmail.com> wrote:

> 2. synchronize_one_slot
>
> + /*
> + * Sanity check: Make sure that concerned WAL is received and flushed
> + * before syncing slot to target lsn received from the primary server.
> + *
> + * This check should never pass as on the primary server, we have waited
> + * for the standby's confirmation before updating the logical slot.
> + */
> + latestFlushPtr = GetStandbyFlushRecPtr(NULL);
> + if (remote_slot->confirmed_lsn > latestFlushPtr)
> + {
> + ereport(LOG,
> + errmsg("skipping slot synchronization as the received slot sync"
> +    " LSN %X/%X for slot \"%s\" is ahead of the standby position %X/%X",
> +    LSN_FORMAT_ARGS(remote_slot->confirmed_lsn),
> +    remote_slot->name,
> +    LSN_FORMAT_ARGS(latestFlushPtr)));
> +
> + return false;
> + }
>
> Previously in v65 this was an elog, but now it is an ereport. But
> since this is a sanity check condition that "should never pass" wasn't
> the elog the more appropriate choice?

We realized that this scenario can be frequently hit when the user has
not set standby_slot_names on primary. And thus ereport makes more
sense. But I agree that this comment is misleading. We will adjust the
comment in the next version.

thanks
Shveta