Re: Synchronizing slots from primary to standby

On Wed, Dec 6, 2023 at 4:53 PM shveta malik <shveta.malik@gmail.com> wrote:
>
> v43-001:
> 1) Support of  'failover' dump in pg_dump. It was missing earlier.
>

Review v43-0001
================
1.
+ * However, we do not enable failover for slots created by the table sync
+ * worker. This is because the table sync slot might not be fully synced on the
+ * standby.

The reason for not enabling failover for table sync slots is not
clearly mentioned.

2.
During syncing, the local restart_lsn and/or local catalog_xmin of
+ * the newly created slot on the standby are typically ahead of those on the
+ * primary. Therefore, the standby needs to wait for the primary server's
+ * restart_lsn and catalog_xmin to catch up, which takes time.

I think this part of the comment should be moved to 0002 patch. We can
probably describe a bit more about why slot on standby will be ahead
and about waiting time.

3.
validate_standby_slots()
{
...
+ slot = SearchNamedReplicationSlot(name, true);
+
+ if (!slot)
+ goto ret_standby_slot_names_ng;
+
+ if (!SlotIsPhysical(slot))
+ {
+ GUC_check_errdetail("\"%s\" is not a physical replication slot",
+ name);
+ goto ret_standby_slot_names_ng;
+ }

Why the first check (slot not found) doesn't have errdetail? The
goto's in this function look a bit odd, can we try to avoid those?

4.
+ /* Verify syntax and parse string into list of identifiers */
+ if (!SplitIdentifierString(rawname, ',', &elemlist))
+ {
+ /* syntax error in name list */
+ GUC_check_errdetail("List syntax is invalid.");
...
...
+ if (!SplitIdentifierString(standby_slot_names_cpy, ',', &standby_slots))
+ {
+ /* This should not happen if GUC checked check_standby_slot_names. */
+ elog(ERROR, "invalid list syntax");

Both are checking the same string but giving different error messages.
I think the error message should be the same in both cases. The first
one seems better.

5. In WalSndFilterStandbySlots(), the comments around else if checks
should move inside the checks. It is hard to read the code in the
current format. I have tried to change the same in the attached.

Apart from the above, I have changed the comments and made some minor
cosmetic changes in the attached. Kindly include in next version if
you are fine with it.

--
With Regards,
Amit Kapila.