RE: Synchronizing slots from primary to standby

On Tuesday, November 28, 2023 8:07 PM Drouvot, Bertrand <bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:

Hi,

> On 11/27/23 9:57 AM, Zhijie Hou (Fujitsu) wrote:
> > On Monday, November 27, 2023 4:51 PM shveta malik
> <shveta(dot)malik(at)gmail(dot)com> wrote:
> >
> > Here is the updated version(v39_2) which include all the changes made in
> 0002.
> > Please use for review, and sorry for the confusion.
> >
>
> Thanks!
>
> As far v39_2-0001:
>
> "
> Altering the failover option of the subscription is currently not
> permitted. However, this restriction may be lifted in future versions.
> "
>
> Should we mention that we can alter the related replication slot?

Will add.

>
> + <para>
> + The implementation of failover requires that replication
> + has successfully finished the initial table synchronization
> + phase. So even when <literal>failover</literal> is enabled for a
> + subscription, the internal failover state remains
> + temporarily <quote>pending</quote> until the initialization
> phase
> + completes. See column
> <structfield>subfailoverstate</structfield>
> + of <link
> linkend="catalog-pg-subscription"><structname>pg_subscription</structna
> me></link>
> + to know the actual failover state.
> + </para>
>
> I think we have a corner case here. If one alter the replication slot on the
> primary then "subfailoverstate" is not updated accordingly on the subscriber.
> Given the 2 remarks above would that make sense to prevent altering a
> replication slot associated to a subscription?

Thanks for the review!

I think we could not distinguish the user created logical slot or subscriber
created slot as there is no related info in slot's data. And user could change
the slot on subscription by "alter sub set (slot_name)", so maintaining this info
would need some efforts.

Besides, I think this case overlaps the previous discussed "alter sub set
(slot_name)" issue[1]. Both the cases are because the slot's failover is
different from the subscription's failover setting. I think we could handle
them similarly that user need to take care of not changing the failover to
wrong value. Or do you prefer another approach that mentioned in that thread[1]
? (always alter the slot at the startup of apply worker).

[1] https://www.postgresql.org/message-id/564b195a-180c-42e9-902b-b1a8b50218ee%40gmail.com

Best Regards,
Hou zj