RE: Synchronizing slots from primary to standby

On Monday, December 11, 2023 3:52 PM Drouvot, Bertrand <bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:

Hi,

> On 12/8/23 10:06 AM, Amit Kapila wrote:
> > On Wed, Dec 6, 2023 at 4:53 PM shveta malik <shveta(dot)malik(at)gmail(dot)com>
> wrote:
> >>
> >> PFA v43, changes are:
> >>
> >
> > I wanted to discuss 0003 patch about cascading standby's. It is not
> > clear to me whether we want to allow physical standbys to further wait
> > for cascading standby to sync their slots. If we allow such a feature
> > one may expect even primary to wait for all the cascading standby's
> > because otherwise still logical subscriber can be ahead of one of the
> > cascading standby.
>
> I've the same feeling here. I think it would probably be expected that the
> primary also wait for all the cascading standby.
>
> > I feel even if we want to allow such a behaviour we can do it later
> > once the main feature is committed.
>
> Agree.
>
> > I think it would
> > be good to just allow logical walsenders on primary to wait for
> > physical standbys represented by GUC 'standby_slot_names'.
>
> That makes sense for me for v1.
>
> > If we agree
> > on that then it would be good to prohibit setting this GUC on standby
> > or at least it should be a no-op even if this GUC should be set on
> > physical standby.
>
> I'd prefer to completely prohibit it on standby (to make it very clear it's not
> working at all) as long as one can enable it without downtime once the standby
> is promoted (which is the case currently).

I think we could not check if we are in a standby server in the GUC check_hook,
because the XLogCtl(which is checked in RecoveryInProgress) may have not been
initialized yet. Besides, other GUCs like synchronous_standby_names also don't
work on standby but it will be no-op. So I feel we can also ignore
standby_slot_names on standby. What do you think ?

Here is the V46 patch set which changed the following things:

V46-0001:
* Address Peter[1] and Amit's[2] comments.
* Fix one CFbot failure in meson build.
* Ignore the standby_slot_names on a standby server since we don't support
syncing slots to cascade standby.

V46-0002:
1) Fix for CFBot make warning.
2) Cascading support removal. Now we do not need to check 'sync_state != 'i''
in the query while fetching failover slots. This check was needed on the
cascading standby to fetch failover slots from the first standby.
3) Test correction and optimization.

0003 patch is removed since we agreed not to support syncing slots to cascading
standby.

Thanks Shveta for working on the changes in V46-0002 and thanks Ajin for
working on the test optimization.

--
TODO

There are few pending comments that mentioned in [3][4][5] which are still in
progress.

[1] https://www.postgresql.org/message-id/CAHut%2BPsf9z132WNgy0Gr10ZTnonpNjvTBj74wG8kSxXU4rOD7g%40mail.gmail.com
[2] https://www.postgresql.org/message-id/CAA4eK1%2BCXpfiTLbYRaOoUBP9Z1-xJZdX6QOp14rCdaF5E2gsgQ%40mail.gmail.com
[3] https://www.postgresql.org/message-id/CAJpy0uDaGMNpgmdxie-MgHmMhnD4ET_LDjQNEe76xJ%2BMLqRQ8Q%40mail.gmail.com
[4] https://www.postgresql.org/message-id/CAJpy0uDcOf5Hvk_CdCCAbfx9SY%2Bog%3D%3D%3DtgiuhWKzkYyqebui9g%40mail.gmail.com
[5] https://www.postgresql.org/message-id/CAJpy0uC-8mrn6jakcFjSVmbJiHZs-Okq8YKxGfrMLPD-2%3DwOqQ%40mail.gmail.com

Best Regards,
Hou zj