Date: Tue, 11 Jun 2024 10:00:46 +0000
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
To: Nathan Bossart <nathandbossart@gmail.com>
Cc: John H <johnhyvr@gmail.com>, pgsql-hackers@postgresql.org
Subject: Re: Allow logical failover slots to wait on synchronous replication
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Hi,

On Mon, Jun 10, 2024 at 09:25:10PM -0500, Nathan Bossart wrote:
> On Mon, Jun 10, 2024 at 03:51:05PM -0700, John H wrote:
> > The existing 'standby_slot_names' isn't great for users who are running
> > clusters with quorum-based synchronous replicas. For instance, if
> > the user has  synchronous_standby_names = 'ANY 3 (A,B,C,D,E)' it's a
> > bit tedious to have to reconfigure the standby_slot_names to set it to
> > the most updated 3 sync replicas whenever different sync replicas start
> > lagging. In the event that both GUCs are set, 'standby_slot_names' takes
> > precedence.
> 
> Hm.  IIUC you'd essentially need to set standby_slot_names to "A,B,C,D,E"
> to get the desired behavior today.  That might ordinarily be okay, but it
> could cause logical replication to be held back unnecessarily if one of the
> replicas falls behind for whatever reason.  A way to tie standby_slot_names
> to synchronous replication instead does seem like it would be useful in
> this case.

FWIW, I have the same understanding and also think your proposal would be
useful in this case.

Regards,

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