RE: Synchronizing slots from primary to standby

On Monday, April 29, 2024 5:11 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
>
> On Mon, Apr 29, 2024 at 11:38 AM shveta malik <shveta(dot)malik(at)gmail(dot)com>
> wrote:
> >
> > On Mon, Apr 29, 2024 at 10:57 AM Zhijie Hou (Fujitsu)
> > <houzj(dot)fnst(at)fujitsu(dot)com> wrote:
> > >
> > > On Friday, March 15, 2024 10:45 PM Bertrand Drouvot
> <bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
> > > >
> > > > Hi,
> > > >
> > > > On Thu, Mar 14, 2024 at 02:22:44AM +0000, Zhijie Hou (Fujitsu) wrote:
> > > > > Hi,
> > > > >
> > > > > Since the standby_slot_names patch has been committed, I am
> > > > > attaching the last doc patch for review.
> > > > >
> > > >
> > > > Thanks!
> > > >
> > > > 1 ===
> > > >
> > > > + continue subscribing to publications now on the new primary
> > > > + server
> > > > without
> > > > + any data loss.
> > > >
> > > > I think "without any data loss" should be re-worded in this
> > > > context. Data loss in the sense "data committed on the primary and
> > > > not visible on the subscriber in case of failover" can still occurs (in case
> synchronous replication is not used).
> > > >
> > > > 2 ===
> > > >
> > > > + If the result (<literal>failover_ready</literal>) of both above steps is
> > > > + true, existing subscriptions will be able to continue without data
> loss.
> > > > + </para>
> > > >
> > > > I don't think that's true if synchronous replication is not used.
> > > > Say,
> > > >
> > > > - synchronous replication is not used
> > > > - primary is not able to reach the standby anymore and
> > > > standby_slot_names is set
> > > > - new data is inserted into the primary
> > > > - then not replicated to subscriber (due to standby_slot_names)
> > > >
> > > > Then I think the both above steps will return true but data would
> > > > be lost in case of failover.
> > >
> > > Thanks for the comments, attach the new version patch which reworded
> > > the above places.
> >
> > Thanks for the patch.
> >
> > Few comments:
> >
> > 1) Tested the steps, one of the queries still refers to
> > 'conflict_reason'. I think it should refer 'conflicting'.

Thanks for catching this. Fixed.

> >
> > 2) Will it be good to mention that in case of planned promotion, it is
> > recommended to run pg_sync_replication_slots() as last sync attempt
> > before we run failvoer-ready validation steps? This can be mentioned
> > in high-availaibility.sgml of current patch
>
> I recall now that with the latest fix, we cannot run
> pg_sync_replication_slots() unless we disable the slot-sync worker.
> Considering that, I think it will be too many steps just to run the SQL function at
> the end without much value added. Thus we can skip this point, we can rely on
> slot sync worker completely.

Agreed. I didn't change this.

Here is the V3 doc patch.

Best Regards,
Hou zj