Re: Synchronizing slots from primary to standby

On Fri, Dec 1, 2023 at 9:31 PM Nisha Moond <nisha(dot)moond412(at)gmail(dot)com> wrote:
>
> On Fri, Dec 1, 2023 at 5:40 PM Nisha Moond <nisha(dot)moond412(at)gmail(dot)com> wrote:
> >
> > Review for v41 patch.
> >
> > 1.
> > ======
> > src/backend/utils/misc/postgresql.conf.sample
> >
> > +#enable_syncslot = on # enables slot synchronization on the physical
> > standby from the primary
> >
> > enable_syncslot is disabled by default, so, it should be 'off' here.
> >
> > ~~~
> > 2.
> > IIUC, the slotsyncworker's connection to the primary is to execute a
> > query. Its aim is not walsender type connection, but at primary when
> > queried, the 'backend_type' is set to 'walsender'.
> > Snippet from primary db-
> >
> > datname | usename | application_name | wait_event_type | backend_type
> > ---------+-------------+------------------+-----------------+--------------
> > postgres | replication | slotsyncworker | Client | walsender
> >
> > Is it okay?
> >
> > ~~~
> > 3.
> > As per current logic, If there are slots on primary with disabled
> > subscriptions, then, when standby is created it replicates these slots
> > but can't make them sync-ready until any activity happens on the
> > slots.
> > So, such slots stay in 'i' sync-state and get dropped when failover
> > happens. Now, if the subscriber tries to enable their existing
> > subscription after failover, it gives an error that the slot does not
> > exist.
>
> Is this behavior expected? If yes, then is it worth documenting about
> disabled subscription slots not being synced?
>

This is expected behavior because even if we would retain such slots
(state 'i'), we won't be able to make them in the 'ready' state after
failover because we can't get the required WAL from the primary.

--
With Regards,
Amit Kapila.