Re: Minimal logical decoding on standbys

On Thu, Jan 12, 2023 at 11:46 PM Andres Freund <andres@anarazel.de> wrote:
>
> Hi,
>
> On 2023-01-12 20:08:55 +0530, Ashutosh Sharma wrote:
> > I previously participated in the discussion on "Synchronizing the
> > logical replication slots from Primary to Standby" and one of the
> > purposes of that project was to synchronize logical slots from primary
> > to standby so that if failover occurs, it will not affect the logical
> > subscribers of the old primary much. Can someone help me understand
> > how we are going to solve this problem with this patch? Are we going
> > to encourage users to do LR from standby instead of primary to get rid
> > of such problems during failover?
>
> It only provides a building block towards that. The "Synchronizing the logical
> replication slots from Primary to Standby" project IMO needs all of the
> infrastructure in this patch. With the patch, a logical rep solution can
> e.g. maintain one slot on the primary and one on the standby, and occasionally
> forward the slot on the standby to the position of the slot on the primary. In
> case of a failover it can just start consuming changes from the former
> standby, all the necessary changes are guaranteed to be present.
>
>
> > Also, one small observation:
> >
> > I just played around with the latest (v38) patch a bit and found that
> > when a new logical subscriber of standby is created, it actually
> > creates two logical replication slots for it on the standby server.
> > May I know the reason for creating an extra replication slot other
> > than the one created by create subscription command? See below:
>
> That's unrelated to this patch. There's no changes to the "higher level"
> logical replication code dealing with pubs and subs, it's all on the "logical
> decoding" level.
>
> I think this because logical rep wants to be able to concurrently perform
> ongoing replication, and synchronize tables added to the replication set. The
> pg_16399_sync_16392_7187728548042694423 slot should vanish after the initial
> synchronization.
>

Thanks Andres. I have one more query (both for you and Bertrand). I
don't know if this has already been answered somewhere in this mail
thread, if yes, please let me know the mail that answers this query.

Will there be a problem if we mandate the use of physical replication
slots and hot_standby_feedback to support minimum LD on standby. I
know people can do a physical replication setup without a replication
slot or even with hot_standby_feedback turned off, but are we going to
have any issue if we ask them to use a physical replication slot and
turn on hot_standby_feedback for LD on standby. This will reduce the
code changes required to do conflict handling for logical slots on
standby which is being done by v50-0001 and v50-0002* patches
currently.

IMHO even in normal scenarios i.e. when we are not doing LD on
standby, we should mandate the use of a physical replication slot.

--
With Regards,
Ashutosh Sharma.