Date: Tue, 26 Mar 2024 12:31:08 +0000
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
To: shveta malik <shveta.malik@gmail.com>
Cc: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>,
	Amit Kapila <amit.kapila16@gmail.com>,
	Nathan Bossart <nathandbossart@gmail.com>,
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Introduce XID age and inactive timeout based replication slot
 invalidation
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Hi,

On Tue, Mar 26, 2024 at 04:49:18PM +0530, shveta malik wrote:
> On Tue, Mar 26, 2024 at 4:35 PM Bharath Rupireddy
> <bharath.rupireddyforpostgres@gmail.com> wrote:
> >
> > On Tue, Mar 26, 2024 at 4:18 PM shveta malik <shveta.malik@gmail.com> wrote:
> > >
> > > > What about another approach?: inactive_since gives data synced from primary for
> > > > synced slots and another dedicated field (could be added later...) could
> > > > represent what you suggest as the other option.
> > >
> > > Yes, okay with me. I think there is some confusion here as well. In my
> > > second approach above, I have not suggested anything related to
> > > sync-worker. We can think on that later if we really need another
> > > field which give us sync time.  In my second approach, I have tried to
> > > avoid updating inactive_since for synced slots during sync process. We
> > > update that field during creation of synced slot so that
> > > inactive_since reflects correct info even for synced slots (rather
> > > than copying from primary). Please have a look at my patch and let me
> > > know your thoughts. I am fine with copying it from primary as well and
> > > documenting this behaviour.
> >
> > I took a look at your patch.
> >
> > --- a/src/backend/replication/logical/slotsync.c
> > +++ b/src/backend/replication/logical/slotsync.c
> > @@ -628,6 +628,7 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid
> > remote_dbid)
> >          SpinLockAcquire(&slot->mutex);
> >          slot->effective_catalog_xmin = xmin_horizon;
> >          slot->data.catalog_xmin = xmin_horizon;
> > +        slot->inactive_since = GetCurrentTimestamp();
> >          SpinLockRelease(&slot->mutex);
> >
> > If we just sync inactive_since value for synced slots while in
> > recovery from the primary, so be it. Why do we need to update it to
> > the current time when the slot is being created?
> 
> If we update inactive_since  at synced slot's creation or during
> restart (skipping setting it during sync), then this time reflects
> actual 'inactive_since' for that particular synced slot.  Isn't that a
> clear info for the user and in alignment of what the name
> 'inactive_since' actually suggests?
> 
> > We don't expose slot
> > creation time, no?
> 
> No, we don't. But for synced slot, that is the time since that slot is
> inactive  (unless promoted), so we are exposing inactive_since and not
> creation time.
> 
> >Aren't we fine if we just sync the value from
> > primary and document that fact? After the promotion, we can reset it
> > to the current time so that it gets its own time. Do you see any
> > issues with it?
> 
> Yes, we can do that. But curious to know, do we see any additional
> benefit of reflecting primary's inactive_since at standby which I
> might be missing?

In case the primary goes down, then one could use the value on the standby
to get the value coming from the primary. I think that could be useful info to
have.

Regards,

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