Re: Introduce XID age and inactive timeout based replication slot invalidation

Hi,

On Tue, Mar 26, 2024 at 01:37:21PM +0530, Amit Kapila wrote:
> On Tue, Mar 26, 2024 at 1:15 PM Bertrand Drouvot
> <bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
> >
> > 2 ===
> >
> > It looks like inactive_since is set to the current timestamp on the standby
> > each time the sync worker does a cycle:
> >
> > primary:
> >
> > postgres=# select slot_name,inactive_since from pg_replication_slots where failover = 't';
> > slot_name | inactive_since
> > -------------+-------------------------------
> > lsub27_slot | 2024-03-26 07:39:19.745517+00
> > lsub28_slot | 2024-03-26 07:40:24.953826+00
> >
> > standby:
> >
> > postgres=# select slot_name,inactive_since from pg_replication_slots where failover = 't';
> > slot_name | inactive_since
> > -------------+-------------------------------
> > lsub27_slot | 2024-03-26 07:43:56.387324+00
> > lsub28_slot | 2024-03-26 07:43:56.387338+00
> >
> > I don't think that should be the case.
> >
>
> But why? This is exactly what we discussed in another thread where we
> agreed to update inactive_since even for sync slots.

Hum, I thought we agreed to "sync" it and to "update it to current time"
only at promotion time.

I don't think updating inactive_since to current time during each cycle makes
sense (I mean I understand the use case: being able to say when slots have been
sync, but if this is what we want then we should consider an extra view or an
extra field but not relying on the inactive_since one).

If the primary goes down, not updating inactive_since to the current time could
also provide benefit such as knowing the inactive_since of the primary slots
(from the standby) the last time it has been synced. If we update it to the current
time then this information is lost.

> In each sync
> cycle, we acquire/release the slot, so the inactive_since gets
> updated. See synchronize_one_slot().

Right, and I think we should put an extra condition if in recovery.

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
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