Re: Introduce XID age and inactive timeout based replication slot invalidation - Mailing list pgsql-hackers

Hi,

On Tue, Mar 26, 2024 at 03:17:36PM +0530, shveta malik wrote:
> On Tue, Mar 26, 2024 at 1:54 PM Bertrand Drouvot
> <bertranddrouvot.pg@gmail.com> wrote:
> >
> > 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.pg@gmail.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 think there may have been some misunderstanding here.

Indeed ;-)

> But now if I
> rethink this, I am fine with 'inactive_since' getting synced from
> primary to standby. But if we do that, we need to add docs stating
> "inactive_since" represents primary's inactivity and not standby's
> slots inactivity for synced slots.

Yeah sure.

> The reason for this clarification
> is that the synced slot might be generated much later, yet
> 'inactive_since' is synced from the primary, potentially indicating a
> time considerably earlier than when the synced slot was actually
> created.

Right.

> Another approach could be that "inactive_since" for synced slot
> actually gives its own inactivity data rather than giving primary's
> slot data. We update inactive_since on standby only at 3 occasions:
> 1) at the time of creation of the synced slot.
> 2) during standby restart.
> 3) during promotion of standby.
> 
> I have attached a sample patch for this idea as.txt file.

Thanks!

> I am fine with any of these approaches.  One gives data synced from
> primary for synced slots, while another gives actual inactivity data
> of synced slots.

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.

Another cons of updating inactive_since at the current time during each slot
sync cycle is that calling GetCurrentTimestamp() very frequently
(during each sync cycle of very active slots) could be too costly.

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
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