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

On Mon, Mar 25, 2024 at 1:37 PM Bertrand Drouvot
<bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
>
> Hi,
>
> On Mon, Mar 25, 2024 at 12:59:52PM +0530, Amit Kapila wrote:
> > On Mon, Mar 25, 2024 at 12:43 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> > >
> > > On Mon, Mar 25, 2024 at 11:53 AM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> > > >
> > > > On Mon, Mar 25, 2024 at 10:33 AM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> > > > >
> > > > > On Sun, Mar 24, 2024 at 3:06 PM Bharath Rupireddy
> > > > > <bharath(dot)rupireddyforpostgres(at)gmail(dot)com> wrote:
> > > > > >
> > > > > > I've attached the v18 patch set here.
> > >
> > > I have one concern, for synced slots on standby, how do we disallow
> > > invalidation due to inactive-timeout immediately after promotion?
> > >
> > > For synced slots, last_inactive_time and inactive_timeout are both
> > > set.
>
> Yeah, and I can see last_inactive_time is moving on the standby (while not the
> case on the primary), probably due to the sync worker slot acquisition/release
> which does not seem right.
>
> > Let's say I bring down primary for promotion of standby and then
> > > promote standby, there are chances that it may end up invalidating
> > > synced slots (considering standby is not brought down during promotion
> > > and thus inactive_timeout may already be past 'last_inactive_time').
> > >
> >
> > This raises the question of whether we need to set
> > 'last_inactive_time' synced slots on the standby?
>
> Yeah, I think that last_inactive_time should stay at 0 on synced slots on the
> standby because such slots are not usable anyway (until the standby gets promoted).
>
> So, I think that last_inactive_time does not make sense if the slot never had
> the chance to be active.
>
> OTOH I think the timeout invalidation (if any) should be synced from primary.

Yes, even I feel that last_inactive_time makes sense only when the
slot is available to be used. Synced slots are not available to be
used until standby is promoted and thus last_inactive_time can be
skipped to be set for synced_slots. But once primay is invalidated due
to inactive-timeout, that invalidation should be synced to standby
(which is happening currently).

thanks
Shveta