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

Hi,

On Fri, Mar 29, 2024 at 09:39:31AM +0530, Amit Kapila wrote:
> On Wed, Mar 27, 2024 at 9:00 PM Bharath Rupireddy
> <bharath(dot)rupireddyforpostgres(at)gmail(dot)com> wrote:
> >
> >
> > Thanks. I'm attaching v29 patches. 0001 managing inactive_since on the
> > standby for sync slots.
> >
>
> Commit message states: "why we can't just update inactive_since for
> synced slots on the standby with the value received from remote slot
> on the primary. This is consistent with any other slot parameter i.e.
> all of them are synced from the primary."
>
> The inactive_since is not consistent with other slot parameters which
> we copy. We don't perform anything related to those other parameters
> like say two_phase phase which can change that property. However, we
> do acquire the slot, advance the slot (as per recent discussion [1]),
> and release it. Since these operations can impact inactive_since, it
> seems to me that inactive_since is not the same as other parameters.
> It can have a different value than the primary. Why would anyone want
> to know the value of inactive_since from primary after the standby is
> promoted?

I think it can be useful "before" it is promoted and in case the primary is down.
I agree that tracking the activity time of a synced slot can be useful, why
not creating a dedicated field for that purpose (and keep inactive_since a
perfect "copy" of the primary)?

> Now, the other concern is that calling GetCurrentTimestamp()
> could be costly when the values for the slot are not going to be
> updated but if that happens we can optimize such that before acquiring
> the slot we can have some minimal pre-checks to ensure whether we need
> to update the slot or not.

Right, but for a very active slot it is likely that we call GetCurrentTimestamp()
during almost each sync cycle.

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
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