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

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? 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.

[1] - https://www.postgresql.org/message-id/OS0PR01MB571615D35F486080616CA841943A2%40OS0PR01MB5716.jpnprd01.prod.outlook.com

--
With Regards,
Amit Kapila.