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

On Mon, Mar 25, 2024 at 12:43 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
>
> 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. 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').
>

On standby, if we decide to maintain valid last_inactive_time for
synced slots, then invalidation is correctly restricted in
InvalidateSlotForInactiveTimeout() for synced slots using the check:

if (RecoveryInProgress() && slot->data.synced)
return false;

But immediately after promotion, we can not rely on the above check
and thus possibility of synced slots invalidation is there. To
maintain consistent behavior regarding the setting of
last_inactive_time for synced slots, similar to user slots, one
potential solution to prevent this invalidation issue is to update the
last_inactive_time of all synced slots within the ShutDownSlotSync()
function during FinishWalRecovery(). This approach ensures that
promotion doesn't immediately invalidate slots, and henceforth, we
possess a correct last_inactive_time as a basis for invalidation going
forward. This will be equivalent to updating last_inactive_time during
restart (but without actual restart during promotion).
The plus point of maintaining last_inactive_time for synced slots
could be, this can provide data to the user on when last time the sync
was attempted on that particular slot by background slot sync worker
or SQl function. Thoughts?

thanks
Shveta