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

On Tue, Mar 26, 2024 at 4:35 PM Bharath Rupireddy
<bharath(dot)rupireddyforpostgres(at)gmail(dot)com> wrote:
>
> On Tue, Mar 26, 2024 at 4:18 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> >
> > > 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.
> >
> > Yes, okay with me. I think there is some confusion here as well. In my
> > second approach above, I have not suggested anything related to
> > sync-worker. We can think on that later if we really need another
> > field which give us sync time. In my second approach, I have tried to
> > avoid updating inactive_since for synced slots during sync process. We
> > update that field during creation of synced slot so that
> > inactive_since reflects correct info even for synced slots (rather
> > than copying from primary). Please have a look at my patch and let me
> > know your thoughts. I am fine with copying it from primary as well and
> > documenting this behaviour.
>
> I took a look at your patch.
>
> --- a/src/backend/replication/logical/slotsync.c
> +++ b/src/backend/replication/logical/slotsync.c
> @@ -628,6 +628,7 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid
> remote_dbid)
> SpinLockAcquire(&slot->mutex);
> slot->effective_catalog_xmin = xmin_horizon;
> slot->data.catalog_xmin = xmin_horizon;
> + slot->inactive_since = GetCurrentTimestamp();
> SpinLockRelease(&slot->mutex);
>
> If we just sync inactive_since value for synced slots while in
> recovery from the primary, so be it. Why do we need to update it to
> the current time when the slot is being created?

If we update inactive_since at synced slot's creation or during
restart (skipping setting it during sync), then this time reflects
actual 'inactive_since' for that particular synced slot. Isn't that a
clear info for the user and in alignment of what the name
'inactive_since' actually suggests?

> We don't expose slot
> creation time, no?

No, we don't. But for synced slot, that is the time since that slot is
inactive (unless promoted), so we are exposing inactive_since and not
creation time.

>Aren't we fine if we just sync the value from
> primary and document that fact? After the promotion, we can reset it
> to the current time so that it gets its own time. Do you see any
> issues with it?

Yes, we can do that. But curious to know, do we see any additional
benefit of reflecting primary's inactive_since at standby which I
might be missing?

thanks
Shveta