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

On Fri, Mar 22, 2024 at 3:23 PM Bertrand Drouvot
<bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
>
> On Fri, Mar 22, 2024 at 02:59:21PM +0530, Amit Kapila wrote:
> > On Fri, Mar 22, 2024 at 2:27 PM Bertrand Drouvot
> > <bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
> > >
> > > On Fri, Mar 22, 2024 at 01:45:01PM +0530, Bharath Rupireddy wrote:
> > >
> > > > 0001 Track invalidation_reason in pg_replication_slots
> > > > 0002 Track last_inactive_at in pg_replication_slots
> > > > 0003 Allow setting inactive_timeout for replication slots via SQL API
> > > > 0004 Introduce new SQL funtion pg_alter_replication_slot
> > > > 0005 Allow setting inactive_timeout in the replication command
> > > > 0006 Add inactive_timeout based replication slot invalidation
> > > >
> > > > 1. Keep it last_inactive_at as a shared memory variable, but always
> > > > set it at restart if the slot's inactive_timeout has non-zero value
> > > > and reset it as soon as someone acquires that slot so that if the slot
> > > > doesn't get acquired till inactive_timeout, checkpointer will
> > > > invalidate the slot.
> > > > 4. last_inactive_at should also be set to the current time during slot
> > > > creation because if one creates a slot and does nothing with it then
> > > > it's the time it starts to be inactive.
> > >
> > > I did not look at the code yet but just tested the behavior. It works as you
> > > describe it but I think this behavior is weird because:
> > >
> > > - when we create a slot without a timeout then last_inactive_at is set. I think
> > > that's fine, but then:
> > > - when we restart the engine, then last_inactive_at is gone (as timeout is not
> > > set).
> > >
> > > I think last_inactive_at should be set also at engine restart even if there is
> > > no timeout.
> >
> > I think it is the opposite. Why do we need to set 'last_inactive_at'
> > when inactive_timeout is not set?
>
> I think those are unrelated, one could want to know when a slot has been inactive
> even if no timeout is set. I understand that for this patch series we have in mind
> to use them both to invalidate slots but I think that there is use case to not
> use both in correlation. Also not setting last_inactive_at could give the "false"
> impression that the slot is active.
>

I see your point and agree with this. I feel we can commit this part
first then, probably that is the reason Bharath has kept it as a
separate patch. It would be good add the use case for this patch in
the commit message.

A minor comment:

if (SlotIsLogical(s))
pgstat_acquire_replslot(s);

+ if (s->data.persistency == RS_PERSISTENT)
+ {
+ SpinLockAcquire(&s->mutex);
+ s->last_inactive_at = 0;
+ SpinLockRelease(&s->mutex);
+ }
+

I think this part of the change needs a comment.

--
With Regards,
Amit Kapila.