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

Hi,

On Fri, Mar 22, 2024 at 01:45:01PM +0530, Bharath Rupireddy wrote:
> On Fri, Mar 22, 2024 at 12:39 PM Bertrand Drouvot
> <bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
> >
> > > > Please find the v14-0001 patch for now.
> >
> > Thanks!
> >
> > > LGTM. Let's wait for Bertrand to see if he has more comments on 0001
> > > and then I'll push it.
> >
> > LGTM too.
>
> Thanks. Here I'm implementing the following:

Thanks!

> 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 don't think we should link both. Changing my mind here on this
subject due to the testing.

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com