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

On Wed, Mar 20, 2024 at 1:51 PM Bertrand Drouvot
<bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
>
> On Wed, Mar 20, 2024 at 12:48:55AM +0530, Bharath Rupireddy wrote:
> >
> > 2. last_inactive_at and inactive_timeout are now tracked in on-disk
> > replication slot data structure.
>
> Should last_inactive_at be tracked on disk? Say the engine is down for a period
> of time > inactive_timeout then the slot will be invalidated after the engine
> re-start (if no activity before we invalidate the slot). Should the time the
> engine is down be counted as "inactive" time? I've the feeling it should not, and
> that we should only take into account inactive time while the engine is up.
>

Good point. The question is how do we achieve this without persisting
the 'last_inactive_at'? Say, 'last_inactive_at' for a particular slot
had some valid value before we shut down but it still didn't cross the
configured 'inactive_timeout' value, so, we won't be able to
invalidate it. Now, after the restart, as we don't know the
last_inactive_at's value before the shutdown, we will initialize it
with 0 (this is what Bharath seems to have done in the latest
v13-0002* patch). After this, even if walsender or backend never
acquires the slot, we won't invalidate it. OTOH, if we track
'last_inactive_at' on the disk, after, restart, we could initialize it
to the current time if the value is non-zero. Do you have any better
ideas?

--
With Regards,
Amit Kapila.