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

On Thu, Mar 21, 2024 at 8:47 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> 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?

This sounds reasonable to me at least.

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