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

On Sat, Mar 16, 2024 at 3:55 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> > What's proposed with max_slot_xid_age is that during checkpoint we
> > look at slot's xmin and catalog_xmin, and the current system txn id.
> > Then, if the XID age of (xmin, catalog_xmin) and current_xid crosses
> > max_slot_xid_age, we invalidate the slot.
> >
>
> I can see that in your patch (in function
> InvalidatePossiblyObsoleteSlot()). As per my understanding, we need
> something similar for slot xids in ComputeXidHorizons() as we are
> doing WAL in KeepLogSeg(). In KeepLogSeg(), we compute the minimum LSN
> location required by slots and then adjust it for
> 'max_slot_wal_keep_size'. On similar lines, currently in
> ComputeXidHorizons(), we compute the minimum xid required by slots
> (procArray->replication_slot_xmin and
> procArray->replication_slot_catalog_xmin) but then don't adjust it for
> 'max_slot_xid_age'. I could be missing something in this but it is
> better to keep discussing this

After invalidating slots because of max_slot_xid_age, the
procArray->replication_slot_xmin and
procArray->replication_slot_catalog_xmin are recomputed immediately in
InvalidateObsoleteReplicationSlots->ReplicationSlotsComputeRequiredXmin->ProcArraySetReplicationSlotXmin.
And, later the XID horizons in ComputeXidHorizons are computed before
the vacuum on each table via GetOldestNonRemovableTransactionId.
Aren't these enough? Do you want the XID horizons recomputed
immediately, something like the below?

/* Invalidate replication slots based on xmin or catalog_xmin age */
if (max_slot_xid_age > 0)
{
if (InvalidateObsoleteReplicationSlots(RS_INVAL_XID_AGE,
0, InvalidOid,
InvalidTransactionId))
{
ComputeXidHorizonsResult horizons;

/*
* Some slots have been invalidated; update the XID horizons
* as a side-effect.
*/
ComputeXidHorizons(&horizons);
}
}

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