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

Hi,

On Fri, Mar 22, 2024 at 03:56:23PM +0530, Amit Kapila wrote:
> On Fri, Mar 22, 2024 at 3:15 PM Bertrand Drouvot
> <bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
> >
> > On Fri, Mar 22, 2024 at 01:45:01PM +0530, Bharath Rupireddy wrote:
> >
> > 1 ===
> >
> > @@ -691,6 +699,13 @@ ReplicationSlotRelease(void)
> > ConditionVariableBroadcast(&slot->active_cv);
> > }
> >
> > + if (slot->data.persistency == RS_PERSISTENT)
> > + {
> > + SpinLockAcquire(&slot->mutex);
> > + slot->last_inactive_at = GetCurrentTimestamp();
> > + SpinLockRelease(&slot->mutex);
> > + }
> >
> > I'm not sure we should do system calls while we're holding a spinlock.
> > Assign a variable before?
> >
> > 2 ===
> >
> > Also, what about moving this here?
> >
> > "
> > if (slot->data.persistency == RS_PERSISTENT)
> > {
> > /*
> > * Mark persistent slot inactive. We're not freeing it, just
> > * disconnecting, but wake up others that may be waiting for it.
> > */
> > SpinLockAcquire(&slot->mutex);
> > slot->active_pid = 0;
> > SpinLockRelease(&slot->mutex);
> > ConditionVariableBroadcast(&slot->active_cv);
> > }
> > "
> >
> > That would avoid testing twice "slot->data.persistency == RS_PERSISTENT".
> >
>
> That sounds like a good idea. Also, don't we need to consider physical
> slots where we don't reserve WAL during slot creation? I don't think
> there is a need to set inactive_at for such slots.

If the slot is not active, why shouldn't we set inactive_at? I can understand
that such a slots do not present "any risks" but I think we should still set
inactive_at (also to not give the false impression that the slot is active).

> > 5 ===
> >
> > Most of the fields that reflect a time (not duration) in the system views are
> > xxxx_time, so I'm wondering if instead of "last_inactive_at" we should use
> > something like "last_inactive_time"?
> >
>
> How about naming it as last_active_time? This will indicate the time
> at which the slot was last active.

I thought about it too but I think it could be missleading as one could think that
it should be updated each time WAL record decoding is happening.

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
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