Re: Replication slot stats misgivings

Hi,

On 2021-03-25 17:12:31 +0530, Amit Kapila wrote:
> On Thu, Mar 25, 2021 at 11:36 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > On Wed, Mar 24, 2021 at 7:06 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > >
> > >
> > > Leaving aside restart case, without some sort of such sanity checking,
> > > if both drop (of old slot) and create (of new slot) messages are lost
> > > then we will start accumulating stats in old slots. However, if only
> > > one of them is lost then there won't be any such problem.
> > >
> > > > Perhaps we could have RestoreSlotFromDisk() send something to the stats
> > > > collector ensuring the mapping makes sense?
> > > >
> > >
> > > Say if we send just the index location of each slot then probably we
> > > can setup replSlotStats. Now say before the restart if one of the drop
> > > messages was missed (by stats collector) and that happens to be at
> > > some middle location, then we would end up restoring some already
> > > dropped slot, leaving some of the still required ones. However, if
> > > there is some sanity identifier like name along with the index, then I
> > > think that would have worked for such a case.
> >
> > Even such messages could also be lost? Given that any message could be
> > lost under a UDP connection, I think we cannot rely on a single
> > message. Instead, I think we need to loosely synchronize the indexes
> > while assuming the indexes in replSlotStats and
> > ReplicationSlotCtl->replication_slots are not synchronized.
> >
> > >
> > > I think it would have been easier if we would have some OID type of
> > > identifier for each slot. But, without that may be index location of
> > > ReplicationSlotCtl->replication_slots and slotname combination can
> > > reduce the chances of slot stats go wrong quite less even if not zero.
> > > If not name, do we have anything else in a slot that can be used for
> > > some sort of sanity checking?
> >
> > I don't see any useful information in a slot for sanity checking.
> >
>
> In that case, can we do a hard check for which slots exist if
> replSlotStats runs out of space (that can probably happen only after
> restart and when we lost some drop messages)?

I suggest we wait doing anything about this until we know if the shared
stats patch gets in or not (I'd give it 50% maybe). If it does get in
things get a good bit easier, because we don't have to deal with the
message loss issues anymore.

Greetings,

Andres Freund