Re: Failed transaction statistics to measure the logical replication progress

On Wed, Sep 29, 2021 at 11:35 AM osumi(dot)takamichi(at)fujitsu(dot)com
<osumi(dot)takamichi(at)fujitsu(dot)com> wrote:
>
> Hi,
>
>
> Thank you, Amit-san and Sawada-san for the discussion.
> On Tuesday, September 28, 2021 7:05 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > > Another idea could be to have a separate view, say
> > > pg_stat_subscription_xact but I'm not sure it's a better idea.
> > >
> >
> > Yeah, that is another idea but I am afraid that having three different
> > views for subscription stats will be too much. I think it would be
> > better if we can display these additional stats via the existing view
> > pg_stat_subscription or the new view pg_stat_subscription_errors (or
> > whatever name we want to give it).
> pg_stat_subscription_errors specializes in showing an error record.
> So, it would be awkward to combine it with other normal xact stats.
>
>
> > > > > Then, if, we proceed in this direction, the place to implement
> > > > > those stats would be on the LogicalRepWorker struct, instead ?
> > > > >
> > > >
> > > > Or, we can make existing stats persistent and then add these stats
> > > > on top of it. Sawada-San, do you have any thoughts on this matter?
> > >
> > > I think that making existing stats including received_lsn and
> > > last_msg_receipt_time persistent by using stats collector could cause
> > > massive reporting messages. We can report these messages with a
> > > certain interval to reduce the amount of messages but we will end up
> > > seeing old stats on the view.
> > >
> >
> > Can't we keep the current and new stats both in-memory and persist on disk?
> > So, the persistent stats data will be used to fill the in-memory counters after
> > restarting of workers, otherwise, we will always refer to in-memory values.
> I felt this isn't impossible.
> When we have to update the values of the xact stats is
> the end of message apply for COMMIT, COMMIT PREPARED, STREAM_ABORT and etc
> or the time when an error happens during apply. Then, if we want,
> we can update xact stats values at such moments accordingly.
> I'm thinking that we will have a hash table whose key is a pair of subid + relid
> and entry is a proposed stats structure and update the entry,
> depending on the above timings.
>

Are you thinking of a separate hash table then what we are going to
create for Sawada-San's patch related to error stats? Isn't it
possible to have stats in the same hash table and same file?

> Here, one thing a bit unclear to me is
> whether we should move existing stats of pg_stat_subscription
> (such as last_lsn and reply_lsn) to the hash entry or not.
>

I think we should move it to hash entry. I think that is an
improvement over what we have now because now after restart those
stats gets lost.

--
With Regards,
Amit Kapila.