Re: Question about initial logical decoding snapshot

On Tue, Jan 3, 2023 at 4:44 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Fri, Dec 30, 2022 at 11:57 PM Chong Wang <chongwa(at)vmware(dot)com> wrote:
> >
> > I'm studying the source code about creation of initial logical decoding snapshot. What confused me is that why must we process 3 xl_running_xacts before we get to the consistent state. I think we only need 2 xl_running_xacts.
> >
> > I think we can get to consistent state when we meet the 2nd xl_running_xact with its oldestRunningXid > 1st xl_running_xact's nextXid, this means the active transactions in 1st xl_running_xact all had commited, and we have all the logs of transactions who will commit afterwards, so there is consistent state in this time point and we can export a snapshot.
> >
>
> Yeah, we will have logs for all transactions in such a case but I
> think we won't have a valid snapshot by that time. Consider a case
> that there are two transactions 723,724 in the 2nd xl_running_xact
> record for which we have waited to finish and then consider that point
> as a consistent point and exported that snapshot. It is quite possible
> that by that time the commit record of one or more of those xacts (say
> 724) wouldn't have been encountered by decoding process and that means
> it won't be recorded in the xip list of the snapshot (we do that in
> DecodeCommit->SnapBuildCommitTxn). So, during export in function
> SnapBuildInitialSnapshot(), we will consider 723 as committed and 724
> as running. This could not lead to inconsistent data on the client
> side that imports such a snapshot and use it for copy and further
> replicating the other xacts.
>
> OTOH, currently, before marking snapshot state as consistent we wait
> for these xacts to finish and for another xl_running_xact where
> oldestRunningXid >= builder->next_phase_at to appear which means the
> commit for both 723 and 724 would have appeared in the snapshot.
>
> Does that makes sense to you or am, I missing something here?
>

You can also refer to the discussion in the thread [1] which is
related to your question.

[1] - https://www.postgresql.org/message-id/c94be044-818f-15e3-1ad3-7a7ae2dfed0a%40iki.fi

--
With Regards,
Amit Kapila.