Re: Skipping logical replication transactions on subscriber side

On Tue, Jan 18, 2022 at 12:20 PM osumi.takamichi@fujitsu.com
<osumi.takamichi@fujitsu.com> wrote:
>
> On Monday, January 17, 2022 9:52 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> > Thank you for the comments!
> ..
> > > (2) Minor improvement suggestion of comment in
> > > src/backend/replication/logical/worker.c
> > >
> > > + * reset during that.  Also, we don't skip receiving the changes in
> > > + streaming
> > > + * cases, since we decide whether or not to skip applying the changes
> > > + when
> > >
> > > I sugguest that you don't use 'streaming cases', because what
> > > "streaming cases" means sounds a bit broader than actual your
> > implementation.
> > > We do skip transaction of streaming cases but not during the spooling phase,
> > right ?
> > >
> > > I suggest below.
> > >
> > > "We don't skip receiving the changes at the phase to spool streaming
> > transactions"
> >
> > I might be missing your point but I think it's correct that we don't skip receiving
> > the change of the transaction that is sent via streaming protocol. And it doesn't
> > sound broader to me. Could you elaborate on that?
> OK. Excuse me for lack of explanation.
>
> I felt "streaming cases" implies "non-streaming cases"
> to compare a diffference (in my head) when it is
> used to explain something at first.
> I imagined the contrast between those, when I saw it.
>
> Thus, I thought "streaming cases" meant
> whole flow of streaming transactions which consists of messages
> surrounded by stream start and stream stop and which are finished by
> stream commit/stream abort (including 2PC variations).
>
> When I come back to the subject, you wrote below in the comment
>
> "we don't skip receiving the changes in streaming cases,
> since we decide whether or not to skip applying the changes
> when starting to apply changes"
>
> The first part of this sentence
> ("we don't skip receiving the changes in streaming cases")
> gives me an impression where we don't skip changes in the streaming cases
> (of my understanding above), but the last part
> ("we decide whether or not to skip applying the changes
> when starting to apply change") means we skip transactions for streaming at apply phase.
>
> So, this sentence looked confusing to me slightly.
> Thus, I suggested below (and when I connect it with existing part)
>
> "we don't skip receiving the changes at the phase to spool streaming transactions
> since we decide whether or not to skip applying the changes when starting to apply changes"
>
> For me this looked better, but of course, this is a suggestion.

Thank you for your explanation.

I've modified the comment with some changes since "the phase to spool
streaming transaction" seems not commonly be used in worker.c.

>
> > >
> > > (3) in the comment of apply_handle_prepare_internal, two full-width
> > characters.
> > >
> > > 3-1
> > > +        * won’t be resent in a case where the server crashes between
> > them.
> > >
> > > 3-2
> > > +        * COMMIT PREPARED or ROLLBACK PREPARED. But that’s okay
> > > + because this
> > >
> > > You have full-width characters for "won't" and "that's".
> > > Could you please check ?
> >
> > Which characters in "won't" are full-width characters? I could not find them.
> All characters I found and mentioned as full-width are single quotes.
>
> It might be good that you check the entire patch once
> by some tool that helps you to detect it.

Thanks!

>
> > > (5)
> > >
> > > I can miss something here but, in one of the past discussions, there
> > > seems a consensus that if the user specifies XID of a subtransaction,
> > > it would be better to skip only the subtransaction.
> > >
> > > This time, is it out of the range of the patch ?
> > > If so, I suggest you include some description about it either in the
> > > commit message or around codes related to it.
> >
> > How can the user know subtransaction XID? I suppose you refer to streaming
> > protocol cases but while applying spooled changes we don't report
> > subtransaction XID neither in server log nor pg_stat_subscription_workers.
> Yeah, usually subtransaction XID is not exposed to the users. I agree.
>
> But, clarifying the target of this feature is only top-level transactions
> sounds better to me. Thank you Amit-san for your support
> about how we should write it in [1] !

Yes, I've included the sentence proposed by Amit in the latest patch.

>
> > > (6)
> > >
> > > I feel it's a better idea to include a test whether to skip aborted
> > > streaming transaction clears the XID in the TAP test for this feature,
> > > in a sense to cover various new code paths. Did you have any special
> > > reason to omit the case ?
> >
> > Which code path is newly covered by this aborted streaming transaction tests?
> > I think that this patch is already covered even by the test for a
> > committed-and-streamed transaction. It doesn't matter whether the streamed
> > transaction is committed or aborted because an error occurs while applying the
> > spooled changes.
> Oh, this was my mistake.  What I expressed as a new patch is
> apply_handle_stream_abort -> clear_subscription_skip_xid.
> But, this was totally wrong as you explained.
>
>
> > >
> > > (7)
> > >
> > > I want more explanation for the reason to restart the subscriber in
> > > the TAP test because this is not mandatory operation.
> > > (We can pass the TAP tests without this restart)
> > >
> > > From :
> > > # Restart the subscriber node to restart logical replication with no
> > > interval
> > >
> > > IIUC, below would be better.
> > >
> > > To :
> > > # As an optimization to finish tests earlier, restart the subscriber
> > > with no interval, # rather than waiting for new error to laucher a new apply
> > worker.
> >
> > I could not understand why the proposed sentence has more information.
> > Does it mean you want to mention "As an optimization to finish tests earlier"?
> Yes, exactly. The point is to add "As an optimization to finish tests earlier".
>
> Probably, I should have asked a simple question "why do you restart the subscriber" ?
> At first sight, I couldn't understand the meaning for the restart and
> you don't explain the reason itself.

I thought "to restart logical replication with no interval" explains
the reason why we restart the subscriber. I left this part but we can
change it later if others also want to do that change.

Regards,

--
Masahiko Sawada
EDB:  https://www.enterprisedb.com/