Re: Perform streaming logical transactions by background workers and parallel apply

On Fri, Jun 17, 2022 at 12:47 PM wangw(dot)fnst(at)fujitsu(dot)com
<wangw(dot)fnst(at)fujitsu(dot)com> wrote:
>
> On Wed, Jun 15, 2022 at 8:13 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > Few questions/comments on 0001
> > ===========================
> Thanks for your comments.
>
> > 1.
> > In the commit message, I see: "We also need to allow stream_stop to
> > complete by the apply background worker to avoid deadlocks because
> > T-1's current stream of changes can update rows in conflicting order
> > with T-2's next stream of changes."
> >
> > Thinking about this, won't the T-1 and T-2 deadlock on the publisher
> > node as well if the above statement is true?
> Yes, I think so.
> I think if table's unique index/constraint of the publisher and the subscriber
> are consistent, the deadlock will occur on the publisher-side.
> If it is inconsistent, deadlock may only occur in the subscriber. But since we
> added the check for these (see patch 0004), so it seems okay to not handle this
> at STREAM_STOP.
>
> BTW, I made the following improvements to the code (#a, #c are improved in 0004
> patch, #b, #d and #e are improved in 0001 patch.) :
> a.
> I added some comments in the function apply_handle_stream_stop to explain why
> we do not need to allow stream_stop to complete by the apply background worker.
>

I have improved the comments in this and other related sections of the
patch. See attached.

>
>
> > 3.
> > +
> > + <para>
> > + Setting streaming mode to <literal>apply</literal> could export invalid LSN
> > + as finish LSN of failed transaction. Changing the streaming mode and making
> > + the same conflict writes the finish LSN of the failed transaction in the
> > + server log if required.
> > + </para>
> >
> > How will the user identify that this is an invalid LSN value and she
> > shouldn't use it to SKIP the transaction? Can we change the second
> > sentence to: "User should change the streaming mode to 'on' if they
> > would instead wish to see the finish LSN on error. Users can use
> > finish LSN to SKIP applying the transaction." I think we can give
> > reference to docs where the SKIP feature is explained.
> Improved the sentence as suggested.
>

You haven't answered first part of the comment: "How will the user
identify that this is an invalid LSN value and she shouldn't use it to
SKIP the transaction?". Have you checked what value it displays? For
example, in one of the case in apply_error_callback as shown in below
code, we don't even display finish LSN if it is invalid.
else if (XLogRecPtrIsInvalid(errarg->finish_lsn))
errcontext("processing remote data for replication origin \"%s\"
during \"%s\" in transaction %u",
errarg->origin_name,
logicalrep_message_type(errarg->command),
errarg->remote_xid);

--
With Regards,
Amit Kapila.