RE: Time delayed LR (WAS Re: logical replication restrictions)

On Tuesday, January 24, 2023 3:58 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > send_feedback():
> > + * If the subscriber side apply is delayed (because of time-delayed
> > + * replication) then do not tell the publisher that the received latest
> > + * LSN is already applied and flushed, otherwise, it leads to the
> > + * publisher side making a wrong assumption of logical replication
> > + * progress. Instead, we just send a feedback message to avoid a
> publisher
> > + * timeout during the delay.
> > */
> > - if (!have_pending_txes)
> > + if (!have_pending_txes && !in_delayed_apply)
> > flushpos = writepos = recvpos;
> >
> > Honestly I don't like this wart. The reason for this is the function
> > assumes recvpos = applypos but we actually call it while holding
> > unapplied changes, that is, applypos < recvpos.
> >
> > Couldn't we maintain an additional static variable "last_applied"
> > along with last_received?
> >
>
> It won't be easy to maintain the meaning of last_applied because there are
> cases where we don't apply the change directly. For example, in case of
> streaming xacts, we will just keep writing it to the file, now, say, due to some
> reason, we have to send the feedback, then it will not allow you to update the
> latest write locations. This would then become different then what we are
> doing without the patch.
> Another point to think about is that we also need to keep the variable updated
> for keep-alive ('k') messages even though we don't apply anything in that case.
> Still, other cases to consider are where we have mix of streaming and
> non-streaming transactions.
Agreed. This will change some existing behaviors. So, didn't conduct this change in the latest patch [1].

[1] - https://www.postgresql.org/message-id/TYCPR01MB8373DC1881F382B4703F26E0EDC99%40TYCPR01MB8373.jpnprd01.prod.outlook.com

Best Regards,
Takamichi Osumi