RE: Synchronizing slots from primary to standby

On Tuesday, March 5, 2024 2:35 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
>
> On Tue, Mar 5, 2024 at 9:15 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Tue, Mar 5, 2024 at 6:10 AM Peter Smith <smithpb2250(at)gmail(dot)com>
> wrote:
> > >
> > > ======
> > > src/backend/replication/walsender.c
> > >
> > > 5. NeedToWaitForWal
> > >
> > > + /*
> > > + * Check if the standby slots have caught up to the flushed
> > > + position. It
> > > + * is good to wait up to the flushed position and then let the
> > > + WalSender
> > > + * send the changes to logical subscribers one by one which are
> > > + already
> > > + * covered by the flushed position without needing to wait on every
> > > + change
> > > + * for standby confirmation.
> > > + */
> > > + if (NeedToWaitForStandbys(flushed_lsn, wait_event)) return true;
> > > +
> > > + *wait_event = 0;
> > > + return false;
> > > +}
> > > +
> > >
> > > 5a.
> > > The comment (or part of it?) seems misplaced because it is talking
> > > WalSender sending changes, but that is not happening in this function.
> > >
> >
> > I don't think so. This is invoked only by walsender and a static
> > function. I don't see any other better place to mention this.
> >
> > > Also, isn't what this is saying already described by the other
> > > comment in the caller? e.g.:
> > >
> >
> > Oh no, here we are explaining the wait order.
>
> I think there is a scope of improvement here. The comment inside
> NeedToWaitForWal() which states that we need to wait here for standbys on
> flush-position(and not on each change) should be outside of this function. It is
> too embedded. And the comment which states the order of wait (first flush and
> then standbys confirmation) should be outside the for-loop in
> WalSndWaitForWal(), but yes we do need both the comments. Attached a
> patch (.txt) for comments improvement, please merge if appropriate.

Thanks, I have slightly modified the top-up patch and merged it.

Attach the V106 patch which addressed above and Peter's comments[1].

[1] https://www.postgresql.org/message-id/CAHut%2BPsATK8z1TEcfFE8zWoS1hagqsvaWYCgom_zYtScfwO7uQ%40mail.gmail.com

Best Regards,
Hou zj