Re: Implement waiting for wal lsn replay: reloaded

Hi!

On Sun, Nov 16, 2025 at 8:37 PM Alexander Korotkov <aekorotkov(at)gmail(dot)com> wrote:
>
> On Wed, Nov 12, 2025 at 9:20 AM Xuneng Zhou <xunengzhou(at)gmail(dot)com> wrote:
> > On Wed, Nov 5, 2025 at 5:51 PM Alexander Korotkov <aekorotkov(at)gmail(dot)com> wrote:
> > > On Mon, Nov 3, 2025 at 5:13 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > > > On 2025-11-03 16:06:58 +0100, Álvaro Herrera wrote:
> > > > > On 2025-Nov-03, Alexander Korotkov wrote:
> > > > >
> > > > > > I'd like to give this subject another chance for pg19. I'm going to
> > > > > > push this if no objections.
> > > > >
> > > > > Sure. I don't understand why patches 0002 and 0003 are separate though.
> > > >
> > > > FWIW, I appreciate such splits. Even if the functionality isn't usable
> > > > independently, it's still different type of code that's affected. And the
> > > > patches are each big enough to make that worthwhile for easier review.
> > >
> > > Thank you for the feedback, pushed.
> > >
> > > > One thing that'd be nice to do once we have WAIT FOR is to make the common
> > > > case of wait_for_catchup() use this facility, instead of polling...
> > >
> > > The draft patch for that is attached. WAIT FOR doesn't handle all the
> > > possible use cases of wait_for_catchup(), but I've added usage when
> > > it's appropriate.
> >
> > I tested the patch using make check-world, and it worked well. I also
> > made a few adjustments:
> >
> > - Added an unconditional chomp($isrecovery) after querying
> > pg_is_in_recovery() to prevent newline mismatches when $target_lsn is
> > accidently defined.
> > - Added chomp($output) to normalize the result from WAIT FOR LSN
> > before comparison.
> >
> > At the moment, the WAIT FOR LSN command supports only the replay mode.
> > If we intend to extend its functionality more broadly, one option is
> > to add a mode option or something similar. Are users expected to wait
> > for flush(or others) completion in such cases? If not, and the TAP
> > test is the only intended use, this approach might be a bit of an
> > overkill.
>
> I would say that adding mode parameter seems to be a pretty natural
> extension of what we have at the moment. I can imagine some
> clustering solution can use it to wait for certain transaction to be
> flushed at the replica (without delaying the commit at the primary).
>
> ------
> Regards,
> Alexander Korotkov
> Supabase

Makes sense. I'll play with it and try to prepare a follow-up patch.

--
Best,
Xuneng