Re: [HACKERS] make async slave to wait for lsn to be replayed

On Fri, Mar 15, 2024 at 4:20 PM Alexander Korotkov <aekorotkov@gmail.com>
wrote:

> On Mon, Mar 11, 2024 at 12:44 PM Alexander Korotkov
> <aekorotkov@gmail.com> wrote:
> > I've decided to put my hands on this patch.
> >
> > On Thu, Mar 7, 2024 at 2:25 PM Amit Kapila <amit.kapila16@gmail.com>
> wrote:
> > > +1 for the second one not only because it avoids new words in grammar
> > > but also sounds to convey the meaning. I think you can explain in docs
> > > how this feature can be used basically how will one get the correct
> > > LSN value to specify.
> >
> > I picked the second option and left only the AFTER clause for the
> > BEGIN statement.  I think this should be enough for the beginning.
> >
> > > As suggested previously also pick one of the approaches (I would
> > > advocate the second one) and keep an option for the second one by
> > > mentioning it in the commit message. I hope to see more
> > > reviews/discussions or usage like how will users get the LSN value to
> > > be specified on the core logic of the feature at this stage. IF
> > > possible, state, how real-world applications could leverage this
> > > feature.
> >
> > I've added a paragraph to the docs about the usage.  After you made
> > some changes on primary, you run pg_current_wal_insert_lsn().  Then
> > connect to replica and run 'BEGIN AFTER lsn' with the just obtained
> > LSN.  Now you're guaranteed to see the changes made to the primary.
> >
> > Also, I've significantly reworked other aspects of the patch.  The
> > most significant changes are:
> > 1) Waiters are now stored in the array sorted by LSN.  This saves us
> > from scanning of wholeper-backend array.
> > 2) Waiters are removed from the array immediately once their LSNs are
> > replayed.  Otherwise, the WAL replayer will keep scanning the shared
> > memory array till waiters wake up.
> > 3) To clean up after errors, we now call WaitLSNCleanup() on backend
> > shmem exit.  I think this is preferable over the previous approach to
> > remove from the queue before ProcessInterrupts().
> > 4) There is now condition to recheck if LSN is replayed after adding
> > to the shared memory array.  This should save from the race
> > conditions.
> > 5) I've renamed too generic names for functions and files.
>
> I went through this patch another time, and made some minor
> adjustments.  Now it looks good, I'm going to push it if no
> objections.
>

The revised patch version with cosmetic fixes proposed by Alexander Lakhin.

------
Regards,
Alexander Korotkov