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

On Mon, Mar 11, 2024 at 12:44 PM Alexander Korotkov
<aekorotkov(at)gmail(dot)com> wrote:
> I've decided to put my hands on this patch.
>
> On Thu, Mar 7, 2024 at 2:25 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)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.

------
Regards,
Alexander Korotkov