Re: logical replication empty transactions

On Thu, Feb 17, 2022 at 4:12 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Mon, Jan 31, 2022 at 6:18 PM Ajin Cherian <itsajin(at)gmail(dot)com> wrote:
> >
>
> Few comments:
> =============
>

One more comment:
@@ -1546,10 +1557,11 @@ WalSndWaitForWal(XLogRecPtr loc)
* otherwise idle, this keepalive will trigger a reply. Processing the
* reply will update these MyWalSnd locations.
*/
- if (MyWalSnd->flush < sentPtr &&
+ if (force_keepalive_syncrep ||
+ (MyWalSnd->flush < sentPtr &&
MyWalSnd->write < sentPtr &&
- !waiting_for_ping_response)
- WalSndKeepalive(false);
+ !waiting_for_ping_response))
+ WalSndKeepalive(false);

Will this allow syncrep to proceed in case we are skipping the
transaction? Won't we need to send a feedback message with
'requestReply' true in this case as we release syncrep waiters while
processing standby message, see
ProcessStandbyReplyMessage->SyncRepReleaseWaiters. Without
'requestReply', the subscriber might not send any message and the
syncrep won't proceed. Why do you decide to delay sending this message
till WalSndWaitForWal()? It may not be called for each transaction.

I feel we should try to device a test case to test this sync
replication mechanism such that without this particular change the
sync rep transaction waits momentarily but with this change it doesn't
wait. I am not entirely sure whether we can devise an automated test
as this is timing related issue but I guess we can at least manually
try to produce a case.

--
With Regards,
Amit Kapila.