On Tue, Nov 12, 2024 at 12:43 PM Ashutosh Bapat <ashutosh.bapat....@gmail.com> wrote: > > On Tue, Nov 12, 2024 at 12:02 PM Masahiko Sawada <sawada.m...@gmail.com> > wrote: > > > > On Mon, Nov 11, 2024 at 2:08 PM Tomas Vondra <to...@vondra.me> wrote: > > > > > > > > > But neither of those fixes prevents backwards move for confirmed_flush > > > LSN, as enforced by asserts in the 0005 patch. I don't know if this > > > assert is incorrect or now. It seems natural that once we get a > > > confirmation for some LSN, we can't move before that position, but I'm > > > not sure about that. Maybe it's too strict. > > > > Hmm, I'm concerned that it might be another problem. I think there are > > some cases where a subscriber sends a flush position older than slot's > > confirmed_flush as a feedback message. > >
Right, it can happen for cases where subscribers doesn't have to do anything (for example DDLs) like I have explained in one of my emails [1] > > But it seems to be dangerous if > > we always accept it as a new confirmed_flush value. It could happen > > that confirm_flush could be set to a LSN older than restart_lsn. > > Possible, though I haven't tried to reproduce such a case. But, will it create any issues? I don't know if there is any benefit in allowing to move confirmed_flush LSN backward. AFAIR, we don't allow such backward values to persist. They will temporarily be in memory. I think as a separate patch we should prevent it from moving backward. > > If confirmed_flush LSN moves backwards, it means the transactions > which were thought to be replicated earlier are no longer considered > to be replicated. This means that the restart_lsn of the slot needs to > be at least far back as the oldest of starting points of those > transactions. Thus restart_lsn of slot has to be pushed further back. > I don't see a reason to move restart_lsn backward. Why do you think so? [1] - https://www.postgresql.org/message-id/CAA4eK1%2BzWQwOe5G8zCYGvErnaXh5%2BDbyg_A1Z3uywSf_4%3DT0UA%40mail.gmail.com -- With Regards, Amit Kapila.
